ERE5.2 | Faults and fractures in geoenergy applications 1: Monitoring, laboratory and field work results
Faults and fractures in geoenergy applications 1: Monitoring, laboratory and field work results
Co-organized by EMRP1
Convener: Roberto Emanuele RizzoECSECS | Co-conveners: Sarah WeihmannECSECS, Nathaniel Forbes InskipECSECS, Reza Jalali
Orals
| Wed, 17 Apr, 08:30–10:15 (CEST)
 
Room K2
Posters on site
| Attendance Wed, 17 Apr, 16:15–18:00 (CEST) | Display Wed, 17 Apr, 14:00–18:00
 
Hall X4
Orals |
Wed, 08:30
Posters on site
Wed, 16:15
Faults and fracture zones are fundamental features of geological reservoirs that control the physical properties of the rock. As such, understanding their role in in-situ fluid behaviour and fluid-rock interactions can generate considerable advantages during exploration and management of reservoirs and repositories.

Physical properties such as frictional strength, cohesion and permeability of the rock impact deformation processes, rock failure and fault/fracture (re-)activation. Faults and fractures create fluid pathways for fluid flow and allow for increased fluid-rock interaction.

The presence of fluids circulating within a fault or fracture network can expose the host rocks to significant alterations of the mechanical and transport properties. This in turn can either increase or decrease the transmissibility of a fracture network, which has implications on the viability and suitability of subsurface energy and storage projects. Thus, it is important to understand how fluid-rock interactions within faults and fractures may alter the physical properties of the system during the operation of such projects. This is of particular interest in the case of faults as the injection/ remobilisation of fluids may affect fault/fracture stability, and therefore increase the risk of induced seismicity and leakage.

Fieldwork observations, monitoring and laboratory measurements foster fundamental understanding of relevant properties, parameters and processes, which provide important inputs to numerical models (see session “Faults and fractures in geoenergy applications 1: Numerical modelling and simulation”) in order to simulate processes or upscale to the reservoir scale. A predictive knowledge of fault zone structures and transmissibility can have an enormous impact on the viability of geothermal, carbon capture, energy and waste storage projects.

We encourage researchers on applied or interdisciplinary energy studies associated with low carbon technologies to come forward for this session. We look forward to interdisciplinary studies which use a combination of methods to analyse rock deformation processes and the role of faults and fractures in subsurface energy systems, including but not restricted to outcrop studies, monitoring studies, subsurface data analysis and laboratory measurements. We are also interested in research across several different scales and addressing the knowledge gap between laboratory scale measurements and reservoir scale processes.

Orals: Wed, 17 Apr | Room K2

Chairpersons: Roberto Emanuele Rizzo, Sarah Weihmann, Reza Jalali
08:30–08:35
5-minute convener introduction
08:35–08:55
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EGU24-4423
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solicited
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Highlight
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On-site presentation
A comprehensive petrophysical databank of crystalline reservoirs for assessing deep geothermal exploration targets in Finland and abroad 
Michael Heap, Alan Bischoff, Toni Luoto, Thierry Reuschlé, Satu Vuoriainen, Marion Spitz, and Marianne Leon-Stackow

Context

As part of the Deep-HEAT-Flows project (https://deep-heat-flows.voog.com), we have collected a comprehensive geological and petrophysical dataset of crystalline reservoirs formed within fault zones and at the contact of igneous intrusions across Finland, evaluating their potential as deep geothermal reservoirs. Our investigations involve a range of laboratory-based experiments encompassing measurements of rock density, elastic wave velocity, electric resistivity, porosity, and permeability under various confining pressures, and the thermal properties of 120+ samples collected from diverse crystalline rocks. Additionally, we apply mineral and pore space caracterization techniques including petrography, micro-XRF spectrometry, SEM-EDS, hyperspectral imaging, and CT scans to understand the processes that control crystalline reservoir formation.

Findings

Our findings highlight a common trend among various petrophysiscal parameters: rock density, resistivity, elastic wave velocity, thermal conductivity, and heat capacity typically reduce as the porosity increases, a characteristic observed across many sedimentary and volcanic rocks. Reservoir quality is primarily determined by the morphology of the pore network, encompassing fractures and interconnected moldic, sieve, and interparticle pores. The most promising reservoir properties were observed in rocks intersected by regional shear zones and therefore affected by intense brecciation, cataclasis, and hydrothermal alteration, leading to a notable porosity of ~20% and permeability in the order of 10−12 m2 (1 darcy). Moreover, the contact margin of rapakivi intrusions also include fractured and hydrothermally altered rocks that have significantly high porosity and permeability. In detail, rocks dominated by fractures typically have little porosity (<4%) and exhibit extremely high permeability (~10−12 m2) only at low confining pressures, which sharply decreases to ~10−19 m2 as the confining pressure surpasses 20–30 MPa (corresponding to depths around 700–1000 m). From our dataset, only fractures linked to mineral dissolution have the potential to sustain permeability above 10−16 m2 at 50 MPa confining pressure (simulating depths of ~2 km). Conversely, rocks that underwent cataclasis and hydrothermal alteration exhibit comparatively milder permeability reductions, maintaining high values even when subjected to high confining pressures of 50 MPa. Throughout the entire dataset, a consistent observation emerges: mafic minerals are commonly substituted by chlorite and epidote, suggesting hydrothermal alteration processes occurring at relatively high temperatures (200–300 °C).

