Hydrothermal fluids can alter the chemical and physical properties of the materials through which they pass and can therefore modify the efficiency of fluid circulation. The role of hydrothermal alteration in the development of geothermal and epithermal mineral resources, systems that require the efficient hydrothermal circulation provided by fracture networks, is investigated here from a petrophysical standpoint using samples collected from a well exposed and variably altered palaeo-hydrothermal system hosted in the Ohakuri ignimbrite deposit in the Taupō Volcanic Zone (New Zealand). Our new laboratory data show that, although quartz and adularia precipitation reduces matrix porosity and permeability, it increases the uniaxial compressive strength, Young’s modulus, and propensity for brittle behaviour. The fractures formed in highly altered rocks containing quartz and adularia are also more planar than those formed in their less altered counterparts. All of these factors combine to enhance the likelihood that a silicified rock-mass will host permeability-enhancing fractures. Indeed, the highly altered silicified rocks of the Ohakuri ignimbrite deposit are much more fractured than less altered outcrops. By contrast, smectite alteration at the margins of the hydrothermal system does not significantly increase strength or Young’s modulus, or significantly decrease permeability, and creates a relatively unfractured rock-mass. Using our new laboratory data, we provide permeability modelling that shows that the equivalent permeability of a silicified rock-mass will be higher than that of a less altered rock-mass or a rock-mass characterised by smectite alteration, the latter of which provides a low-permeability cap required for an economically viable hydrothermal resource. Our new data show, using a petrophysical approach, how hydrothermal alteration can produce rock-masses that are both suitable for geothermal energy exploitation (high-permeability reservoir and low-permeability cap) and more likely to host high-grade epithermal mineral veins, such as gold and silver (localised fluid flow).

How to cite: Heap, M., Gravley, D., Kennedy, B., Gilg, A., Bertolett, E., and Barker, S.: Upscaling laboratory measurements: Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7237, https://doi.org/10.5194/egusphere-egu2020-7237, 2020.

Shales are layered sedimentary rocks, which can be almost impermeable for fluids and act as seals and cap-rock, or, if a shale layer hosts a fracture network, it can act as a fluid reservoir and/or a conduit.  Organic-rich shales contain organic matter - kerogen, which can transform from solid-state to oil and gas during shale burial and exposure to heat. When the organic matter is decomposing into lighter molecular weight hydrocarbons, the pore-pressure inside the shale rock increases and can drive propagation of hydraulic fractures and strongly modify the permeability of these tight rocks. Density, geometry, extension, and connectivity of the final fracture network depend on the combination of the heating conditions and history of external loading experienced by the shale reservoir. Here, we have performed a series of rock physics experiments where organic-.rich shale samples were heated, under in situ conditions, and the development of microfractures was imaged through time. We used the high-energy X-ray beam produced at the European Synchrotron Radiation Facility to acquire dynamic microtomography images and monitor different modes of the shale deformation in-situ in 3D. We reproduce natural conditions of the shale deformation processes using a combination of vertical load, confining and heating of the shale samples. Shales feature natural mineral and silt lamination and hydraulic fractures easily propagate parallel to these laminae if no overburden stress is applied. However, if the principal external load becomes vertical, perpendicular to the shale lamination, the fracture propagation direction can deviate from the horizontal one. Together horizontal and vertical fractures form a three-dimensional connected fracture network, which provides escaping pathways for generated hydrocarbons. Our experiments demonstrate that tight shale rocks, which are often considered as impermeable, could have hosted transient episodes of micro-fracturing and high permeability during burial history.

How to cite: Kobchenko, M., Pluymakers, A., Cordonnier, B., and Renard, F.: Time-lapse X-ray imaging of deformation modes in organic-rich Green River Shale heated under confinement , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5417, https://doi.org/10.5194/egusphere-egu2020-5417, 2020.

How to cite: Moaven, A., Massart, T. J., and Zlotnik, S.: Real time solutions of Thermo-Hydro Mechanical problems with application to the design of Engineered Barriers via Reduced Order Methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10171, https://doi.org/10.5194/egusphere-egu2020-10171, 2020.

Maximizing heat exploitation in geothermal systems is crucial for the economic efficiency of many geothermal systems. As the hydraulic flow in most geothermal systems is primarily due to fracture flow, heat transfer processes along the fracture surfaces are essential. However, while flow and mass transport in a single fracture have been studied experimentally and theoretically to a great extent, heat transfer processes have been rarely investigated. Laboratory experiments show the influence of the fracture surface morphology on flow and heat transfer processes, though a physical interpretation has been missing so far. Further, in many geothermal systems but also in many natural hydrothermal systems, the solid and fluid phases are not in local thermal equilibrium. Parameterization of local thermal non-equilibrium models was originally developed for porous media and adoptions to fractures have been cumbersome. In this work, I present a numerical study on heat transfer processes across rough fracture surfaces. Using a three-dimensional steady-state flow model, heat transfer across the fracture surface is studied for both scenarios: assuming and neglecting a thermal equilibrium across phase boundaries. Also, separate fracture morphologies have been studied using natural sandstone probes as well as synthetically generated fractures. The numerical simulations results are compared to laboratory experiments using artificially generated and 3D-printed fracture surfaces of various fracture morphologies for code validation. The full three-dimensional simulations reveal the role of flow channeling effects on the heat transfer taking place along rough surfaces, which is not captured by simulations with reduced spatial dimensions. The simulations results suggest a re-examination of the effective heat transfer coefficient for fractured reservoirs under local thermal non-equilibrium conditions incorporating characteristics of fracture morphology. The simulations results can also be linked to thermal stress generation and possibly explaining the deformations of fracture surfaces observed in the laboratory. However, parameterization of surface roughness is neither distinct nor trivial. Various parameters exist, such as the joint roughness coefficient, Hurst exponent or statistical descriptions, but none has been successfully linked to flow, transport or transfer characteristics. Relating fracture morphology with results of numerical simulations and laboratory findings regarding transfer and transport processes indicate a shortfall of conventional roughness parameterizations to sufficiently describe the observed variation in heat transfer parameters.

How to cite: Heinze, T.: Numerical study of heat transfer across rough fracture surfaces, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4195, https://doi.org/10.5194/egusphere-egu2020-4195, 2020.

The carbon storage and energy development activities in deep geologic zones that potentially affect water quality in shallow aquifers are of central importance in the energy-water nexus. The extraction of natural gas involves hydraulically fracturing deep shale formations. The storing of carbon dioxide in deep geologic formations is pursued to mitigate global warming. Both these activities have the potential to contaminate the shallow aquifers used for potable water and return the greenhouse gases to the atmosphere. In the case of carbon storage, both during and post-injection phases, it is possible for the CO2 and formation brine to leak through natural faults, pressure-induced fractures, or failed well casings. Two scientific challenges have to be addressed to safely store the carbon in the deep formation while protecting the shallow aquifers. The first, characterizing the affected geologic formations, and the second is monitoring the leakage. Monitoring involves determining leakage locations and tracking of the gas and brine plume through the geologic formation between the deep confining layer used for storage and the shallow aquifer. Challenges to the characterization derive from the limitations and sparsity of observational data in deep formations.  Effective monitoring poses both scientific and engineering challenges as the leakage locations not known, and the resulting pathways cannot be predicted easily. This paper presents two studies where intermediate-scale testing systems were used to understand the processes that occur during the leakage of stored supercritical CO2.  The focus of the first study was to better understand the process of CO2 gas exsolution after a leak from the deep confining formation. The second study addresses the issue of monitoring brine leakage from the confined formation where supercritical CO2 is stored. The improved understanding of these leakage processes will help to develop assessment and monitoring systems for storage permeance and protecting shallow sources of potable groundwater. It is not feasible to conduct experiments in the field to obtain both the fundamental process understanding and test and validate developed modeling and monitoring tools due to lack of control of boundary and initial conditions and expense in fully characterizing deep formations. Tests in intermediate scale synthetic aquifers where highly controlled experiments can be conducted to obtain accurate data provide an alternative to overcome this challenge. However, designing test systems and performing tests under ambient laboratory conditions different types of challenges. This paper will present some of the challenges and how they were overcome. The results on new process insights, how the data was used to assess the natural capacity of the aquifer attenuation of leaking gas, and validating inversion methods for site characterization and leakage detection will be presented. Even though this study focused on CO2 leakage, the results will be of value in problems of natural has leakage during hydraulic fracturing in alternate energy development.

How to cite: Illangasekare, T. and Askar, A.: Understanding the leakage of greenhouse gasses from seep geologic formations during geologic carbon storage and hydraulic fracturing: intermediate scale testing challenges , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19592, https://doi.org/10.5194/egusphere-egu2020-19592, 2020.

Residual trapping in which ganglia of fluid are isolated and immobilised in porous media by capillary forces is innate to several subsurface engineering applications including carbon geo-sequestration. Residual trapping is highly significant in carbon dioxide (CO₂) sequestration, as entrapment of supercritical CO₂ in rock pore spaces, limits upward migration of the buoyant CO₂ plume and enhances long-term CO₂ storage security. It is estimated that residual trapping contributes up to 40% of overall trapping CO₂ in the first century following injection (1). The amount of residual trapping depends largely on the wettability of the porous rock.

Brine filled saline aquifers have been identified as having the largest potential for CO₂ storage with an estimated cumulative storage capacity of 10⁴ Giga-tons of CO₂ (2). Likewise, the focus of many studies has been devoted to investigating residual trapping in water-wet, brine filled sandstone reservoirs, and little attention has been given to intermediate-wet and oil-wet carbonate reservoirs. However, until CO₂ storage technology reaches maturity, initial CO₂ sequestration projects will most likely be conducted in depleted and oil producing carbonate reservoirs due to economic benefits associated with CO₂ enhanced oil recovery and the existence of installed infrastructure which can be reassigned for CO₂ injection purposes (3). 

Accordingly, in this work, the intrinsically water-wetting surfaces of laser fabricated glass micromodels (4); which are two-dimensional representations of natural porous rock structures, were chemically modified to imitate intermediate-wet reservoir conditions through a silanization procedure. Imbibition experiments were conducted in the micromodels using two proxy, CO₂-brine fluid pairs; deionized (DI) water and n-decane as well as DI water and air.

Fluid displacement under intermediate wettability was analysed and compared with water-wet conditions and residual fluid saturations were quantified for different porous structures. The Volume of Fluid method was used to simulate the experiments in OpenFOAM. Results from the micromodel experiments were used to validate the simulations.

This work has demonstrated that fluid displacement during the imbibition process occurs through a series of cooperative pore-filling events under intermediate-wet conditions and the presence of dead-end pores was found to enhance residual trapping of the non-wetting fluid. Coupling experimental and simulation studies provides a unique insight to multiphase flow under intermediate wet conditions.  

Acknowledgements

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (MILEPOST, Grant agreement no: 695070). This paper reflects only the authors’ view and ERC is not responsible for any use that may be made of the information it contains.

References

1. Li X, Akbarabadi M, Karpyn ZT, Piri M, Bazilevskaya E, Experimental Investigation of Carbon Dioxide Trapping Due to Capillary Retention in Saline Aquifers, Geofluids, 2015;15(4):563–76.
2. Benson; GEA; Iiasa. Chapter 13: Carbon Capture and Storage. Global Energy Asssessment. 2012.
3. Al-Menhali AS, Menke HP, Blunt MJ, Krevor SC. Pore Scale Observations of Trapped CO₂ in Mixed-Wet Carbonate Rock: Applications to Storage in Oil Fields. Environ Sci Technol 2016;50(18):10282–90.
4. Wlodarczyk KL, Carter RM, Jahanbakhsh A, Lopes AA, Mackenzie MD, Maier RRJ, Hand DP, and Maroto-Valer MM, Rapid Laser Manufacturing of Microfluidic Devices from Glass Substrates. Micromachines. 2018; 9(8)

How to cite: Nhunduru, R., Jahanbakhsh, A., Shahrokhi, O., Wlodarczyk, K., Hand, D., MacPherson, W., Garcia, S., and Maroto-Valer, M.: A Pore-Scale Investigation of Fluid Displacement and Residual Trapping Under Intermediate-Wet Conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-805, https://doi.org/10.5194/egusphere-egu2020-805, 2020.

Nowadays Hydraulic Fracturing (HF) is one of the most effective stimulation technique for hydrocarbon extraction from unconventional reservoirs, as well as enhanced geothermal applications. Practical applications of HF can have different aims. In one case, we need to stop cracks inside the host rock to avoid some HF breakthroughs into other formations and possible groundwater pollutions. The second situation is when we need to fracture several bedding planes in a reservoir which has a complex structure, especially in case of the presence of multiple natural fractures in unconventional reservoir. It is important to study hydraulic fracturing, its propagation and conditions of interaction with interfaces in laboratory conditions before expensive field application.

The present work demonstrates the results of a laboratory study designed to understand fracture interaction with artificial interfaces. For the first series of experiments, we used some natural materials such as shales, sandstones, dolomites and limestones with different porosity, permeability and mechanical properties. During these experiments we initiated hydraulic fracturing in homogeneous specimens with and without artificial surfaces, modelling natural fractures or bedding planes in unconventional reservoirs. For the second series of experiments, we used a combination of different materials to understand HF propagation in heterogeneous media, to study conditions of HF crossing or arrest at the boundaries between different types of rock. These laboratory experiments were done to create HF simulating natural processes in fractured and heterogeneous rocks or reservoirs.

Series of hydraulic fracturing experiments under uniaxial load conditions were conducted using the multifunctional system MTS 815.04. Before testing, samples were scanned by 3D CT System to characterize the rock fabric, and after testing, CT scanning was repeated to characterize 3D shape of created HF. The dynamics of HF initiation and propagation was monitored by Acoustic Emission (AE) technique, using piezoelectric sensors glued to the surface of the rock to record elastic waves radiated during the process of HF propagation. The experiments were made with different injection rates and fluid viscosities. Changes in radial strain, injection pressure and microseismic data over time were recorded.

As the result, these experiments indicate significant factors (rock heterogeneity, porosity, permeability, fluid viscosity and injection rate), influencing cracks initiation, propagation or arrest on the artificial interface. The fracture propagation and opening are characterized by measured radial deformation, fluid pressure and geometrical orientation in the sample volume. The experiments demonstrated, that fracture easily crossed artificial surface in the homogeneous limestone samples. And cracks initiated in limestone were arrested on the border with shale. In all cases combination of the AE and deformation monitoring allows to indicate fracture initiation, propagation and arrest.

How to cite: Shevtsova, A., Filev, E., Bobrova, M., Stanchits, S., and Stukachev, V.: Laboratory study of Hydraulic Fracture interaction with artificial interfaces., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9408, https://doi.org/10.5194/egusphere-egu2020-9408, 2020.

Geophysical surveying (both gravity and magnetic) is of great help in 3D modeling of granitic bodies at depth. As in any potential-field geophysics study, petrophysical data (density [r], magnetic susceptibility [k] and remanence) are of key importance to reduce the uncertainty during the modeling of rock volumes. Several works have already demonstrated that ∂¹⁸O or [SiO₂] display a negative correlation to density and to magnetic susceptibility. These relationships are particularly stable (and linear) in the so-called “non-magnetic” granites (susceptibilities falling within the paramagnetic range; between 0 and 500 10^-6 S.I.) and usually coincident with calc-alcaline (CA) compositions (very common in Variscan domains). In this work we establish robust correlations between density and magnetic susceptibility at different scales in CA granites from the Pyrenees. Other plutons from Iberia were also considered (Veiga, Monesterio). The main goal is to use the available and densely sampled nets of anisotropy of magnetic susceptibility (AMS) data, performed during the 90’s and early 2000’s, together with new data acquired in the last few years, as an indirect measurement of density in order to carry out the 3D modelling of the gravimetric signal.

We sampled some sections covering the main range of variability of magnetic susceptibility in the Mont Louis-Andorra, Maladeta and Marimanha granite bodies (Pyrenees), all three characterized by even and dense nets of AMS sites (more than 550 sites and 2500 AMS measurements). We performed new density and susceptibility measurements along two main cross-sections (Maladeta and Mont Louis-Andorra). In these outcrops, numerous measurements (usually more than 50) were taken in the field with portable susceptometers (SM20 and KT20 devices). Density data were derived from the Arquimedes principle applied on large hand samples cut in regular cubes weighting between 0.3 and 0.6 kg (whenever possible). These samples were subsampled and measured later on with a KLY-3 susceptibility bridge in the laboratory. Additionally, some density data were derived from the geometry and weighting of AMS samples.

After the calibration of portable and laboratory susceptometers, density and magnetic susceptibility were plotted together. Regressions were derived for every granite body and they usually followed a linear function similar to: r = 2600 kg/m³ + (0.5 * k [10^-6 S.I.]). As previously stated, this relationship is only valid in CA and paramagnetic granites, where iron is mostly fractioned in iron-bearing phyllosilicates and the occurrence of magnetite is negligible (or at least its contribution to the bulk susceptibility). These relationships allow transforming magnetic susceptibility data into density data helping in the 3D modelling of the gravimetric signal when density data from rock samples are scarce. Given the large amount of AMS studies worldwide, together with the quickness and cost-effectiveness of susceptibility measurements with portable devices, this methodology allows densifying and homogenizing the petrophysical data when modelling granite rock volumes based on both magnetic and gravimetric signal.

How to cite: Pueyo, E. L., Román-Berdiel, M. T., Ayala, C., Loi, F., Soto, R., Beamud, E., Fernandez de Arévalo, E., Gimeno, A., Galán, L., Schamuells, S., Bach-Oller, N., Clariana, P., Rubio, F. M., Casas-Sainz, A. M., Oliva-Urcia, B., García-Lobón, J. L., Rey, C., and Martí, J.: Density and magnetic susceptibility relationships in non-magnetic granites; a “wildcard” for modeling potential fields geophysical data., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8736, https://doi.org/10.5194/egusphere-egu2020-8736, 2020.

Testing the efficiency of ferrofluid impregnation in porous media – recommendations for future magnetic pore fabric studies

Michele Pugnetti*, Yi Zhou*, Andrea R. Biedermann*

* Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland (michele.pugnetti@geo.unibe.ch)

In AMS (anisotropy of magnetic susceptibility)-based pore fabric studies, the role of ferrofluid impregnation is crucial to ensure significant magnetic measurements. However, no standard methods to test the ferrofluid impregnation of porous media have been proposed so far. The details of fluid behaviour in porous media are important in many fields of natural sciences, but nanoparticle distribution in the fluid is particularly important for magnetic measurements. In this study methods to test the impregnation efficiency of ferrofluid in porous media, and nanoparticle distribution are proposed, using different materials: wood, agarose and TEOS (tetraethylorthosilicate) gel, and synthetic samples of given composition and grain size, as well as natural rocks. Magnetic pore fabric measurements are normally performed on natural porous samples to correlate the direction of maximum magnetic susceptibility with the direction of preferred pore elongation, and preferred flow direction. The advantage of using artificial samples is the possibility to control and adjust some physical parameters, including porosity and pore size, to keep them more uniform or fix them to a given value. This allows investigating the nanoparticle distribution in ideal samples without the influence of additional heterogeneities inherent to natural samples and to determine the lowest porosity value and smallest pore size that is possible to impregnate with ferrofluid. In particular, the agarose and TEOS gel have a uniform porous structure controlled by the gel concentration or chemical agents used in sample preparation. The wood has a wider range of porosity compared to rocks and a known intrinsically anisotropic structure. The synthetic samples have a uniform grain size, mineralogy and structure. First the porosity of the samples was measured, then to impregnate the samples different methods were developed and tested, (1) percolation, (2) standard vacuum impregnation, (3) flow-through impregnation, (4) diffusion process in gel structure. Impregnation efficiency was evaluated both optically and magnetically. Different impregnation methods provide different impregnation efficiency depending also on the investigated material; in particular porosity plays an important role in limiting the impregnation efficiency. Initial experiments indicate that in general, flow-through impregnation is more efficient than vacuum impregnation because it combines the effect of vacuum with the pressure applied to the fluid that is pushed through the sample. The best results on natural samples were obtained using calcarenites with relatively high porosity. These results and the methods proposed here will help advance magnetic pore fabrics studies and impregnation processes in general.   

How to cite: Pugnetti, M., Zhou, Y., and Biedermann, A.: Testing the efficiency of ferrofluid impregnation in porous media – recommendations for future magnetic pore fabric studies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5434, https://doi.org/10.5194/egusphere-egu2020-5434, 2020.

Unconventional oils are emerging as an alternative hydrocarbon reserve since conventional oil is depleting nowadays. A kind of unconventional oil is bitumen, which is characterized by high density, high viscosity and API gravity less than 10° and these physical properties are temperature sensitive. Therefore, an accurate assessment of variation in petrophysical properties of bitumen as a function of temperature and pressure is interesting in oil exploration industry.

In this work we investigated the role of heavy hydrocarbons (HHC) in changing petrophysical properties of carbonate-bearing rocks of the Majella reservoir performing seismic wave velocity measurements at increasing temperature. The investigated lithology belongs to the Bolognano formation that outcrops naturally in saturated and unsaturated conditions in the northwest sector of Majella Mountain (in Central Italy).

We conducted ultrasonic measurements of compressional and shear wave velocities on HHC-bearing carbonate samples showing different bitumen content and porosity between 10% and 19%. Firstly, we characterized bitumen density by HCl dissolution of the hosting rock, that resulted to be included between 1.14 and 1.26 gr/cm³ at ambient temperature. Then, we calculated HHC content of our samples, spanning from 2% (low HHC-bearing sample) to 16% (high HHC-bearing sample). Our acoustic velocities point out an inverse relationship with temperature. P- and S-wave velocities depict a distinct trend with increasing temperature depending on the amount of HHC content. Indeed, samples with the highest HHC content show a larger gradient of velocity changes in the temperature range of about 60°-50° C, suggesting that bitumen can be in a fluid state. Conversely, below about 50° C the velocity gradient is lower because, at this temperature, bitumen can change its phase in a solid state. Currently, we are analysing the coupling effect of temperature and pressure on HHC-bearing carbonate samples to test the acoustic response of the investigated samples simulating the reservoir conditions.

Our preliminary results highlight a strongly temperature dependence for HHC-bearing carbonate properties and bitumen influences the acoustic response of carbonate rocks. Such petrophysical characterization would provide a better link between seismic parameters and the hydrocarbon properties with important implications for reservoir characterization from seismic data and for production monitoring. 

How to cite: Ruggieri, R. and Trippetta, F.: Combination effects of temperature and pressure on the petrophysical properties of bitumen-bearing carbonate rocks: insight for the Majella reservoir (Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14960, https://doi.org/10.5194/egusphere-egu2020-14960, 2020.

Carbonate rocks are common in many parts of the world including the Eastern Mediterranean where they host significant groundwater supplies and are used widely in engineering as building and ornamental stones. Porosity of carbonate rocks plays a critical role in fluid storage and retrieval. The pore structure connectivity, in particular, controls many properties of rocks, and the relationships between the characteristics of individual minerals and the gross behavior of the rock. To study the relationships between porosity, rock properties, pore structure, pore size, and their impact on reservoir characteristics, several carbonate rock samples were collected from four stratigraphic sections exposed near Sidon, south Lebanon. The studied carbonate rocks are related to marine deposits of different ages (e.g., Upper Cretaceous, Eocene and Upper Miocene). In order to understand the pore connectivity, the MICP (mercury injection capillary pressure) is conducted on ten representative samples. Results from the SEM analysis indicate the dominance of very fine and fine pore sizes with various categories ranging in diameter from 0.1 to10 µm. The MICP data revealed that the pore throat radii vary widely from 0.001 to 1.4µm, and that all samples are dominated by micropore throats. The grain size analysis indicated that the studied rocks have significant amounts of silt- and clay-size grains with respect to the coarser sand-size particles; suggesting a high proportion of microporosity. Obtained results such as the poorly-sorted nature of grains, high microporosity, and the high percentage of micropore throats justify the observed low mean hydraulic radius, the high entry pressure, and the very low permeability of the studied samples. These results suggest that the carbonate rocks near Sidon (south of Lebanon) are possibly classified as non-reservoir facies.

How to cite: Salah, M.: Pore Structure and Petrophysical Characterization of Carbonate Rocks from Southern Lebanon , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1920, https://doi.org/10.5194/egusphere-egu2020-1920, 2020.