Implications for geothermal exploration

Exploring deep geothermal resources in crystalline settings offers a promising solution for direct space heating, industrial applications, and electricity generation. However, the typically low porosity and low permeability of crystalline rocks remain a key obstacle in deep geothermal exploration. The identification of hydrothermally altered rocks as potential deep geothermal reservoirs could mark a substantial shift in geothermal exploration within crystalline regions, broadening target prospects beyond the conventional focus on volcanic and rifting areas. Brecciation, cataclasis, fracturing, and mineral dissolution collectively contribute to the creation of exceptional reservoir properties, which have been widely overlooked in deep and ancient (over a billion years) crystalline settings. Our results hold paramount importance for identifying highly productive permeable zones within crystalline settings and also to the advancement of Enhanced Geothermal Systems that could prioritize the creation of more intricate fracture networks through thermal and chemical stimulation.

How to cite: Heap, M., Bischoff, A., Luoto, T., Reuschlé, T., Vuoriainen, S., Spitz, M., and Leon-Stackow, M.: A comprehensive petrophysical databank of crystalline reservoirs for assessing deep geothermal exploration targets in Finland and abroad, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4423, https://doi.org/10.5194/egusphere-egu24-4423, 2024.

08:55–09:05
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EGU24-13098
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On-site presentation
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Heat Sensitive Epoxy Foam for Permeability Alteration in Fractured Geothermal Fields  
Dani Or, Rishi Parashar, Ying Yang, Manish Bishwokarma, and Satish Karra

Geothermal energy plays a growing role in the transition to renewable and carbon free energy sources. A challenge for many geothermal fields is how to enhance water-rock heat exchange either by creation of new fractures, or by blocking short-circuiting large conduits. Here we report a novel approach for blocking large conduits (faults and large fractures) using heat sensitive epoxy resin foam designed to be transported as discrete resin droplets to specific regions that are then activated (foamed and cure) in-situ at targeted temperatures. In contrast with alternative methods for reducing geothermal rock permeability such as silicate gels or heat responsive polymer microbeads targeting small aperture fractures < 0.1 mm, the epoxy foam can reduce the permeability of fractures with apertures up to several millimeters. Results from laboratory 2-D glass fracture model provide insights by visualizing the transport phase and subsequent temperature-sensitive foaming and curing transformations with associated flow pathway blocking. Modeling results for transport and foaming in simple fracture networks considering rheological properties and foaming (volume expansion) behavior will be presented. On going activities of rheological resin characterization; tuning of the foaming-curing to different temperature ranges; and consideration of resin dispersion using small droplets for enhanced transport will be discussed.  

How to cite: Or, D., Parashar, R., Yang, Y., Bishwokarma, M., and Karra, S.: Heat Sensitive Epoxy Foam for Permeability Alteration in Fractured Geothermal Fields , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13098, https://doi.org/10.5194/egusphere-egu24-13098, 2024.

Vimeo: EGU24-13098-material-r0
09:05–09:15
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EGU24-8226
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ECS
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On-site presentation
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Fault core structure affects fault slip during fluid injection: insights from laboratory friction experiments 
Stefano Aretusini, Chiara Cornelio, Giuseppe Volpe, Giacomo Pozzi, Elena Spagnuolo, and Massimo Cocco

Natural faults when subjected to stimulation by fluid injection may result in slip acceleration because pore pressure (Pf) increases in the rock volumes inside and surrounding the fault zone leading to reduction of effective normal stress (σn’). Slip mode ranges from aseismic creep to seismic ruptures defining a spectrum of fault-slip behavior. Fault stimulation experiments will be conducted in the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG, Switzerland) to understand fault reactivation processes on a target well-identified fault zone, fully instrumented to monitor deformation and seismicity during both fluid injection and fault reactivation. This is envisioned in the ERC-Synergy FEAR (Fault Activation and Earthquake Rupture) project. In BULGG, the target fault zone has both a sub-centimetric fault core containing fault gouge and granite asperities in contact and other fractures in the surrounding rock volume.  Therefore, it becomes important to define the frictional properties and slip mode of both gouges and bare rock surfaces taking advantage of a laboratory controlled experimental environment.

Fault stimulation by increasing Pf was simulated in laboratory following an injection protocol suitable for the BULGG fluid stimulation. Experiments were performed on the target fault gouge and on bare rock surfaces made of nearby Rotondo Granite. We employed a rotary shear apparatus (SHIVA) allowing the fluid injection under a controlled shear stress. First, we imposed the stresses measured at depth in the underground laboratory, halved due to apparatus limitations: 7.5 MPa σn’, 7.5 MPa confining pressure and 2.5 MPa Pf. Second, we imposed a slip rate of 10-5 m/s for 0.01 m to have a reference texture. Third, we applied a shear stress so that an equivalent slip tendency of 0.35 (equal to the one measured in the target fault) is achieved (ca. 2.7 MPa) keeping it constant. We then increased stepwise the pore fluid pressure by 0.1 MPa every 150 s. After fault slip initiation, the maximum allowed slip velocity was 0.1 m/s. Between each of the experimental stages, permeability and transmissivity were measured with the gradient or Pf oscillations methods.

We show that reactivation occurs at lower Pf in bare rock surfaces (4.7 MPa) with respect to MC fault gouge (6.5 MPa), suggesting that the effective coefficient of friction, the ratio of shear stress and σn’, is larger in gouge (0.58) than in bare rock surfaces (0.49). Moreover, upon the application of last Pf step, reactivation is slower in fault gouge (150 s delay) with respect to bare rock surfaces (10 s delay), consistently with the lower hydraulic transmissivity measured for target fault gouge with respect to bare rock surfaces (i.e., 10-19 vs 10-17 m3). Our experiments also show that creep and dilatancy precede reactivation in fault gouge, whereas reactivation is sudden and not preceded by dilatancy in bare rock surfaces.