Understanding the deformation-related thermomechanical state of reservoir rocks under in-situ conditions is essential for modelling the stress distribution and stability of subsurface structures, for example associated with aftershock activity and induced seismicity. Commonly, reservoir modelling approaches make use of the generalized friction criterion according to Byerlee, which distinguishes between depths below and above approximately 6 km. However, numerous studies have shown that thermomechanical rock properties under elevated pressure and temperature conditions differ significantly from those at the surface and among rock types. The significant influence of the geothermal gradient on elastic and inelastic rock properties has already been demonstrated for temperature variations as low as 150 °C. Studies on the effect of in-situ stress and temperature conditions on post-failure behaviour and frictional properties are completely lacking.

In our experimental study we determined the thermomechanical properties of porous Ruhr sandstone samples during conventional triaxial deformation tests to derive stress- and temperature-dependent failure and friction criteria. Effective confining pressures and temperatures applied in the tests cover the range of in-situ conditions equivalent to depths up to three kilometres. Simultaneously, ultrasonic P- and S-wave measurements were performed to determine properties of ultrasound wave propagation (i.e. dynamic elastic properties) as a function of in-situ conditions. Triaxial deformation experiments were conducted at various strain rates to investigate the deformation-rate dependence of the failure and friction criteria and the correlation between dynamic and static elastic properties.

How to cite: Ahrens, B., Duda, M., and Saenger, E. H.: Experimental investigations on the temperature and strain-rate dependence of failure and friction criteria for a porous sandstone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15134, https://doi.org/10.5194/egusphere-egu2020-15134, 2020.

While the deep granitic basement in the Upper Rhine Graben is currently being exploited as a geothermal reservoir at numerous geothermal sites, the Permo-Triassic sandstones that lie directly above the granite are critical to continued regional hydrothermal convection. Here we investigate the propensity for variably sealed fractures to be reactivated during deformation and the role this fracture reactivation plays on permeability enhancement in geothermal reservoirs. We source un-fractured, bedded sandstones and the same bedded sandstones containing a single, variably-sealed fracture from a 400 m-thick unit of Permo-Triassic sandstone sampled from the EPS-1 exploration well near Soultz-sous-Forêts (France) in the Upper Rhine Graben.

31 cylindrical samples (20 mm in diameter and 40 mm long) were cored such that their dominant structural feature (i.e. bedding or natural fracture) was oriented parallel, perpendicular, or at 30° to the sample axis. The initial permeability of the un-fractured samples ranged between 2.5×10^-17 and 5.6×10^-16 m² and between 3.6×10^-16 and 3.3×10^-14 m² for naturally fractured samples. In un-fractured samples, permeability decreases as a function of increased bedding angle; fracture orientation, however, does not appear to have a discernable influence on permeability. Samples were water-saturated and deformed until failure under pressure conditions appropriate to the Soultz-sous-Forêts geothermal system - P_eff of 14.5 MPa - and at a strain rate of 10^-6 s^-1. All samples developed through-going shear fractures, however, only in samples containing partially sealed fractures did the experimentally produced fractures take advantage of the pre-existing features. In samples containing a fully-sealed fracture, the experimentally induced fracture developed in a previously undeformed part of the sandstone matrix. Further, post-deformation permeability measurements indicate that while sample permeability increased by up to one order of magnitude for a given sample, this increase is generally independent of feature orientation.

Therefore, formations containing sealed fractures may not necessarily be weaker and, as a consequence, may not be more apt to significant permeability increases during stimulation than un-fractured formations. These data can contribute to the development and optimization of stimulation techniques used in the Upper Rhine Graben.

How to cite: Kushnir, A., Heap, M., Baud, P., and Reuschlé, T.: Fracture reactivation for permeability enhancement in geothermal systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7273, https://doi.org/10.5194/egusphere-egu2020-7273, 2020.

Global energy demand is expected to grow significantly as the world population and the standard of living increase in the coming decades. As a potential source of energy, gas hydrate, which is a crystalline compound of gas-water mixture formed in stable of high pressure and low temperature, has been intensively investigated in the past few decades. In this work, a new analytical model is derived to study the effect of hydrate saturation on stress-dependent relative permeability behavior of hydrate-bearing sediments. The proposed relative permeability model solves the steady-state Navier-Stokes equations for gas-water two-phase flow in porous media with hydrates. It considers water saturation, hydrate saturation, viscosity ratio and hydrate-growth pattern, and is adequately validated with the experimental results in existing literatures. The model demonstrates that gas-water relative permeability in wall coating hydrates (WC hydrates) is larger than that in pore filling hydrates (PF hydrates). For WC hydrates, water phase relative permeability monotonically decreases as gas saturation increases. However, for PF hydrates, water phase relative permeability firstly increases and then decreases with the increase of gas saturation, which can be explained by the “lubricative” effect of the gas phase that exists between the water phase and hydrates. This work constitutes a comprehensive investigation of stress-dependent relative permeability in deformable hydrate-bearing sediments, which is a key issue for sustainable gas production. It not only provides theoretical foundations for quantifying relative permeability in hydrate-bearing sediments, but also can be used to estimate pore-scale parameters and rock lithology of gas hydrate-bearing sediments using inverse modeling.

How to cite: Lei, G., Liao, Q., and Shirish, P.: New Predictive Model for Relative Permeability of Deformable Gas Hydrate-Bearing Sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2148, https://doi.org/10.5194/egusphere-egu2020-2148, 2020.

Fluid flow along fractures or in fractured rock is of great importance in Enhanced Geothermal System, since natural fracture networks generally affect the permeability of the reservoir rocks and therefore the hydraulic performance. The cubic law commonly estimates the permeability of a single fracture, which is only valid for the flow through two smooth parallel plates. In fact, the flow performance is strongly influenced by the aperture fluctuations, which are related to the fracture surface roughness, the fluid-rock interaction process, and the amount of flow exchange between the matrix and the fracture itself, etc.

To quantify the hydraulic performance and get the better knowledge of the more real fracture flow, we conduct numerical simulations of fluid flow in a fracture-rock system hosting one single rough fracture from laboratory to field scales. As an example, a 2D self-affine rough surface is synthetically generated (Candela et al, 2012), with two anisotropic roughness exponents H_// = 0.6 along the slip direction, H_perp = 0.8 in the perpendicular direction and a RMS amplitude of 0.1m at the 512m scale. Based on this surface generation, the opening geometry of a rough fracture is obtained as an input structure for finite element mesh generation. On one hand, we apply a lubrication approximation and limit the fracture opening to spatially variable 2D features with lower-dimensional element embedded in a saturated porous. On the other hand, we consider the full 3D features of the fracture opening as the space between two surfaces symmetrical about the mean fracture plane. The simulations are performed in the framework of the Mutiphysics Object Oriented Simulation Environment (MOOSE) combined with a MOOSE-based application GOLEM dedicated to modeling coupled Thermal-Hydraulic-Mechanical (THM) process in fractured geothermal reservoirs.

For the lubrication case, the mass balance equation for a saturated porous medium is described in terms of volumetric averaged mass conservation equations for the fluid phase, with Darcy’s law governing the momentum conservation equation. For the 3D fracture case, the incompressible Navier-Stokes equation is solved for the dynamic pressure and the velocity field inside the fracture only.

We compare the 2D and 3D cases and assess the effects of the nonlinear inertial term (u•∇)u in 3D case especially when the Reynolds number is high. The objective is to evaluate the large-scale hydraulic diffusivity of the fractured domain and its anisotropy owing to the strong contrast between the fluctuating fracture opening, and the homogeneous bulk porosity. The results show that the long-range aperture variations significantly affect the fluid flow, like the channeling effect and the hydraulic diffusivity anisotropy (i.e., along and perpendicular to the fault), which may have strong implications on the spatial distribution of fluid-induced seismic events in faulted reservoir.

How to cite: Deng, Q., Schimittbuhl, J., Blocher, G., and Cacace, M.: Fluid flow along a rough fracture: impact on hydraulic diffusivity , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16747, https://doi.org/10.5194/egusphere-egu2020-16747, 2020.

When two mineral surfaces are in close contact, nanometers to microns apart, the proximity of another surface can significantly influence the pathways of chemical reactions happening in the interfacial region. Apart from affecting the kinetics of dissolution and nucleation reactions in spatial confinement, the proximity of charged surfaces can lead to electrochemically induced recrystallization processes. The latter may happen in an asymmetric system, in which two surfaces have a dissimilar surface charge. The charge and mass transferred during electrochemical reactions can induce dissolution or growth of solids and can significantly affect the local topography of surfaces, causing them to smooth out or to roughen. In this work, we present the experimental study of reactive mineral interfaces, immersed in geologically relevant electrolyte solutions, obtained with the electrochemical surface forces apparatus (EC-SFA). EC-SFA setup consists of one mineral surface and one gold surface (working electrode), the surface charge of which is controlled by applying an electrical potential. EC-SFA can, therefore, monitor electrochemically induced surface recrystallization processes. As the SFA technique is based on white light interferometry measurements, the changes in mineral thickness during recrystallization can be determined with an accuracy better than a nanometer over micrometer-large contact regions. Moreover, SFA allows in situ measurement of surface forces acting between mineral surfaces, which can provide additional information about how the surface reactivity influences the cohesion between mineral surfaces by modifying adhesive and repulsive forces acting between them at small separations.

How to cite: Dziadkowiec, J., Cheng, H.-W., Røyne, A., and Valtiner, M.: Interfacial processes at dissimilarly charged mineral surfaces in contact – a surface forces apparatus study , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5982, https://doi.org/10.5194/egusphere-egu2020-5982, 2020.

Electrochemical interactions of calcite with brines in natural subsurface settings have received ample attention in the last decades due to the broad range of their applications. These interactions can be described by an electrical property termed the zeta potential. Many numerical simulation studies using surface complexation modelling (SCM) have been performed to investigate the relationship between the zeta potential and a wide range of salinities and complex brine compositions. Although most of the simulated results, especially in low salinity conditions, successfully match the experimentally measured zeta potential, the simulated zeta potential for high salinity conditions is still poorly understood.

In this study, we present a new approach of SCM to simulate the zeta potential by considering the actual molecular-scale phenomena at the calcite-brine interface. Unlike previous SCM studies, our model considers the hydrated diameter of ions as the distance of approach, which depends on salinity. We also consider the permittivity of the Stern layer as a function of salinity, which is consistent with previous unrelated studies. We calculate the capacitance for each salinity based on the relationship between the hydrated diameter of ions and the permittivity of the Stern layer. Moreover, all calcite-brine surface reactions are described by new equilibrium constants independent of salinity and composition of brines.

Our results show that the simulated zeta potential which is obtained from our SCM at a broad range of salinities is successfully matched with the published experimental data for two different carbonate rock samples as long as the salinity dependence of the hydration diameter and electrical permittivity is accounted for. We find that the potential determining ions (Ca²⁺, Mg²⁺, SO₄^2-, HCO₃^-,CO₃^2-) play a dominating role compared to the indifferent ions (Na⁺, Cl^-) in the calcite-brine surface reactions. The Implications of our findings are significant for wettability evaluation, characterisation of shallow and deep aquifers and CO₂ geological sequestration.

How to cite: Hidayat, M., Vinogradov, J., Iglauer, S., and Sarmadivaleh, M.: Evolution of Modelling Zeta Potential: Impact of Brine Compositions and Concentration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20857, https://doi.org/10.5194/egusphere-egu2020-20857, 2020.

In this study, zeta potential has been measured by using the streaming potential method for the intact sandstone in contact with CaCl₂ electrolytes. The experimental results show that a positive zeta potential has been observed for the first time for the intact Fontainebleau sandstone under high salinity of CaCl₂, and its magnitude increases with increasing ionic strength. It cannot be explained by the Gouy-Chapman theory anticipating a constant potential for high salinities due to the collapse of the electrical double layer. Meanwhile, the brine effluents after the completion of the streaming potential measurements were collected and then pH and brine composition were analysed suggesting that those variations of pH and chemical composition are negligible and cannot explain the polarity change at high salinity. The anomalous positive potential of the intact Fontainebleau sandstone is due to that overcharge of calcium ions sorbed into the mineral surface, which is consistence with previous literature data.

How to cite: Li, S. and Jackson, M.: Critical role of the structure of the mineral-water interface in the zeta potential measured by streaming potential method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22077, https://doi.org/10.5194/egusphere-egu2020-22077, 2020.