We suggest that well-oriented and smooth bare rock surfaces might be easily reactivated similarly to what observed for fault gouge during fluid stimulation. Our data and observations will contribute to shed light on the mechanics of faults and induced earthquakes by fluid stimulation experiments.

How to cite: Aretusini, S., Cornelio, C., Volpe, G., Pozzi, G., Spagnuolo, E., and Cocco, M.: Fault core structure affects fault slip during fluid injection: insights from laboratory friction experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8226, https://doi.org/10.5194/egusphere-egu24-8226, 2024.

09:15–09:25
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EGU24-11723
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On-site presentation
Spatio-temporal evolution of permeability during quasi-static fault growth in granite 
Nicolas Brantut, Frans Aben, and Ado Farsi

In tight crystalline basement rocks such as granite, faults are known to be substantially more hydraulically conductive than the rock matrix. However, most of our knowledge of rock permeability in the laboratory and in the field relies on indirect inference, static measurements, or before/after datasets, and the spatio-temporal evolution of the permeability field during faulting remains unknown. Specifically, we would like to determine at which stage of the faulting process does permeability change most, and the degree of permeability heterogeneity along shear faults.

We conducted a series of triaxial deformation experiments in initially intact Westerly granite, where faulting was stabilised by monitoring the acoustic emission rate. At many stages from pre- to post-failure states, we paused deformation and imposed macroscopic fluid flow to characterise the overall permeability of the material. In addition, we measured the pore pressure distribution in the sample, and estimated apparent permeability at different locations along the fault, from the intact ligaments to damaged regions. We monitored the propagation of the macroscopic shear fault by locating acoustic emissions.

We find that average permeability increases dramatically (by around 3 orders of magnitude) near the peak stress, where the fault (as seen by acoustic emission locations) is not yet through-going. Post-peak evolution shows a more gradual increase in overall permeability, with local heterogeneities remaining along the fault, primarily controlled by small-scale fault geometry and the existence of undamaged regions as imaged by acoustic emission locations.

We conclude that permeability change and fluid flow focussing occurs at very early stages of faulting, and do not require substantial slip. Our results highlight the key role of fault geometry in the fine-scale permeability structure of basement rocks.

 

How to cite: Brantut, N., Aben, F., and Farsi, A.: Spatio-temporal evolution of permeability during quasi-static fault growth in granite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11723, https://doi.org/10.5194/egusphere-egu24-11723, 2024.

09:25–09:35
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EGU24-19263
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ECS
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Virtual presentation
The evolution of crack transmissivity under normal and shear stress before and after slip 
Lining Yang, Julian Mecklenburgh, and Ernest Rutter

Understanding the flow of fluids in the subsurface is of vital importance to geo-energy exploitation and disposal of waste fluids. In most situations, individual cracks can be more effective for fluid transport than fluids flowing through the porous matrix of the rock. The enhancement of a single crack in a low permeable rock can be over 1000 times. However, for both porous matrix flow and crack flow, the bulk permeability and crack transmissivity are all affected by the stress state in the lithosphere.

This study aims to investigate the influence of the Terzaghi effective normal and shear stress on the transmissivity of cleavage cracks under upper crustal conditions. Penrhyn slate was selected as samples for the experimental study because of its low porosity (<1%), permeability and slaty cleavage. The matrix permeability of Penrhyn slate is very low, ranging from 10-20 to 10-22 m2 when the effective pressure is in the range of 10 to 53 MPa measured by the oscillating pore pressure method (OPPM). The crack transmissivity ranged from 10-18 to 10-23 m3 when the effective pressure changed from 10 to 280 MPa. The experimental results show that the evolution of crack transmissivity of a single fracture under several cycles of pressurization and depressurization is similar to the trend found in permeability. The first application of the normal stress on fracture surfaces always produces a nonrecoverable loss in crack transmissivity. In the subsequent pressurization and depressurization, partially recoverable variations in transmissivity were observed, suggesting a linear elastic behaviour of crack closure. The highest peak effective pressure attained in the stress history affects the extent of subsequent recoverable crack transmissivity. When the fracture surface is subjected to a new higher peak stress, the crack transmissivity will no longer recover to its former low level but to a lower level, indicating a permanent transmissivity loss. Thus, the transmissivity has a memory of the previous maximum stress the fractured rock was subjected to.

The influence of shear stress on crack transmissivity was studied in Solnhofen limestone and Carrara marble samples with saw-cut ground smooth fractures and compared with the rough cleaved fractures of Penrhyn slate. The influence of shear stress was studied in two situations: (a) the stable (no-slip) condition at shear stress less than needed to promote slip on fracture (b) at shear stresses high enough to yield slip on the fractures. In situation (a), the cyclic increase and decrease of shear stress led to a continuous decrease in crack transmissivity. The magnitudes of this decrease in crack transmissivity decrease with more cycling. The transmissivity tended to decrease to a lowest value eventually but this lowest value can be regenerated by slip on the fracture. In situation (b), a single slip can decrease crack transmissivity. The decrease in crack transmissivity can be attributed to the formation and smearing of frictional wear products or gouges. Under the progressive compaction, there exists a lowest level of crack transmissivity which is independent of the normal stress.

How to cite: Yang, L., Mecklenburgh, J., and Rutter, E.: The evolution of crack transmissivity under normal and shear stress before and after slip, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19263, https://doi.org/10.5194/egusphere-egu24-19263, 2024.

09:35–09:45
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EGU24-8619
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ECS
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On-site presentation
Hydraulic Stimulation Experiments in a Decimeter-scale True Triaxial Compressive Apparatus 
Julian Osten, Mohammadreza Jalali, Alexander Cadmus, Leonie Welsing, Tom Schaber, Paul Cook, Yves Guglielmi, Raul Fuentes, and Florian Amann

Geothermal energy is considered a sustainable energy source for the transition to a carbon-neutral economy. In Central Europe, sufficiently hot source rocks are buried deep underground and comprise tight crystalline basement formations. To extract their thermal energy, hydraulic stimulation is used to create efficient heat exchangers in the context of Enhanced Geothermal Systems (EGS). Successful geothermal reservoir initiation requires a broad understanding of the hydro-mechanical coupling in fractured rock masses. For this reason, a decimeter-scale true-triaxial setup has been developed to conduct injection-driven shear tests under various stress conditions.

To gain a deeper insight into the hydro-mechanical processes involved in hydraulic stimulation, a true triaxial compressive apparatus at the decimeter scale is employed. The experimental setup consists of 30 x 30 x 45 cm cuboidal granite specimens, each containing an oblique saw-cut laboratory fracture with different surface properties. The fracture is crossed by two boreholes equipped with packers to isolate a fracture interval. Fluid injection into the isolated intervals follows the typical HTPF (hydraulic testing of pre-existing fractures) scheme, including stepwise pressure increases and decreases. Stress boundary conditions are introduced by three sets of oil-filled flatjacks, contained within a steel frame which allows a more realistic and accurate replication of the stress conditions experienced by geological formations during hydraulic stimulation experiments. Stresses for individual tests are manipulated from hydrostatic to strike-slip conditions to test for different end member states of slip tendency. Fracture and rock deformations are recorded by 16 linear variable differential transformer (LVDT) sensors mounted externally along the edges of the specimen, volume changes in the flatjacks and a newly developed borehole deformation probe (mini-SIMFIP).

How to cite: Osten, J., Jalali, M., Cadmus, A., Welsing, L., Schaber, T., Cook, P., Guglielmi, Y., Fuentes, R., and Amann, F.: Hydraulic Stimulation Experiments in a Decimeter-scale True Triaxial Compressive Apparatus, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8619, https://doi.org/10.5194/egusphere-egu24-8619, 2024.

09:45–09:55
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EGU24-19710
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On-site presentation
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Evolution of Microcracks in Damaged Natural Salt: Insights from 4D imaging 
Yuntao Ji, Christopher Spiers, Suzanne Hangx, Hans de Bresser, and Martyn Drury

Rocksalt formations are critical candidates for storing natural gas, hydrogen, compressed air energy, and radioactive waste. While pure, undisturbed rock salt deposits exhibit low porosity and impermeability when buried deeply, excavation leads to near-field microcracking and dilatancy in the salt, increasing porosity and permeability. Over time, the connectivity of brine- or water-vapor-filled microcrack networks in deformation-damaged salt is expected to decrease, partly due to dissolution-precipitation healing. In this study, we employ 4D (time-resolved 3D) microtomography to investigate the long-term evolution of dilated grain boundary and microcrack networks developed in deformation-damaged natural salt through brine-assisted processes. Our findings reveal substantial microstructural modification and healing occurring over periods ranging from days to a few months. Cracks and dilated grain boundaries undergo crystallographic faceting, necking, and migration, effectively "recrystallizing" the material and resulting in increased tortuosity and decreased connectivity of the crack network. Understanding the complex interplay between microcracking, healing, and permeability changes in deformation-damaged rock salt is of utmost importance for optimizing storage and disposal applications in geomechanics and physical chemistry. Our research contributes valuable insights to this field and informs the sustainable development and management of rock salt formations for diverse energy storage and waste management needs.

How to cite: Ji, Y., Spiers, C., Hangx, S., de Bresser, H., and Drury, M.: Evolution of Microcracks in Damaged Natural Salt: Insights from 4D imaging, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19710, https://doi.org/10.5194/egusphere-egu24-19710, 2024.

09:55–10:05
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EGU24-7157
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ECS
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On-site presentation
Reconstructing the geothermal reservoir model and estimating the structural permeability variation in the metamorphic terrane in Ruisui, Taiwan 
Yung-Ching Huang, Jian-Cheng Lee, Gong-Ruei Ho, Chih-Wen Chiang, Yi-Chia Lu, Sheng-Rong Song, Chih-Hao Yang, Chi-Hsuan Chen, and Yue-Gau Chen Chen

    This study identifies the key geothermal features in the metamorphic terrane within the Yuli belt (a metamorphic mélange) in Ruisui, eastern Taiwan, including fault-related pathways for hot fluids, cap rocks, and naturally fractured reservoirs, and builds a structural permeability distribution model. A preliminary geothermal reservoir model is created for a depth of 3 km, by integrating geological field analysis, magnetotelluric (MT) surveys, hot spring geochemistry, and data from 3 exploration wells. The low resistivity zones shown in MT results are beneath a unit of amphibole-albite schist, containing numerous ultra-mafic blocks at different sizes. We tentatively interpret the four MT low-resistivity zones as four possible reservoirs, which are associated with high-density jointed and faulted quartz-mica schists, underneath cap rocks of amphibole-albite schists, which reveal poor development of joints. The geochemical analysis of hot spring water indicates a high concentration of sodium ions, potentially originating from the amphibole-albite schist. We also observe surface exposures where water up to 50-60oC flows up through NW-SE trending sub-vertical faults with the downhole temperature up to 200oC at a depth of 0.9km.