We numerically study the effects that roughness in the walls of cracks has on the P-wave modulus dispersion and attenuation due to squirt flow. We emulate the deformation caused by a seismic P-wave by applying an oscillatory relaxation test on numerical rock models having two perpendicular fluid-filled cracks interconnected and embedded in a cubic elastic background. The deformation caused by the P-wave induces a fluid pressure gradient and then, during the consequent fluid pressure diffusion process, the friction between fluid particles dissipate seismic wave energy. In this work, we consider P-wave deformation normal to one of the cracks. We first consider binary aperture distribution for the cracks to analyse where the energy dissipation process takes place. Then, more complex geometries for the roughness of the walls are also considered. In both cases, the cracks have finite length and square-shape and no contact areas between the walls of the cracks were allowed to occur. We show that the arithmetic mean of the apertures controls the P-wave modulus magnitudes at the low- and high-frequency limits. Additionally, two attenuation peaks and modulus dispersion regimes may occur associated with squirt flow. In general, at low-frequencies, the energy dissipation tends to happen inside the minimum aperture of the cracks, and consequently, the minimum aperture determines the frequency at which the low-frequency attenuation peak occurs. For the considered models, we observed that when the percentage of minimum aperture in the cracks is lower than 10$\%$, a second attenuation peak at high frequencies become dominant. The characteristic frequency of this attenuation process is controlled by an effective hydraulic aperture. Finally, we simulate an increase in confining pressure by reducing the crack apertures by a constant value, allowing for contact areas occurrence. In this scenario, the stiffness of the cracks can not longer be explained with the arithmetic mean of the aperture, as the stiffening effect of the distribution of the contact areas plays a much stronger role. In general, from the analysis of the local energy dissipation, different apertures seem to control the energy dissipation process at each frequency, which means that a frequency-dependent hydraulic aperture might be needed to describe the squirt flow process in cracks with rough walls.

How to cite: Lissa, S., Barbosa, N. D., Caspari, E., Alkhimenkov, Y., and Quintal, B.: Seismic attenuation and velocity dispersion due to squirt flow in cracks with rough walls, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1424, https://doi.org/10.5194/egusphere-egu2020-1424, 2020.

The use of passive seismic techniques to monitor geothermal reservoirs allows to assess the risks associated with their exploitation and stimulation. One key characteristic of geothermal reservoirs is the degree of fracture connectivity and its evolution. The reason for this is that changes in the interconnectivity of the prevailing fractures affect the permeability and, thus, the productivity of the system. An increasing number of studies indicates that the Rayleigh wave velocity can be sensitive to changes in the mechanical and hydraulic properties of geothermal reservoirs. In this work, we explore the effects of fracture connectivity on Rayleigh wave velocity dispersion accounting for wave-induced fluid pressure diffusion effects. To this end, we consider a 1D layered model consisting of a surficial sandstone formation overlying a fractured and water-saturated granitic layer, which, in turn, is underlain by a compact granitic half-space. For the stochastic fracture network prevailing in the upper granitic layer, we consider varying levels of fracture connectivity, ranging from entirely unconnected to fully interconnected. We use an upscaling approach based on Biot’s poroelasticity theory to determine the effective properties associated with these scenarios. This procedure allows to obtain the frequency-dependent seismic body wave velocities accounting for fluid pressure diffusion effects. Finally, using these parameters, we compute the corresponding Rayleigh wave velocity dispersion. Our results show that Rayleigh wave phase and group velocities exhibit a significant sensitivity to the degree of fracture connectivity, which is mainly due to a reduction of the stiffening effect of the fluid residing in connected fractures in response to wave-induced fluid pressure diffusion. This suggests that time-lapse observations of Rayleigh wave velocity changes, which so far are commonly associated with changes in the fracture density, could also be related to changes in the interconnectivity of pre-existing fractures.

How to cite: Quiroga, G., Rubino, J. G., Solazzi, S., Barbosa, N., and Holliger, K.: Effects of fracture connectivity on Rayleigh wave velocity dispersion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14762, https://doi.org/10.5194/egusphere-egu2020-14762, 2020.

Elastic waves are commonly studied in geophysics. They are used for example for prospecting, to follow the exploitation of hydrocarbon reservoirs, to study the effect of fluid injection (CO₂ storage)… However, the wave frequencies used in the field (sonic – seismic measurements) are not the same as the ones commonly used in the laboratory (ultrasonic measurements), and fluid-saturated rocks are known to be dispersive, i.e the P- and S- wave velocity in fluid-saturated rock change with frequency. The comparison between field and laboratory measurements is therefore not straightforward.

In the ENS facilities, it is possible to subject samples, under pressure (1 to 30 MPa) to forced - oscillations varying from 0.01 Hz to 1 kHz (field frequencies) and 1 MHz (ultrasonic frequencies) using a triaxial cell. Axial and radial strain gauges are installed to record the resulting strains on the sample. Forced-oscillation can be done on 1) confining pressure to get the bulk modulus as function of frequency or on 2) axial stress to get the Young modulus and Poisson ratio as function of frequency.  With this information, it is thus possible to deduce the P- and S- wave velocities with frequency.

The elastic properties were measured on different samples from the Libra oil field, for which logging measurements are available. Thus, the measurements obtained in the laboratory can be compared to the measurements in the field at the same frequency. In addition, the evolution of the velocity with frequency measured in the laboratory allows us to discuss the mechanisms at the origin of the dispersion.  

How to cite: Gallagher, A.: Comparison of the elastic properties of reservoir rocks in the field and the laboratory: link between seismic, sonic and ultrasonic measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1335, https://doi.org/10.5194/egusphere-egu2020-1335, 2020.

Field observations and laboratory experiments have demonstrated strain localisation can develop in porous rocks in response to an applied stress field. Shear fractures and compaction bands are strain localisation features that can form at relatively low confinement during brittle deformation and at higher confinement during shear-enhanced compaction, respectively. Previous experimental studies suggested that the formation and geometry of compaction bands also depends on the microstructural attributes of the rock.

We investigated the influence of microstructure on compaction localisation in porous rocks using sintered glass bead samples, which allowed for a tight control on grain size and shape and sample porosity. During the fabrication process, populations of solid glass microspheres of predetermined size and size distribution are heated above their glass transition temperature. Above this temperature, the glass beads act as viscous liquid droplets. Time-dependent coalescence of droplets that share contact then causes the bead-pack to evolve into a connected system, producing a porous granular material of known microstructural geometries and final porosity.

We previously conducted hydrostatic compaction and triaxial compression tests on synthetic samples of porosity ranging from 10 to 38% with a monodisperse grainsize (diameter ranging from 0.15 to 1.3 mm). Experimental results showed remarkable reproducibility for the same experimental conditions and concurrence with the phenomenology of mechanical behaviour of natural sandstones. After these validation tests, we conducted systematic experiments on monodisperse synthetic samples of 25 and 35% of porosity prepared using glass beads of mean diameter 0.25, 0.525 and 1.15 mm. Triaxial deformation tests were conducted on water-saturated samples, in drained conditions (with a fixed pore pressure of 10 MPa), at room temperature, at a constant strain-rate and at effective pressures corresponding to the regime of formation of compaction bands. Our mechanical data provide indirect evidence for compaction localisation. We have focused our attention on the influence of porosity and grain size on the formation and microstructural attributes (such as thickness, length and tortuosity) of the compaction bands.

How to cite: Carbillet, L., Heap, M. J., Wadsworth, F. B., Baud, P., and Reuschlé, T.: Microstructural control on compaction localisation in granular materials, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2697, https://doi.org/10.5194/egusphere-egu2020-2697, 2020.

How to cite: Eichheimer, P., Thielmann, M., Fujita, W., Golabek, G. J., Nakamura, M., Okumura, S., Nakatani, T., and Kottwitz, M. O.: Combined numerical and experimental study of microstructure and permeability in porous granular media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2992, https://doi.org/10.5194/egusphere-egu2020-2992, 2020.

Scanning electron microscopy (SEM) and helium ion microscopy (HIM) are two of the fundamental tools in the study of the microstructures of shale. A comprehensive comparison of these two techniques in the application of organic pore structure characterization is presented in this work. Owing to the small wavelength of the helium ion, the spot size of the ion beam is not restricted by diffraction aberration, and the convergence angle of helium ion beam can be much smaller than of the electron beam. The microscopic images and reconstruction models indicate that HIM has higher spatial resolution and increased depth of field than SEM. The pores below 10 nm and inner structures of pore networks can be observed via HIM images. The advantages shown in the focused ion beam/helium ion microscopy (FIB/HIM) results are similar to the 2-D HIM images. Smaller pores whose size is beyond the resolution of focused ion beam/scanning electron microscopy (FIB/SEM) can be found, which suggests the connection possibility of the big pores. However, to get reliable pictures, the ion-induced damage on organic matters should be avoided. To lower the beam current and to shorten the dwell time are two effective ways to reduce the beam damage.

How to cite: Zhao, J., Zhang, W., and Zhang, D.: Organic pore structure characterization of shale: a comparison between scanning electron microscopy and helium ion microscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12191, https://doi.org/10.5194/egusphere-egu2020-12191, 2020.

Microtomographic imaging techniques and advanced numerical simulations are combined by digital rock physics (DRP) to obtain effective physical material properties. The numerical results are typically used to complement laboratory investigations with the aim to gain a deeper understanding of physical processes related to transport (e.g. permeability and thermal conductivity) and effective elastic properties (e.g. bulk and shear modulus). The present study focuses on DRP and laboratory techniques applied to a rock called reticulite, which is considered as an end-member material with respect to porosity, stiffness and brittleness of the skeleton. Classical laboratory investigations on effective properties, such as ultrasonic transmission measurements and uniaxial deformation experiments, are very difficult to perform on this class of high-porosity and brittle materials.

Reticulite is a pyroclastic rock formed during intense Hawaiian fountaining events. The open honeycombed network has a porosity of more than 80 % and consists of bubbles that are supported by glassy threads. The natural mineral has a strong analogy to fabricated open-cell foams. By comparing experimental with numerical results and theoretical estimates we demonstrate the potential of digital material methodology with respect to the investigation of porosity, effective elastic properties, thermal conductivity and permeability

We show that the digital rock physics workflow, previously applied to conventional rock types, yields reasonable results for a high-porosity rock and can be adopted for fabricated foam-like materials. Numerically determined effective properties of reticulite are in good agreement with the experimentally determined results. Depending on the fields of application, numerical methods as well as theoretical estimates can become reasonable alternatives to laboratory methods for high porous foam-like materials.

How to cite: Schepp, L. L., Ahrens, B., Balcewicz, M., Duda, M., Nehler, M., Osorno, M., Uribe, D., Steeb, H., Nigon, B., Stöckhert, F., Swanson, D. A., Siegert, M., Gurris, M., and Saenger, E. H.: Digital rock physics and laboratory considerations on a high-porosity volcanic rock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6706, https://doi.org/10.5194/egusphere-egu2020-6706, 2020.

In the lithosphere, the transition from brittle to ductile deformation corresponds to a regime where brittle fracturing and plastic flow coexist, called the semi-brittle deformation zone. Within these different regimes, a large fault slip spectrum has been observed, from fast to slow earthquakes. Studying the parameters controlling fault (un-)stability and strain partitioning across this transition is fundamental to understand how natural faults behave at varying crustal depths.

To investigate semi-brittle deformation and the conditions promoting it, we report here the results of experiments performed on Carrara marble saw-cut faults in triaxial conditions. We studied the influence of the confining pressure, axial loading rates and initial fault roughness on fault (un-)stability. From mechanical data, we performed strain partitioning calculations to infer elastic, frictional and plastic strain contributions during the deformation process.

We conclude that (laboratory) earthquakes may nucleate within a regime where homogeneous plastic deformation of the bulk and dynamic fault slip may coexist. The contribution of plastic strain is promoted with increasing confining pressure and fault roughness.

How to cite: Aubry, J., Passelègue, F., Escartín, J., Deldicque, D., Gasc, J., Marty, S., Page, M., and Schubnel, A.: Fault (un-)stability and strain partitioning across the brittle-ductile transition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4092, https://doi.org/10.5194/egusphere-egu2020-4092, 2020.