    To determine whether the NW-SE vertical fault zones are suitable for open structures, which appear to be the major geothermal up-flows, we measure the orientation of faults and 3 sets of joints, most of which are sub-vertical in the field. The principal stress orientations are adopted as σ1 vertical, σ2 N120oE, and σ3 N30oE, by combining GPS observation, focal mechanisms of shallow earthquakes, and fault slickenlines measured in exposures. We conduct fault-slip inversion and obtain the stress ratio phi=0.51. Utilizing the Mohr-Coulomb failure criterion coupled with selected parameters, such as in-situ principal stresses, fluid pressures, and rock mechanical properties, our model indicates that steeply-dipping NW-SE trending (N120o-130Eo) joints and faults are mechanically prone to open as fluid infiltrating or injecting during thermal events.

    We furthermore measured the fracture length and density at the aforementioned hot spring exposure where 50-60oC hot fluid flows out from an NW-SE trending sub-vertical fault. The measuring result shows that the density of the fractures (or joints) decreases away from the fault core, thus we anticipate the joints tend to form on the microfractures created by the fault, which explains an increase of the permeability toward the fault. As for estimating the structural permeability along the NW-SE fault and open joints, the fracture aperture is calculated using linear elastic fracture mechanics, and then the permeability is estimated by using the cubic law for fluid flow in rock fractures. By doing so we obtain the structural permeability in the NW-SE fault zone, which exponentially decreases away from the fault core, and the permeability value ranges from 10-10 to 10-13 m2 at a distance of 10 m.

How to cite: Huang, Y.-C., Lee, J.-C., Ho, G.-R., Chiang, C.-W., Lu, Y.-C., Song, S.-R., Yang, C.-H., Chen, C.-H., and Chen, Y.-G. C.: Reconstructing the geothermal reservoir model and estimating the structural permeability variation in the metamorphic terrane in Ruisui, Taiwan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7157, https://doi.org/10.5194/egusphere-egu24-7157, 2024.

10:05–10:15
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EGU24-9336
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ECS
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On-site presentation
Fractal diffusion analyses of periodic pumping tests 
Victoria Alegria Jimenez Martinez, Yan Cheng, and Jörg Renner

We investigated the usefulness of the fractal diffusion equation, also known as generalized radial flow (GRF) equation, to characterize hydraulic properties and flow dimensions of the subsurface. Unlike other methods for deriving hydraulic properties that require selecting the flow dimension, analyses based on the GRF equation in principle constrain both, flow dimension and hydraulic properties. We utilized the GFR equation to analyze periodic pumping tests carried out in boreholes penetrating gneiss rocks in the research mine Reiche Zeche, Freiberg, Germany. These tests involved one injection borehole, where flow rate and injection pressure were recorded, and four monitoring boreholes, where pressure responses were monitored. Phase-shifts and amplitude ratios were derived through interference analysis, involving a comparison of the periodic signals of injection and monitoring pressure, as well as injectivity analysis, consisting of a comparison of the periodic flow rate and injection pressure. The pumping tests were conducted at three distinct intervals within the injection borehole, isolated by a double-packer probe and selected based on the characteristics of the fractures intersecting the borehole.  One interval contained a natural fracture zone characterized by a high fracture density with a high mean aperture. The others were previously hydraulically stimulated. While one of them had a single pre-existing fracture, the other was entirely intact before the stimulation that led to an induced fracture with feather geometry, as typical for a borehole that does not follow a principal stress axis. Several observations suggest that the gneiss volume is hydraulically heterogeneous: a) the hydraulic properties and flow dimensions vary with pumping period; b) estimated diffusivity values and flow dimensions differ for interference and injectivity analyses; c) discernible differences in diffusivity values and flow dimensions along diverse hydraulic paths, as determined by interference analysis. Furthermore, pressure dependence in hydraulic properties and flow dimensions are observed for all intervals. The hydraulic response of the fault-zone interval exhibits a greater sensitivity to variations in mean pumping pressure than the two stimulated intervals.

How to cite: Jimenez Martinez, V. A., Cheng, Y., and Renner, J.: Fractal diffusion analyses of periodic pumping tests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9336, https://doi.org/10.5194/egusphere-egu24-9336, 2024.

Posters on site: Wed, 17 Apr, 16:15–18:00 | Hall X4

Display time: Wed, 17 Apr, 14:00–Wed, 17 Apr, 18:00
Chairpersons: Roberto Emanuele Rizzo, Sarah Weihmann, Reza Jalali
X4.127
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EGU24-6192
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ECS
Structural, petrophysical, and geophysical characterization of a fault zone in southern Finland – application for subsurface fluid flow in granitic settings 
Jon Engström, Alan Bischoff, Evgenii Kortunov, Mira Markovaara-Koivisto, Nikolas Ovaskainen, Nicklas Nordbäck, and Markku Paananen

Southern Finland crystalline basement was formed and modified during the 1.9–1.8 Ga Svecofennian orogeny, which constitutes a portion of the Fennoscandian Shield. The bedrock comprises supracrustal and early to late-orogenic igneous rocks of mafic to felsic compositions and is characterized by an overall high metamorphic grade associated with high-T and low-P conditions. The bedrock was subjected to multiple tectonic events of distributed deformation, first during a compressional stage, then followed by an extensional stage and finally  a transpressional stage.