Acoustic Emissions (AE), the laboratory analogue to seismic events, recorded during conventional triaxial deformation tests allow for an unprecedented amount of information on the evolution of fractured media within a controlled environment. This study presents the results of a new and robust derivation of first motion polarity focal mechanism solutions (FMS). 4 x 10 cm cylindrical samples of Alzo Granite (AG) and Darley Dale Sandstone (DDS) underwent systematic triaxial deformation testing (5, 10, 20 and 40 MPa) in order to investigate the relationships between increasing confining pressure, deformation and failure mode and role of pre-existing microstructure. With an average of 11 of 12 waveforms picked using a neural network for each AE, high resolution datasets are obtained that can track the evolution of deformation structure through time. Focal mechanisms are solved using a least squares minimisation of the fit between projected polarity measurements and the deviatoric stress field induced by tensile, shearing and collapse/closing type sources. Results reveal a surprisingly limited dependency on the distribution of shear fracturing in the lead up to dynamic failure. Instead, deformation is driven by the competition between the opening and closure of fractures that is strongly related to the coupling of local stress fields with pre-existing damage.Spatio-temporal trends in mechanism type and AE amplitude allow for clear identification of: a) Fracture Enucleation. This phase is characterised by broadly distributed tensile fracturing that becomes preferentially aligned as confining pressure increases; b) Fracture Growth. The onset is characterized by a discrete increase in low amplitude shearing events and cyclic fracture development that evolves from a dominance of collapse to shearing followed by tensile fracturing which then returns to collapse type. Influences in mechanism dominance due to rock type are highlighted by increased tensile fracturing in AG, which is replaced by shearing in DDS. A reduction in low amplitude tensile events at 10 MPa in both rock types further reveals a switch from axial splitting to planar localisation as confinement increases; c) Crack Coalescence. The cyclic fracture growth prior to dynamic failure and the amount of strain of this phase share a positive log-linear relationship with confining pressure, allowing to identify the potential for real-time failure prediction; d) Dynamic Failure: High amplitude events characterize the propagation of fractures. Taken together results highlight that failure of the studied samples is the result of the complex interaction between distinct regions of dilatant and compactant deformation. Although planar localisation and preferentially aligned flaws play a more significant role at higher confining pressures, it is the initial heterogeneity or patchiness of the regions undergoing damage that control dynamic failure occurrence and the eventual fracture plane features.

How to cite: Vinciguerra, S., King, T., Benson, P., and De Siena, L.: Damage indicators and failure prediction in Focal Mechanism solutions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13547, https://doi.org/10.5194/egusphere-egu2020-13547, 2020.

Fractures are ubiquitous in geological sequences, and play an important role in the movement of fluids in the earth’s crust, particularly in fields such as hydrogeology, petroleum geology and volcanology. When predicting or analysing fluid flow, fractures are often simplified as a set of smooth parallel plates. In reality, they exhibit tortuosity on a number of scales: Fine-scale tortuosity, or roughness, is the product of the small-scale (µm – mm) irregularities in the fracture surface, whereas large-scale (> mm) tortuosity occurs as a result of anisotropy and heterogeneity within the host formation that leads to the formation of irregularities in the fracture surfaces. It is important to consider such tortuosity when analysing processes that rely on the movement (or hindrance) of fluids flowing through fractures in the subsurface. Such processes include fluid injection into granitic plutons for the extraction of heat in Engineered Geothermal Systems, or the injection of CO₂ into reservoirs overlain by fine-grained mudrocks acting as seals in Carbon Capture and Storage projects.

Although it is generally assumed that tortuosity is controlled by factors such as grain size, mineralogy and fracture mode, a systematic study of how these factors quantitatively affect tortuosity is currently lacking. Furthermore, in anisotropic rocks the fracture orientation with respect to any inherent anisotropy is also likely to affect tortuosity.

In order to address this gap, we have induced fractures in a selection of different rock types (mudrocks, sandstones and carbonates) using the Brazil disk method, and imaged the fracture surfaces using both a digital optical microscope and X-ray Computed Tomography. Using these methods we are able to characterise both the fine-scale (roughness) and large-scale tortuosity. In order to understand the effect of fracture orientation on tortuosity we have also analysed fractures induced at different angles to bedding in samples of a highly anisotropic mudrock taken from South Wales, UK. Results indicate that fine-scale tortuosity is highly dependent on the fracture orientation with regards to the bedding plane, with fractures normal to bedding being rougher than those induced parallel to bedding. Finally, in order to measure the effect of tortuosity on fluid flow, we have carried out a series of core flooding experiments on a subset of fractured samples showing that fracture transmissivity decreases with increasing tortuosity.

How to cite: Forbes Inskip, N., Phillips, T., Bisdom, K., Borisochev, G., Busch, A., and den Hartog, S.: An investigation into the controls on fracture tortuosity in rock sequences and the impact on fluid flow in the upper crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10236, https://doi.org/10.5194/egusphere-egu2020-10236, 2020.

Fluid-driven fracturing is a key process in enhancing production in both the hydrocarbon and geothermal energy extraction industries. However, whilst a large number of studies have now developed laboratory methods to simulate the process in a range of settings, and across a number of different rock types, data relating the fundamental material parameters (such as fracture toughness) to the overall rock mechanics response as a function of parameters such as confining and pore pressure remain limited. Here we report a new analysis to recover fracture toughness across a range of effective pressures from hydraulic fracturing experiments that use a modified thick-walled cylinder sample mounted in a conventional triaxial deformation apparatus. We use samples that are 90mm in length and 40mm diameter, with a central, axially drilled borehole 12.6 mm in diameter. An array of 16 ports in the engineered, nitrile, sample jacket allows us to record radial strain (4 channels), acoustic emission output (11 channels) and borehole fluid pressure (1 channel) continuously throughout each test. The sample material was Nash Point shale (NPS) from the south coast of Wales, UK, with samples cored both normal and parallel to bedding in order to investigate the effect of anisotropy. Earlier, ambient pressure fracture toughness tests using the Semi-Circular Bend sample geometry had indicated significant anisotropy, values of 0.24 – 0.30 MPa.m^1/2 in the Short-Transverse (ST) orientation, and 0.71 - 0.73 MPa.m^1/2 in the Divider (DIV) orientation.

Here, we present results from a suite of 9 experiments, 6 with samples cored parallel to bedding (ST fracture orientation) and 3 with samples cored normal to bedding (DIV fracture orientation). We find that the fluid injection pressure required to fracture our annular shell samples is significantly higher for DIV samples than for ST samples, and increases significantly with increasing confining pressure in both orientations; ranging from 10 to 36 MPa for ST samples and 30 to 58 MPa for DIV samples as confining pressure is increased from 2.2 to 20.5 MPa. We note that the fluid injection pressure undergoes a number of oscillations between fracture nucleation and the fracture reaching the sample boundary. Such oscillations are more common in ST samples than in DIV samples, and in experiments at lower confining pressures. We use the magnitude of each pressure oscillation to estimate the associated increment of fracture extension via the proportion of AE energy generated relative to the total energy accumulated when the fracture reaches the sample boundary. This analysis produces fracture toughness values ranging from 0.36 to 2.76 MPa.m^1/2 (ST orientation) and 2.98 to 4.05 MPa.m^1/2 (DIV orientation) as confining pressure was increased from 2.2 to 20.5 MPa. We further find that the increase in fracture toughness increases essentially linearly with increasing effective pressure, and this trend appears to be independent of orientation and the material anisotropy.

How to cite: Benson, P., Gehne, S., Forbes Inskip, N., Meredith, P., and Koor, N.: Fluid-driven tensile fracture and fracture toughness in Nash Point Shale at elevated pressure., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14434, https://doi.org/10.5194/egusphere-egu2020-14434, 2020.

We focus on calcareous homogenous shales featuring different degrees of damage along a km-long strain gradient, marked by cleavage development. In a previous study, we used high-resolution X-ray computed tomography (µCT) to document the evolution of the 3D fabric of the fine-grained shales along the strain gradient (Saur et al., JSG, 2020). Our conclusions were based on samples of ~ 2.5 mm³ containing over 10’000 quartz and calcite grains. The objective of the current study is to assess the representativeness of analyses on such small rock samples. To that extent, we first repeat the µCT analysis on multiple samples of the same, limited, volume and assess the variability of the results. These results are then compared to both macroscopic field observations and bulk fabric measurements obtained with AMS (Anisotropy of Magnetic Susceptibility) on larger samples (~ 10 cm³). AMS provides a statistical description of the magnetic susceptibility tensor, and particularly the confidence angle of axis orientation. Generally, this confidence angle is the result of matrix organization and rock magnetism. In this study, AMS is only controlled by the presence of illite particles which reflect the matrix organization. Finally, we perform a subvolume analysis on the µCT images to determine the smallest representative volume characterizing the fine-grained fabric. In light of these analyses we discuss the representativeness of investigated volume of fine grained shales, subjected to different degrees of deformation.

How to cite: Saur, H., Aubourg, C., Moonen, P., Sénéchal, P., Boiron, T., and Derluyn, H.: Representativeness of volume investigated by high-resolution X-ray computed tomography in damaged fine-grained rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20641, https://doi.org/10.5194/egusphere-egu2020-20641, 2020.

Geological and geophysical observations of fault zones reveal that fault cores are surrounded by regions of damaged rocks consist of fractures at a wide range of length scales with decaying intensity with distance from the fault core. The main mechanisms proposed for the development of off-fault damage include slip on faults with geometrical irregularities, migrating process zones, and dynamic damage from the passage of earthquake ruptures. Field observations of relatively deep exhumed fault zones have shown that fault damage zone width scales with the displacement on a fault. In this study, we combine such observations with numerical modeling to test what is the dominant mechanism producing off-fault damage at depth of several kilometres.

The field data [Faulkner et al., 2011] include measurements of micro-fracture damage zone width from small displacement fault zones within the Atacama fault zone in northern Chile that formed at ∼6 km depth within a dioritic protolith. An increase in damage zone width with displacement is clearly seen. We perform simulations of slip on synthetic faults, with roughness properties similar to that of natural faults, and examine how the total slip and roughness characteristics affect the extent and intensity of inelastic deformation to constrain the geometrical and frictional properties that could generate the observed damage. To accurately account for the effects of geometrical irregularities on the fault and allow slip that is large relative to the size the minimum roughness wavelength, we use the mortar finite element method, in which non-matching meshes are allowed across the fault and the contacts are continuously updated. Inelastic deformation of the bulk is modelled with Drucker–Prager viscoplasticity, which is a simple choice for describing cracked medium and is closely related to the Mohr–Coulomb model. Our results indicate that, for the depth and fault lengths in the field data, geometrical irregularities produce the scaling of damage zone width with displacement observed in the field and suggest that this, rather than the other mechanisms, produce most of the damage.

How to cite: Tal, Y. and Faulkner, D.: The effect of geometrical irregularities on damage zone width: Modeling and field observations , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5884, https://doi.org/10.5194/egusphere-egu2020-5884, 2020.

The differences between static and dynamic elastic moduli remain a controversial issue in rock physics. Various empirical correlations can be found in the literature. However, the experimental methods used to derive the static and dynamic elastic moduli differ and may entail substantial part of the discrepancies observed at the laboratory scale. The representativeness and bias of these methods should be fully assessed before applying big data analytics to the numerous datasets available in the literature.

We will illustrate, discuss and analyze the differences inherent to static and dynamic measurements through a series of triaxial and petroacoustic tests performed on an outcrop carbonate. The studied rock formation is Euville limestone, which is a crinoidal grainstone composed of roughly 99% calcite and coming from Meuse department located in Paris Basin. Sister plugs have been cored from the same quarry block and observed under CT-scanner to check their homogeneity levels.

The triaxial device is equipped with an internal stress sensor and provides axial strain measurements both from strain gauges glued to the samples and LVDTs placed inside the confinement chamber. Two measures of the static Young's modulus can thus be derived: the first one from the local strain measurements provided by the strain gauges and the second one from the semi-local strain measurements provided by the LVDTs. The P- and S-wave velocities are measured both through first break picking and the phase spectral ratio method, providing also two different measures of the dynamic Young's modulus.

The triaxial tests have been performed in drained conditions and the measured static elastic moduli correspond to drained elastic moduli. The petroacoustic tests have been performed using the fluid substitution method, which consists in measuring the acoustic velocities for various saturating fluids of different bulk modulus. No weakening or dispersion effects have been observed. Gassmann's equation can then be used to derive the dynamic drained elastic moduli and the solid matrix bulk modulus, which is otherwise either taken from the literature for pure calcite or dolomite samples, or computed using Voigt-Reuss-Hill or Hashin-Shtrikman averaging of the mineral constituents.

For the studied carbonate formation, we obtain similar values for static and dynamic elastic moduli when derived from careful lab experiments. Based on the obtained results, we will finally make recommendations, emphasizing the necessity of using relevant experimental techniques for a consistent characterization of the relation between static and dynamic elastic moduli.

How to cite: Bemer, E., Dubos-Sallée, N., and Rasolofosaon, P. N. J.: Differences between static and dynamic elastic moduli: Importance of experimental methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9982, https://doi.org/10.5194/egusphere-egu2020-9982, 2020.

Material instabilities are critical phenomena which can occur in geomaterials at high stress and temperature conditions. They generally result in the degradation of the microstructure organisation, ultimately leading to material failure. These phenomena are relevant to a large variety of geoscientific and geotechnical applications including earthquake physics, fault mechanics, successful targeting of unconventional georesources and mitigation of induced seismicity. Quantifying and predicting the onset of material degradation upon instability remains a major challenge due to our lack of understanding of the physics controlling the behaviour of porous rocks subject to high temperature and pressure conditions.

In the laboratory, rocks gradually transition from a time-independent brittle behaviour to a transient semi-brittle, semi-ductile behaviour upon an increase in pressure and/or temperature. Furthermore, even when subject to constant subcritical stress conditions rocks have been observed to macroscopically fail due to growth of subcritical processes such as stress corrosion. Brittle creep is a phenomenon observed on a variety of rock types (volcanic and sedimentary) and shows a high sensitivity to temperature and stress conditions. In the field, such subcritical transient processes are difficult to detect and can jeopardise the safety of geothermal projects. Transient failure mechanisms in the reservoir have set back geotechnical projects through induced seismicity occurring days or even weeks after stimulation shut in as observed at the Basel geothermal site in Switzerland or at the Pohang geothermal project in South Korea. These observations demonstrate how conventional techniques fail at describing the physics responsible for fault reactivation, which is controlled by dynamic processes resulting from transient multiphysics coupling.

In this contribution, we detail the theory and procedure to develop a constitutive model for rate-dependent damage poro-elasto-plastic material behaviour suitable for porous rocks. To allow for a generic framework for assessing geomaterials instabilities, this model incorporates the potential for microstructure degradation and a path- and rate-dependence. To that purpose, we rely on thermodynamic principles to derive in the frame of the hyperplasticity theory a coupled hydro-mechanical rate-dependent plasticity and damage rheology. We present numerical examples of this new constitutive model at the laboratory scale using experimental data on brittle creep in sandstones and discuss the implications upon upscaling at the reservoir and lithosphere scale.

How to cite: Jacquey, A., Regenauer-Lieb, K., Parisio, F., and Cacace, M.: Multiphysics of transient deformation processes leading to macroscopic instabilities in geomaterials, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16050, https://doi.org/10.5194/egusphere-egu2020-16050, 2020.

The assessment of susceptibility to failure of soft rock coastal cliffs, along with the associated failure mechanism, is not a simple task. Equilibrium conditions depend on the combination of several factors such as structural setting, rock mechanical strength, weathering processes, the hydro-mechanical action of sea waves, the variation of the rock cliff geometry, to mention some of the most important ones. From a geomechanical perspective, the brittle - strain softening behaviour of the rocks plays a key role in the onset and propagation of failure (Ciantia & Castellanza 2015). In particular, the rapid strength reduction occurring after peak under mechanical loading leading to localised deformations within shear fractures is detrimental for rock cliffs. Taking rock brittleness into account in numerical simulations under the framework of continuum mechanics is not straightforward, due to the problems related to a strong dependence of the numerical results from the adopted mesh when strain-softening laws are implemented (Vermeer and Brinkgreve 1994). Nowadays, several regularization techniques are available to control the size of the localised region and prevent the mesh dependence. Within regularization techniques, the nonlocal integral type solution has the advantage of not changing the field equations which facilitates numerical implementation. In this approach, the chosen nonlocal variables are valuated from spatial averages of the field variables in a neighbourhood, and the constitutive model is updated by replacing a local variable with its nonlocal counterpart. Consequently, the constitutive response of a Gauss point is influenced by all the integration points within a neighbourhood, with the size determined through a characteristic length (Bažant and Jirásek 2002). This contribution addresses the problem of the stability of an ideal 2-D plane strain coastal cliff, 20-m high, by means of the use of a non-local constitutive model implemented in a commercial finite element code (Mánica et al. 2018). The numerical results show insights into the evolution of the strain field and the process of slip surface/fracture propagation in the rock cliff as well as highlight the importance of regularising the finite element solution in the presence of brittle materials.

How to cite: Lollino, P., Fazio, N. L., Perrotti, M., Genco, A., Elia, G., and Ciantia, M. O.: Assessment of the mechanism of fracture propagation of soft rock coastal cliffs by using non-local constitutive models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19783, https://doi.org/10.5194/egusphere-egu2020-19783, 2020.

The onset of dilatancy determines the start of critical fracture growth in rocks under increasing load. For various applications such as the construction of nuclear repositories or dams, a quantitative comprehensive knowledge on the critical conditions leading to dilatancy is required.

Thus, it is important to determine the parameters, which control the dilatant behaviour of rocks, and to analyse their interactions.

We conducted a series of undrained triaxial experiments on two consolidated, fully saturated Opalinus Clay samples from the Mont Terri underground research lab and one sample of Bunter Sandstone from southern Lower Saxony. By testing only a few samples but them extensively, we avoid that the natural material heterogeneity among multiple samples affected our results. Here we show that our approach allows identifying new correlations between different parameters with surprising clarity.

During the experiments, which can take years, the samples are repeatedly exposed to increases in differential stress (σ₁ -σ₃) into the dilatant regime but always well below the point of failure. This we achieve by monitoring the pore pressure during the increase in differential stress. The onset of dilatancy becomes visible as clear drop in pore pressure with increasing differential stress.

In addition to the detection of the onset of dilatancy via the pore pressure evolution, pressure diffusion experiments are performed to determine the onset of dilatancy. For this, in the dilatant regime, the differential stress is kept constant and the pore pressure on one side of the sample is de- and increased repeatedly, while the reaction of the pore pressure on the other side of the sample is monitored. With the pore pressure pulse diffusing though our sample specimen, this controlled pore pressure variation induces a transition between dilatant and subdilatant regimes at constant differential stress.

The values for the onset of dilatancy derived by these two methods permit a comprehensive analysis of the dilatant behaviour not only of the Opalinus Clay samples, but also of the Bunter Sandstone sample. Our results show that dilatant behaviour of the tested materials is not governed by only one parameter but by an intricate interplay of several parameters.  Consequently, the development of an equation of state for the dilatant behaviour of different types of rock is achievable. However, due to the multiple parameter dependencies, it will be a time-consuming undertaking.

How to cite: Schumacher, S. and Gräsle, W.: The onset of dilatancy in rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5397, https://doi.org/10.5194/egusphere-egu2020-5397, 2020.

Not all rocks are perfect. Frequently heterogeneities will be present, either in the form of pre-existing fractures, or in the form of sealed fractures. Tensile strength and strength anisotropy of rocks has been investigated for strongly layered rocks, such as shales, sandstones and gneisses, but data is lacking on the effect of single planar heterogeneities, such as pre-existing fractures or stylolites. We have performed Brazilian Disc tests on limestone samples containing pre-existing fractures and stylolites, investigating Brazilian test Strength (BtS) and fracture orientation. We used Indiana limestone samples, pre-fractured with the Brazilian Disc method, and Treuchtlinger Marmor samples which contained central stylolites. All experiments were filmed. The planar discontinuity was set at different rotation angles of approximately 0–20–30–45–60–90⁰, where 90⁰ is parallel to the principal loading direction, and 0⁰ to the horizontal axis of the sample. Pre-fracturing Indiana limestone samples results in a cohesion-less planar discontinuity, whereas the stylolites in the Treuchtlinger Marmor samples are discontinuities which have some strength.

The results show that our imperfect samples with a planar discontinuity are always weaker than an intact sample. For the Indiana limestone, with a cohesion-less interface, there is 10 to 75% of weakening, which is angle-dependent. Once the angle is 30 or lower there is no influence from the initial fracture for the orientation of the new fracture. The stress-displacement pattern followed the expectation for Brazilian Disc testing. However, in the samples with a stylolite, strength is isotropic and between 25 and 65% of the strength of an intact sample. For all cases several new cracks appeared, of which the orientation is influenced by the orientation of the stylolite. The fracture pattern and associated stress drops are more complex for high angles. Interestingly, in the samples with stylolites, always more than one fracture was formed, whereas in the samples with a cohesionless interface usually only one new fracture formed, which for natural settings suggests a potential for higher fracture density when hydrofracturing a stylolite-rich interval.

A second difference between these datasets is the amplitude of the pre-existing interface. The effect of amplitude will be qualitatively investigated with a 2D Comsol model, to investigate the location of the first fracture occurring, which can then be compared to the camera data of the experiments.

How to cite: Pluymakers, A., Bakker, R., and Barnhoorn, A.: Effect of a heterogeneity on tensile failure: interaction between fractures in a limestone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10115, https://doi.org/10.5194/egusphere-egu2020-10115, 2020.

Digital rock physics (DRP) became a complementary part in reservoir characterization during the last two decades. Deriving transport, thermal, or effective elastic rock properties from a digital twin requires a three-step workflow: (1) Preparation of a high-resolution X-ray computed tomography image, (2) segmentation of pore and grain phases, respectively, and (3) solving equations due to the demanded properties. Despite the high resolution µ-CT images, the numerical predictions of rock properties have their specific uncertainties compared to laboratory measurements. Missing unresolved features in the µ-CT image might be the key issue. These findings indicate the importance of a full understanding of the rocks microfabrics. Most digital models used in DRP treat the rock as a heterogeneous, isotropic, intact medium which neglect unresolved features. However, we expect features within the microfabrics like micro-cracks, small-scale fluid inclusions, or stressed grains which may influence the elastic rock properties but have not been taken into account in DRP, yet. Former studies have shown resolution-issues in grain-to-grain contacts within sandstones or inaccuracies due to micro-porosity in carbonates, this means the micritic phase. Within the scope of this abstract, we image two different sandstone samples, Bentheim and Ruhrsandstone, as well as one carbonate sample. Here, we compare the mentioned difficulties of X-ray visualization with further analytical methods, this means thin section and focused ion beam measurements. This results into a better understanding of the rocks microstructures and allows us to segment unresolved features in the X-ray computed tomography image. Those features can be described with effective properties at the µ-scale in the DRP workflow to reduce the uncertainty of the predicted rock properties at the meso- and fieldscale.

How to cite: Balcewicz, M. and Saenger, E. H.: Digital rock physics: Segmentation of sub-resolution features, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6743, https://doi.org/10.5194/egusphere-egu2020-6743, 2020.

How to cite: Sandrone, F., Pimienta, L., Gastaldo, L., and Violay, M.: Elastic properties of rocks: Why shouldn’t they be constant?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16134, https://doi.org/10.5194/egusphere-egu2020-16134, 2020.

The rocks used to produce curling stones for international competition are only sourced from two localities in the world: Ailsa Craig, Scotland and Trefor, Wales. Curling stones consist of two components: (1) the running band (the ring-shaped bottom surface of the stone which rests on the ice) and (2) the striking band (the convex band on the profile of stones which collides with those of other stones). With a focus on the striking bands, we aim to document the damage evolution of curling stones using synchrotron microtomography (3D characterisation of pristine samples and 4D damage evolution), optical and scanning electron microscopy (quantitative characterisation of pristine samples and microfracture characterisation of damaged striking bands), and petrophysical testing (fracture characteristics and comparative data). These data will be complemented by an on-ice experiment that will determine the mechanics (e.g., stress distribution, contact area, and velocity) of curling stone impacts. Out of four curling stone varieties (from Ailsa Craig and Trefor), we observe the striking bands of three varieties to show macroscopic, incipient to mature, curvilinear fractures. The curvature of these fractures is consistent and does not vary significantly between individual stones and between curling stone varieties. However, the degree of macroscopic fracture development differs between aged striking bands of curling stone types: Blue Trefor (macroscopic fractures not observed), Red Trefor (weakly incipient), Ailsa Craig Common Green (incipient to juvenile), and Ailsa Craig Blue Hone (juvenile to mature). Unfortunately, it is not possible to determine the degree of usage (age) of the selected samples and thus it is not possible to normalize these apparent differences in damage. Given that the striking band limits the lifetime of curling stones, understanding the damage evolution of curling stones can contribute valuable information to the maintenance of curling stones. The rock physics of curling stone impacts is linked to dynamic spalling and more broadly to rock failure, as these processes are ultimately related to the initiation and propagation of fractures.