Hence, the Kopparnäs study site in southern Finland has been subjected to several stages of ductile and brittle deformation. The site has been studied by the Geological Survey of Finland for several years, with special research emphasize on a subvertical E–W orientated multi-core fault zone that intersects granites, amphibolites, and migmatites. A drillhole cuts through this fault zone at 100 m depth. This drillhole has beed studied using downhole instrumentation, such as optical and acoustic imaging and diverse geophysical surveys (fullwave sonic, magnetic susceptibility, gamma density, natural gamma radiation, drillhole caliper and various electrical loggings). In addition, a comprehensive study of the drill core enables detailed geological and petrophysical characterization of the fault architecture, including recognition of fractures, alteration zones, and mineralization across the fault and its host rocks. These studies together with fluid flow measurements with a packer system, enable us to define subsurface properties for this fault zone.

The initial results suggest that faulting strongly impacts the petrophysical characteristics of the rock, typically increasing porosity and reducing bulk density. This change is most likely related to the fracturing at the site being often associated with mineral alteration and dissolution. These events altered and deformed the multiple fault cores in distinctively manner, affecting the subsurface fluid flow which can be observed in fluid chemical composition differences.

These studies are part of the FLOP project (FLOw Pathways within faults and associated fracture systems in crystalline bedrock) and the Deep-HEAT geothermal energy project. 

How to cite: Engström, J., Bischoff, A., Kortunov, E., Markovaara-Koivisto, M., Ovaskainen, N., Nordbäck, N., and Paananen, M.: Structural, petrophysical, and geophysical characterization of a fault zone in southern Finland – application for subsurface fluid flow in granitic settings, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6192, https://doi.org/10.5194/egusphere-egu24-6192, 2024.

X4.128
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EGU24-18458
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ECS
Variations in fracture distribution across Northern Bavaria – Towards large-scale geothermal fracture models 
Ruaridh Smith, Rahul Prabhakaran, Fabian Jakob, and Daniel Koehn

Natural faults and fractures form a critical component of fluid flow in low permeable reservoirs such as tight carbonates for a wide variety of applications including geothermal energy extraction. Fractured systems often control permeability in these reservoirs at the first order where properties of these networks are defined by fracture orientation, intensity, aperture, and connectivity. Accurately quantifying these network properties is vital in generating representations of the fracture networks at reservoir depth.

In regions with limited subsurface data (borehole and seismic), field data and outcrop analogues become an important source for characterising the fracture networks for modelling reservoirs at depth. Outcrops can be used to define several properties of the networks and information on the variation in the fracture distribution across defined areas.

The Franconian Basin is a major tectonic structure in Northern Bavaria containing Mesozoic sediments up to 3500m thick. It is a relatively under-researched region where limited subsurface data is available in comparison to the south in the Molasse Basin where geothermal exploration and production is well established with extensive subsurface datasets widely distributed. Increased geothermal gradients have been identified in Northern Bavaria, including surrounding the major urban areas presenting an opportunity to improve the understanding of the geothermal potential of the region. Several of the identified reservoir units in this region are primarily composed of low permeable carbonates where faults and fractures control primary reservoir flow. These units are also present as outcrop analogues in the Franconian Alb which can be utilised for surface fracture characterisation.

We present results analysing the variations in the fault and fracture systems from across the region captured from 1D measurements and 2D and 3D imaging of quarry and cave outcrops. Using these results, stochastic fracture models of the parts of the region can be generated, providing realisations of the fracture networks which can contribute to assessing the permeability and geothermal potential of the reservoirs in Northern Bavaria.

How to cite: Smith, R., Prabhakaran, R., Jakob, F., and Koehn, D.: Variations in fracture distribution across Northern Bavaria – Towards large-scale geothermal fracture models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18458, https://doi.org/10.5194/egusphere-egu24-18458, 2024.

X4.129
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EGU24-9736
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ECS
Characterization and 3D geometrical modelling of a complex fault zone for earthquake rupture experiments 
Alberto Ceccato, Peter Achtziger-Zupančič, Giacomo Pozzi, Alexis Shakas, Alba Simona Zappone, Daniel Escallon, Marian Hetrich, Mohammadreza Jalali, Xiaodong Ma, Men-Andrin Meier, Julian Osten, Florian Amann, Massimo Cocco, Domenico Giardini, and Stefan Wiemer

Fault zone geological and geometrical complexities are prime parameters playing a fundamental role in controlling the characteristics of both natural and induced seismicity. In the Bedretto tunnel (Switzerland), the Fault Activation and Earthquake Rupture (FEAR) project aims at triggering a Mw = 1 seismic event through fluid injection and stimulation of a natural fault zone situated in a large-scale (> 106 m3) fractured granite reservoir. The limited exposure of the fault zone in the tunnel, however, restricts the possibility to constrain in detail the geometrical and geological characteristics of the experimental target. Therefore, in order to constrain the geological and geometrical characteristics of the target fault zone, we have integrated structural analyses, borehole and core logging, and borehole ground penetrating radar (GPR).

Preliminary field investigations in the tunnel allowed to identify the complex fault structure characteristics, fault rock properties, and slip tendency in the current stress field of the selected fault zone. These results were compared to the structural observations obtained from field surveys and remote sensing, constraining the slip history, and lateral extent of the set of natural fault zones occurring on the surface above the Bedretto tunnel. Indeed, the lateral extent of the selected fault has been confirmed through the logging (optical/acoustic televiewer, fracture intensity, fracture typology) of exploration boreholes and the analyses of the related cores. The comparison between the geological characteristics of fault zones in the cores and the characteristics of the selected fault zone exposed in the tunnel allowed to confirm the occurrence of the same typology of fault zone further away from the exposure in the tunnel. In addition, GPR logging of the exploratory boreholes provided fundamental insights on the lateral continuity of the identified fault zones on the tunnel wall, as well as those identified in the borehole/core logging.