How to cite: Leung, D., Fusseis, F., and Butler, I.: Microscale characterisation of damage evolution in curling stones used in international competition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-902, https://doi.org/10.5194/egusphere-egu2020-902, 2020.

This work proposed an available approach to analyze the property evolution of weak interlayers during immersion softening at micro and macro scales, which combining the advantages of nanoindentation tests and numerical modelling. The weak interlayers has significant impact on the failure process of natural slopes, however, their properties are difficult to be obtained using traditional triaxial compression tests. Because these weak interlayers are consist of clay and rock fragments which leads to the difficult to prepare intact samples. Additionally, the softening properties of these weak interlayers are strongly related to their fillings at micro scale. In this work, the weak interlayers is investigated using nano-scale micromechanical tests and upscaling methodologies, so only small rock fragments are required (see Fig.1). 
In northwestern Hubei China, the mountains often developed several layers of weak interlayers with major lithology as shale which is sedimentary rock with low strength and dense clay particles. We investigated these shale fragments in weak interlayers, which is prone to decrease in strength induced by precipitation erosion. The Gaussian mixture model was used to analyze a large amount of data obtained by statistical grid nanoindentation method. Then the Mori-Tanaka scheme was used to homogenize the elastic properties of the samples and upscale the nanoindentation data to the macroscale. The hardness values which obtain by Berkovich and Cube corner indenter were able to assess the cohesion and friction angle of shale. Finally, these achieved parameters were applied in numerical model, in order to analyze the slope failure caused by the softening of weak interlayers (see Fig. 2).
The results show that: (1) the chlorite and muscovite minerals, which are major proportion of shale, soften or dissolve with the increasing saturation time. The fine mineral particles are gradually stripped from micro structure. As a result, at microscale the compact shale samples sale became loose. The strength of these shale samples are also decrease because water seeped through pores and micro cracks. (2) After water immersion, the friction angle is almost constant, while the elastic modulus and cohesion decrease significantly with increasing saturation time. (3) The shear strength decrease so that the shearing creep occurs along the weak interlayers surface, then bottom sliding surface is cut, which leads to landslide.

How to cite: Xu, J., Feng, Y., and Tang, X.: Investigate the softening properties of weak interlayers in slope failure process using nanoindentation test and simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18394, https://doi.org/10.5194/egusphere-egu2020-18394, 2020.

In situ stress state at shallow depths (<1 km) is important for designing underground systems for various projects such as nuclear waste disposal, carbon dioxide geological sequestration, and geo-resource development. Stress characterization for such projects rely largely on stress measurement data (such as hydraulic fracturing test data). We compile a large number of hydraulic fracturing test data measured in a total of 226 boreholes in South Korea, and attempt to characterize shallow crustal stress over the country. These data are measurements at depths down to 850 m, and classified mostly low-quality based on World Stress Map quality ranking scheme (B-quality: 7%, C: 42%, and D: 51%). We grid the country by 0.25°×0.25°, and find a circular bin size at each grid point using two statistical methods (weighted standard deviation and quasi interquartile range), by which the uniformity of stress orientation can be estimated. As many data are low-quality, we apply this process to two subsets of data (B-C and B-D) to find an optimal stress characterization. Our most optimal characterization results show that bin diameter in most of the country vary between 100 and 200 km, except for southeastern Korea. Bin diameters in southeastern Korea range between 0 and 60 km, which means that stress heterogeneity is especially significant in the region, where lithology varies markedly and several active faults are clustered. The stress orientations in the northeastern part of the country are characterized as intermediate stress uniformity (bin size of ~120 km in diameter) but a systematic horizontal stress rotation (up to ~60°) from that of the deep-seated regional stress. This region is mountainous with altitude as high as 1.4 km. To verify whether the stress rotation is a result of topographic effect, we model stress perturbation using the digital elevation model (DEM) data of the region, which yields stress rotation comparable to measurements. We find that lithology is a particularly important factor that affects stress magnitudes over the country, as the stress magnitudes at the same depth tend to be markedly smaller in sedimentary rocks than in crystalline rocks. Our study, although given data are of fairly low-quality, can provide a basis for shallow stress map of South Korea.

How to cite: Kang, M. and Chang, C.: Effects of topography and lithology variation on in situ stress at shallow depths in South Korea: results from statistical characterization of stress data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8327, https://doi.org/10.5194/egusphere-egu2020-8327, 2020.

Due to the large burial depth of the Pliocene Red Layer in Qingyang, Gansu and its special historical causes, its engineering mechanical characteristics are quite different from those of the southern red clay. Lack of systematic data on the internal forces of the lining structure through the stratum tunnel. Therefore, this paper takes the Yinchuan-Xian High-speed Railway Qingyang Tunnel as the research object, through field measurement and finite element simulation to obtain the space-time distribution characteristics of the internal force of the lining structure, the surrounding rock pressure, the deep displacement of the surrounding rock from 5 to 10 m, and the convergent deformation of the support. The reasons for the stress state of the lining-surrounding rock composite structure reflected in the results are analyzed, and the ABAQUS software is used to simulate the tunnel excavation process to compare and verify the lining structure stressing law. Internal force characteristics. The results show that: 1) The physical and mechanical indicators of the Pliocene red layer in the Neogene in Qingyang, Gansu belong to the extremely hard soil-very soft rock critical category. Due to the long consolidation pressure and long consolidation history, it can be obvious on the saturated flooding fault surface. Observation of the characteristics of layered joints proves that this layer of red clay has a tendency of sedimentary diagenesis. 2) The quality of the surrounding rock of the stratum lining structure is good. The horizontal in-situ stress is twice that of the vertical in-situ stress. It can be optimized for the design of III-IV surrounding rock while increasing the side pressure coefficient. 3) The unclosed initial support cannot effectively limit the deformation of the surrounding rock, and the temporary stress can be used to improve the state of stress. The numerical simulation results are consistent with the field measurement laws. 4) This stratum with severe deformation is the cave diameter range of the excavation boundary to the surrounding rock. The deformation area is mainly concentrated in the vault. Delayed excavation of the inverted arch can effectively reduce the stress on the internal lining structure of the inverted arch.

How to cite: Xie, Z. and Wu, X.: Study on Deformation and Force Characteristics of Deep-buried Large Section Expansive Red Clay Tunnel, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2248, https://doi.org/10.5194/egusphere-egu2020-2248, 2020.

Drilling an ice core through an ice sheet (typically 2000 to 3000 m thick) is a technical challenge that nonetheless generates valuable and unique information on palaeo-climate and ice dynamics. As technically the drilling cannot be done in one run, the core has to be fractured approximately every 3 m to retrieve core sections from the bore hole. This fracture process is initiated by breaking the core with core-catchers which also clamp the engaged core in the drill head while the whole drill is then pulled up with the winch motor.

This standard procedure is known to become difficult and requires extremely high pulling forces (Wilhelms et al. 2007), in the very deep part of the drill procedure, close to the bedrock of the ice sheet, especially when the ice material becomes warm (approximately -2°C) due to the geothermal heat released from the bedrock. Recently, during the EastGRIP (East Greenland Ice coring Project) drilling we observed a similar issue with breaking off cored sections only with extremely high pulling forces, but started from approximately 1800 m of depth, where the temperature is still very cold (approximately -20°C). This has not been observed at other ice drilling sites. As dependencies of fracture behaviour on crystal orientation and grain size are known (Schulson & Duval 2009) for ice, we thus examined the microstructure in the ice samples close to and at the core breaks.

First preliminary results suggest that these so far unexperienced difficulties are due to the profoundly different c-axes orientation distribution (CPO) in the EastGRIP ice core. In contrast to other deep ice cores which have been drilled on ice domes or ice divides, EastGRIP is located in an ice stream. This location means that the deformation geometry (kinematics) is completely different, resulting in a different CPO (girdle pattern instead of single maximum pattern). Evidence regarding additional grain-size dependence will hopefully help to refine the fracturing procedure, which is possible due to a rather strong grain size layering observed in natural ice formed by snow precipitation.

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Wilhelms, F.; Sheldon, S. G.; Hamann, I. & Kipfstuhl, S. Implications for and findings from deep ice core drillings - An example: The ultimate tensile strength of ice at high strain rates. Physics and Chemistry of Ice (The proceedings of the International Conference on the Physics and Chemistry of Ice held at Bremerhaven, Germany on 23-28 July 2006), 2007, 635-639

Schulson, E. M. & Duval, P. Creep and Fracture of Ice. Cambridge University Press, 2009, 401

How to cite: Weikusat, I., Wallis, D., Franke, S., Stoll, N., Westhoff, J., Hansen, S. B., Popp, T. J., Wilhelms, F., and Dahl-Jensen, D.: Issues with fracturing ice during an ice drilling project in Greenland (EastGRIP), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21768, https://doi.org/10.5194/egusphere-egu2020-21768, 2020.

Tunnel excavations are known to perturb the hosting rock mass at long distances, with changes in the hydrogeological flow affecting, as well as deforming the rock mass, inducing subsidence in a zone above the tunnel. During the extension of the Mont Terri Underground Rock Laboratory, we had the unique opportunity to monitor the final part of the excavation of Gallery18 and the final breaktrough.

The joint effort of two experiments (CS-D lead by ETH Zurich and FS-B lead by LBNL) allowed for a detailed characterization of the poro-elastic response of the rock mass and the Mont Terri Main Fault Zone to the excavation. Geophysical, geomechanical, and hydrogeological monitoring include: (1) pressure monitoring in several borehole intervals; (2) deformation at a chain potentiometer and fiber optics grouted in boreholes (normal to bedding and parallel to fault zone), and platform-tilmeters installed at the tunnel floor, as well as detailed 3D displacement at the SIMFIP probe.

All monitoring systems detected major perturbations starting from 15 days before the breakthrough and continuing for several days after it. We summarize the observations and will combine numerical modelling and observed trend to conceptualized the pattern of poro-elastic deformation. The results of the analysis could help shedding light on the poro-elastic behaviour of clay, providing interesting hints for the modeling community and helping in planning of future nuclear waste repositories in such material.

How to cite: Rinaldi, A. P., Guglielmi, Y., Zappone, A., Soom, F., Robertson, M., Cook, P., Kakurina, M., Wenning, Q., Rebscher, D., and Nussbaum, C.: Coupled processes in clay during tunnel excavation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18041, https://doi.org/10.5194/egusphere-egu2020-18041, 2020.

The P-T conditions in extremely-rapid gravity-driven rockslides are difficult to constrain from the descended rock mass itself. Here, we report mineralogical observations from the Koefels rockslide and their interpretation. The Koefels event – happened between 9527-9498 cal BP – comprises 3.9 km³ mainly of muscovite + biotite-bearing orthogneiss, and is one of the few large rockslides in silicate-bearing rocks worldwide. Detached by collapse of a valley flank, the rockslide impacted the opposite valley flank: While the lower part of the mass was sharply stopped, the overriding part propagated farther. This led to shear localization along discrete planes and, in consequence, to transient melting by frictional heating. The resulting frictionites comprise thin glassy levels with floating crystal fragments. The bulk composition of the glassy melt corresponds to the composition of the orthogneiss.

            In the frictionites, ultra-high pressure metamorphosed quartz (UPQ) occurs next to unaffected quartz in a glassy matrix. Micro-Raman spectroscopy of unaffected quartz yielded an intense A1 Raman mode at 464 cm^-1 ; UPQ shows a shift of this band down to 460cm^-1, with some grains showing an internal gradient of up to 3 cm^-1 from the core (463cm^-1) to the rim (460 cm^-1). Some UPQ are rimmed by lechatelierite (SiO₂ glass), which never surrounds unaffected quartz grains. Until now lechatelierite formation in frictionites was considered to be a function of temperature only (Heuberger et al. 1984). Because lechatelierite only rims UPQ with outward decreasing band numbers, we interpret lechatelierite formation to be mainly pressure-driven. The completely molten matrix and the lack of glassy rims at the edges of normal quartz indicates minimum temperatures of 900°C. Experimental investigations have shown that the shifted A1 mode of UPQ equilibrates to 464 cm^-1 at 1100°C, thus giving an upper limit of the temperature range. The Raman shift of the A1 mode and the presence of lechatelierite strongly suggest that a pressure >23 GPa was attained (cf., McMillan et al. 1991, Fritz et al. 2011, Kowitz et al. 2013).