All geological and geometrical information have been integrated into a preliminary 3D geometrical model (in Leapfrog Geo), representing the overall geometry of the selected fault zone. This preliminary geometrical model has been validated against synthetic GPR profiles, computed through GPR forward modelling along the exploration boreholes.

The integrated results define the selected fault zone as a 3-7 m wide zone of higher density (up to 5/m), of variably oriented secondary fractures, and bounded by two main slip surfaces. The slip surfaces are irregularly decorated by phyllosilicate-rich gouge patches, filling the roughness of the fault surface. The lateral extension of each discrete fracture does not exceed 30 m in length, but the overall lateral continuity of the fault zone exceeds several hundreds of meters.

The presented integrated characterization approach allowed us to constrain a geologically-sound, first-order 3D geometrical model of a complex natural fault zone, validated against geophysical forward modelling. These preliminary results have fundamental implications for the expected experimental planning and outcomes, modelling and injection strategies, project logistics, as well as the design and deployment of the monitoring network around the stimulated fault zone.

How to cite: Ceccato, A., Achtziger-Zupančič, P., Pozzi, G., Shakas, A., Zappone, A. S., Escallon, D., Hetrich, M., Jalali, M., Ma, X., Meier, M.-A., Osten, J., Amann, F., Cocco, M., Giardini, D., and Wiemer, S.: Characterization and 3D geometrical modelling of a complex fault zone for earthquake rupture experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9736, https://doi.org/10.5194/egusphere-egu24-9736, 2024.

X4.130
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EGU24-10618
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ECS
Systematic multi-stage hydraulic stimulation experiments in a hectometer-scale fractured rock volume at the Bedretto Underground Laboratory, Switzerland 
Kai Bröker, Xiaodong Ma, Nima Gholizadeh Doonechaly, Antonio Pio Rinaldi, Anne Obermann, Martina Rosskopf, Marian Hertrich, and Domenico Giardini and the BedrettoLab Team

Interest in engineered geothermal systems (EGS) has grown in the last decade due to their recognition as a low-emission, renewable energy source. EGS reservoirs with sufficiently high temperatures are located at depths of several kilometers, where the permeability of the crystalline basement rocks is insufficient for geothermal energy extraction. Permeability enhancement is accomplished through hydraulic stimulation, either by hydraulic shearing of natural fractures or shear zones, or through hydraulic fracturing of intact rock. The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) in Switzerland serves as an in situ test-bed where hectometer-scale hydraulic stimulation experiments are conducted to better understand the seismo-hydromechanical response of fractured crystalline rock masses (Ma et al. 2022).

The geothermal testbed of the BedrettoLab is located in a 100 m long enlarged section of the Bedretto tunnel in the Swiss Central Alps, with an overburden of more than 1000 m of granite. Several characterization, monitoring, and two stimulation boreholes were drilled. One of the stimulation boreholes (referred to as ST1) is 400 m long, 45°-dipping, and was equipped with a multi-packer system that partitions the borehole into 15 intervals.

In this work, we present the structural and seismo-hydromechanical characterization of eight stimulation intervals closely observed using a dense monitoring network (see Plenkers et al. 2023 for the detailed network layout). We injected relatively small fluid volumes (0.35–14 m3) following a standardized injection protocol to compare the response of the targeted geological structures in each interval. Depending on the transmissivity of the interval, the stimulation was conducted pressure- or flow rate-controlled with several steps at constant pressure/flow rate. Despite the similarly oriented structures in each interval, the observed seismo-hydromechanical behavior is complex and heterogeneous. The detected seismicity follows multiple steeply-dipping and NE-SW striking planes (Obermann et al. 2024), which coincides with the direction of known pre-existing fault structures obtained from the geological characterization. In most intervals, a clear bilinear behavior on the pressure vs. flow rate plot marks a strong increase in injectivity above a certain reactivation pressure. Analysis of these reactivation pressures in comparison with the stress field, fracture and seismic cloud orientations implies that the stimulation mechanism is hydraulic shearing of the fractures rather than elastic opening (also known as hydraulic jacking).

References:

Ma, X., Hertrich, M., Amann, F., Bröker, K., Gholizadeh Doonechaly, N., et al. (2022). Multi-disciplinary characterizations of the BedrettoLab -- a new underground geoscience research facility. Solid Earth, 13(2), 301–322. https://doi.org/10.5194/se-13-301-2022

Obermann, A., et al. (2024). Picoseismic response of hectometer-scale fracture systems to stimulation with cm-scale resolution under the Swiss Alps, in the Bedretto Underground laboratory. In preparation for JGR: Solid Earth.

Plenkers, K., Reinicke, A., Obermann, A., Gholizadeh Doonechaly, N., Krietsch, H., et al. (2023). Multi-Disciplinary Monitoring Networks for Mesoscale Underground Experiments: Advances in the Bedretto Reservoir Project. Sensors, 23(6), 3315. https://doi.org/10.3390/s23063315

How to cite: Bröker, K., Ma, X., Gholizadeh Doonechaly, N., Rinaldi, A. P., Obermann, A., Rosskopf, M., Hertrich, M., and Giardini, D. and the BedrettoLab Team: Systematic multi-stage hydraulic stimulation experiments in a hectometer-scale fractured rock volume at the Bedretto Underground Laboratory, Switzerland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10618, https://doi.org/10.5194/egusphere-egu24-10618, 2024.