            The UPQ and lechatelierite rims formed by grain collisions during initial shear localization, when the shear plane was relatively cool. Next, upon rapid frictional heating the glassy frictionite matrix formed and became locally injected into lechatelierite rims. Once formed, the melt prevented high-energy grain collisions. Unaffected quartz (which nevertheless may have seen pressures up to 22 GPa) in the frictionites perhaps escaped UHP overprint due to position in local pressure shadows and/or was sheared out from the adjacent caciritic rock mass into the melt. Our results help to better constrain numerical simulations of P-T-conditions in rockslides. Since our investigation only provides limiting estimates the actual P-T conditions in deep shear levels of rockslides exceeding the volume of the Koefels event might be even higher.

References:

Fritz et al. 2011: International Journal of Impact Engineering, 38:440

Heuberger et al. 1984: Mountain Research and Development, 4:345

Kowitz et al. 2013: Earth and Planetary Science Letters, 384:17

McMillan et al. 1992: Physics and Chemistry of Minerals, 19:71

How to cite: Sanders, D., Joachim-Mrosko, B., Konzett, J., Lanthaler, J., Ostermann, M., and Tropper, P.: Petrological constraints on ultra-high pressure metamorphism and frictionite formation in a catastrophic rockslide: The Koefels event (Eastern Alps). , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4831, https://doi.org/10.5194/egusphere-egu2020-4831, 2020.

Scanner pockmark is an active and continuous methane venting seafloor depression of ~ 900 x 450 m wide and 22 m deep. It is located in the northern North Sea, within the Witch Ground basin where the seafloor and shallow sediments are heavily affected by pockmarks and paleo-pockmarks of various sizes. A seismic chimney structure is present below the Scanner pockmark. It is expressed as a near-vertical column of acoustic blanking below a bright zone of gas-bearing sediments. Seismic chimneys are thought to host connected vertical fractures which may be concentric within the chimney and align parallel to maximum compression outside it. The crack geometry modifies the seismic velocities, and hence, the anisotropy measured inside and outside of the chimney is expected to be different.

We carried out anisotropic P-wave tomography with a GI-gun wide-angle dataset recorded by the 25 Ocean Bottom Seismometers (OBSs) of the CHIMNEY experiment (2017). Travel times of more than 60,000 refracted phases propagating within a volume of 4 x 4 x 2 km were inverted for P-wave velocity and the direction and degree of P-wave anisotropy. The grid is centred on the Scanner Pockmark and has a y-axis parallel to -34^o N. The horizontal node interval is denser in the zone covered by the OBSs and the vertical node interval is denser near the seabed. A 3 iteration inversion leads to a chi² misfit value of 1 and a root-mean-square misfit of <10 ms. The results show a maximum P-wave anisotropy of 5%, and higher degrees of anisotropy correlates well with higher velocities. The fast P-wave velocity orientation, a proxy for fracture orientations, is 46^o N. The top of the chimney possibly links a bright spot mapped at 270 ms in two way travel time using RMS amplitudes of MCS data, to the surface gas emission. The bright spot corresponds to low tomographic P-wave velocity and anisotropy, suggesting that gas is located in a zone with unaligned fractures or porosity. This observation is in good agreement with early multi-channel seismic data interpretations which suggested that the gas is trapped within a sandy clay layer, the Ling Bank Formation, capped by an upper clay layer, the Coal Pit Formation. In the next step, we will invert the travel-times of reflected phases in order to increase the image resolution.  

How to cite: Bayrakci, G., Minshull, T. A., Bull, J. M., Henstock, T. J., Provenzano, G., Birinci, H., Macdonald, C., and Dunn, R.: P-wave velocity anisotropy in an active methane venting pockmark: The Scanner Pockmark, northern North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5125, https://doi.org/10.5194/egusphere-egu2020-5125, 2020.

Scanner pockmark, located in the Witch Ground Graben region of the North Sea, is a ~900 m by 450 m, ~22 m-deep elliptical seafloor depression at which vigorous and persistent methane venting is observed. Previous studies here have indicated the presence of chimney structures which extend to depths of several hundred meters, and which may represent the pathways along which upwards fluid migration occurs. A proposed geometry for the crack networks associated with such chimney structures comprises a background pattern outside the chimney with unconnected vertical fractures preferentially aligned with the regional stress field, and a more connected, possibly concentric fracture system within the chimney. The measurement of seismic anisotropy using shear-wave splitting (SWS) allows the presence, orientation and density of subsurface fracture networks to be determined. If the proposed model for the fracture structure of a chimney feature is correct, we would expect, therefore, to be able to observe variations in the anisotropy measured inside and outside of the chimney.

Here we test this hypothesis, using observations of SWS recorded on ocean bottom seismographs (OBS), with the arrivals generated using two different air gun seismic sources with a frequency range of ~10-200 Hz. We apply a layer-stripping approach based on observations of SWS events and shallow subsurface structures mapped using additional geophysical data to progressively determine and correct for the orientations of anisotropy for individual layers. The resulting patterns are then interpreted in the context of the chimney structure as mapped using other geophysical data. By comparing observations both at the Scanner pockmark and at a nearby reference site, we aim to further contribute to the understanding of the structures and their role in governing fluid migration. Our interpretation will additionally be informed by combining the field observations with analogue laboratory measurements and new and existing rock physics models.

This work has received funding from the NERC (CHIMNEY; NE/N016130/1) and EU Horizon 2020 programme (STEMM-CCS; No.654462).

How to cite: Robinson, A., Bayracki, G., MacDonald, C., Callow, B., Provenzano, G., Minshull, T., Chapman, M., Henstock, T., and Bull, J.: Fracture characterisation using frequency-dependent shear-wave splitting analysis of azimuthal anisotropy: application to fluid flow pathways at the Scanner Pockmark area, North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6669, https://doi.org/10.5194/egusphere-egu2020-6669, 2020.

The spacing of opening-mode fractures in layered materials, such as certain sedimentary rocks and laminated engineering materials, is often proportional to the thickness of fractured layers. Bai, Pollard & Gao (2000) investigated the full stress distribution between such fractures, from which they show that the spacing initially decreases as extensional strain increases in the direction perpendicular to the fractures. But at a certain ratio of spacing to layer thickness, no new fractures form and the additional strain is accommodated by further opening of existing fractures: the spacing then simply scales with layer thickness, which is called fracture saturation. Their conclusion is in marked contrast to existing theories of fracture, such as the stress-transfer theory, which predict that spacing should decrease with increasing strain ad infinitum. Here we show that the principle for 2D equal spaced fracture problem also applies to the 3D polygonal fracture problem. By using 3D mechanical modeling on a spherical shell model under interior expansion, we found that the modeled plate mosaic exactly follows the same principle that the size of formed plates is also proportional to the thickness of the fractured shell. By using a spherical shell model with isotropic, elastic two-layers, we numerically load the shell to fail under a quasistatical, slowly increasing interior pressure in a displacement controlling manner (induced, e.g., by gradual thermal expansion). The fractures only occur in the surface layer. The value at which a particular element breaks is random, but fixed at the start of the fragmentation process (i.e., the disorder is quenched). The probability distribution (PD) of breakdown thresholds is a material property and is known from the start. We account for this local randomness by assigning to each element a failure threshold taken from a Weibull probability distribution (PD), with a parameter defines the degree of material homogeneity, called the homogeneity index. We use a three-dimensional finite element code named RFPA (Rock Failure Process Analysis) to solve the problem. The modeling results show that, under conditions of uniform expansion force from inside the shell, the cracking pattern also follows a global scale law in terms of the thickness of the fractured layer. The numerical modeling demonstrates an important observation that, under conditions of uniform and layer-parallel tension induced by thermal expansion within the spherical shell, surface cracks spontaneously self-organize into quasi-hexagonal tessellations, following the mechanical principle that the hexagonal pattern relieves the greatest strain energy for the least work invested in nucleation and propagation of fractures. If this applies to the problems of Earth tessellations, called Platonics (Anderson, 2002), it implies that the thermal expanded Earth may breakup to form plate-like network as a consequence of thermal-expansion induced rift rather than mantle convective or plutonic causes, and the plate size may be proportional to the thickness of lithosphere. This provides a new explanation on how the plate number should be, and whether there is a pattern in the plate mosaic, issues related to the optimal sizes and shapes of plates in terms of fracture spacing.

How to cite: Chen, T., Tang, C., and Wang, Y.: How many plates?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4707, https://doi.org/10.5194/egusphere-egu2020-4707, 2020.

The continuous high-quality geomagnetic field measurements delivered by the Swarm satellite constellation trio have enabled reliable global mapping of the magnetic signature of ocean tides for several tidal constituents. These signals provide geophysical constraints on the average electrical conductivity profile of the upper mantle below the oceans. In principle, these signals can also sense lateral variations of the electrical conductivity in the oceanic upper mantle, although the amplitude of these effects is small. Additionally, the long-term changes in the climatology of the ocean can be potentially detected by the magnetic satellite signals. Both applications put additional demands on the accuracy and resolution of the extracted signals. This contribution discusses potential ways to meet the required demands and evaluates the feasibility of using the magnetic signature of ocean tides for studying these effects.

How to cite: Grayver, A., Olsen, N., Finlay, C., and Kuvshinov, A.: Oceanic tidal signals in satellite magnetic data: quo vadis?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4651, https://doi.org/10.5194/egusphere-egu2020-4651, 2020.

The Thermal Ion Imagers on Swarm A-C, and the Suprathermal Electron/Ion Imager on ePOP (now “Swarm-E”) provide a unique view of charged particle distribution functions in the ionosphere at high time resolution (up to 100 images/s). Through high resolution, CCD-based imaging (~3000 pixels/image), ion drift velocity is derived from these images at a resolution of 20 m/s or better, and in general agreement with velocities derived from ground based radars [1] and an empirical convection model [2]. This talk reviews recent scientific applications of this technique, which are wide-ranging and include mechanisms of ion heating and upflow [3,4], M-I coupling via Alfven waves [5,6], electron acceleration and heating by Alfven waves [7,8, 9], intense plasma flows associated with “Steve” [10,11], and electrodynamics of large-scale FAC systems[ 12], among others. In addition, future opportunities made possible by these data will be discussed.

[1] Koustov et al. (2019), JGR, https://doi.org/10.1029/2018JA026245

[2] Lomidze et al. (2019), ESS, https://doi.org/10.1029/2018EA000546

[3] Shen and Knudsen (2020a), On O+ ion heating by BBELF waves at low altitude, JGR, in revision.

[4] van Irsel et al. (2020), Highly correlated ion upflow and electron temperature variations in the high latitude topside ionosphere, submitted to JGR.

[5] Pakhotin et al. (2020), JGR, https://doi.org/10.1029/2019JA027277

[6] Wu et al. (2020a), Swarm survey of Alfvenic fluctuations and their relation to nightside field-aligned current and auroral arcs systems, JGR, in revision.

[7] Liang et al. (2019), JGR, https://doi.org/10.1029/2019JA026679

[8] Wu et al. (2020b), e-POP observations of suprathermal electron bursts in the ionospheric Alfven resonator, GRL, submitted.  

[9] Shen and Knudsen (2020b), Suprathermal electron acceleration perpendicular to the magnetic field in the topside ionosphere, JGR, in press.

[10] Archer et al. (2019), JGR, https://doi.org/10.1029/2019GL082687

[11] Nishimura et al. (2019), JGR, https://doi.org/10.1029/2019GL082460

[12] Olifer et al (2020), Swarm observations of dawn/dusk asymmetries between Pedersen conductance in upward and downward FAC regions, submitted to JGR.

How to cite: Knudsen, D.: Recent scientific findings based on high-resolution core plasma imaging of the ionosphere with Swarm and ePOP, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12192, https://doi.org/10.5194/egusphere-egu2020-12192, 2020.

Properties and dynamics of ionosphere-thermosphere coupling may be investigated using observations from the Swarm electric field instruments (EFI). We illustrate this claim using measurements of vertical ion drift and electron temperature made by the EFIs, within the context of ambipolar diffusion parallel to the geomagnetic field. The associated ambipolar electric field is difficult to measure directly. Rather, under conditions where the ambipolar electric field is assume