X4.131
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EGU24-15749
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ECS
Exploring the Hydro-Mechanical Behavior of Fractures Utilizing the mini-SIMFIP Probe 
Tom Schaber, Julian Osten, Mohammadreza Jalali, Alexander Cadmus, Leonie Welsing, Paul Cook, Yves Guglielmi, Raul Fuentes, and Florian Amann

The SIMFIP (Step-rate Injection Method for Fracture In-situ Properties) probe is a downhole displacement tool designed to measure injection-driven fracture displacement in an isolated borehole interval in three dimensions. The resulting displacement and interval pressure data can be used to estimate fracture characteristics such as fracture stiffnesses, strength and hydraulic properties from borehole measurements. SIMFIP results have also been successfully implemented in a stress inversion routine that allows the calculation of the full stress tensor from a single measurement.

As part of the SPINE (Stress Profiling IN Enhanced geothermal systems) project, a laboratory-scale deformation tool, the mini-SIMFIP probe has been developed. The probe, with a diameter of 20 mm and a length of 65 mm, can be installed in laboratory to study the 3D deformation of intact rock and fractures in different type of rocks. Preliminary tests were conducted in a decimeter-scale true triaxial test apparatus containing a cuboid granite sample with an oblique saw-cut laboratory fracture. Two boreholes crossing the fracture can be isolated and equipped with the mini-SIMFIP probe. Fluid injection into the isolated interval opens the fracture or induces hydraulic shearing under anisotropic stress conditions. The resulting dataset can be used to quantify measurement uncertainties associated with the field SIMFIP protocol, to benchmark stress inversion protocols against known stress boundary conditions, and gain better insight into hydro-mechanically coupled processes.

How to cite: Schaber, T., Osten, J., Jalali, M., Cadmus, A., Welsing, L., Cook, P., Guglielmi, Y., Fuentes, R., and Amann, F.: Exploring the Hydro-Mechanical Behavior of Fractures Utilizing the mini-SIMFIP Probe, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15749, https://doi.org/10.5194/egusphere-egu24-15749, 2024.

X4.132
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EGU24-11990
Microstructural Characterization of Fault Rocks from the Groningen Gas Field 
Ernst Willingshofer, Job Arts, Dallyn Rodrigues, Fadi Nader, Martyn Drury, Liviu Matenco, and Andre Niemeijer

Human activities in the subsurface such as geothermal energy production, CO2- and hydrogen storage, and gas extraction can affect the regional stress field and lead to induced seismicity. Gas production from the Groningen gas field in the northeast of the Netherlands has led to more than 300 shallow earthquakes with local magnitudes ML > 1.5 and up to a maximum magnitude of ML 3.6, resulting in substantial damage to buildings. Recent earthquake localization studies show that seismicity dominantly occurs on complex normal fault systems, at the depth of the Permian (Rotliegend) reservoir. These faults were formed during multiple tectonic phases from the Late Paleozoic to Early Cenozoic and may comprise breccias, cataclasites, fault gouges and clay smears. The fault strength and slip behaviour are controlled by its composition and microstructural state (porosity, grain size and shape, and presence of foliation within the fault core). Fluid-rock interactions and diagenetic processes during and after fault activity may have altered these characteristics and, hence, the strength and slip behaviour of the fault. Knowledge on the state and composition is thus required to reliably predict the maximum stress drop and seismic energy release upon fault reactivation. However, such knowledge is still lacking at present day.

With this study, we aim at characterizing the microstructures of fault gouges in the Groningen faults. We assess the mineralogy, porosity, and grain size distribution of natural samples from faulted core samples derived from the Groningen gas field. Well-log data is presented to show the representativeness of these samples in the larger context of the gas field. The observations on natural microstructures are then used to define simplified geometrical representations or scenarios that can be used as input for microphysical models. Microstructural characterization involves optical microscopy for quantitative petrography of both bulk rock and selected regions of interest (ROI) within the fault zone. Scanning Electron Microscopy (SEM) with Backscattered Electron (BSE), Cathodoluminescence (CL), and Energy-Dispersive X-ray Spectroscopy (EDX) is employed to analyse porosity, grain size, shape, and mineralogy of faulted regions.

Preliminary results show that the compositions of fault rocks differ from the host rock and that along-fault variability in mineralogy, cementation, and grain size are important to consider. We distinguish between four main types of fault gouges in the Groningen Rotliegend, based on their microstructural characteristics: (1) gouges consisting of quartz and feldspar grains embedded in a very fine clay matrix, (2) very fine-grained quartz-rich gouges, (3) quartz-cemented gouges, and (4) anhydrite-cemented gouges. We expect that induced fault movement in the first two gouges occurs by reactivation of the earlier produced fault gouges. Since quartz and anhydrite cementation is concentrated in the faults, reactivation of the latter two presumably occurs by cataclastic processes and gouge formation from the adjacent bulk rock rather than the cemented gouge. This suggests that a well constrained fault diagenetic history is required to infer which components of the fault material governs its frictional behaviour and hence the related seismic hazards. 

How to cite: Willingshofer, E., Arts, J., Rodrigues, D., Nader, F., Drury, M., Matenco, L., and Niemeijer, A.: Microstructural Characterization of Fault Rocks from the Groningen Gas Field, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11990, https://doi.org/10.5194/egusphere-egu24-11990, 2024.

X4.133
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EGU24-20381
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ECS
Physical and mechanical characterization of veined rocks: Implications to a porphyry model 
Franco Robbiano, Luis Felipe Orellana, and Marie Violay

The brittle-ductile transition is an important mechanical shift within the Earth's lithosphere and, as a crucial interface between ascending hot magma and colder host rock, plays a fundamental role in fluid migration within hydrothermal systems. Traditionally assumed to occur at temperatures between 350 °C and 400 °C, recent studies challenge this assumption, revealing a temperature transition range dependent on minerals. Experimental and numerical investigations highlight significant variability, ranging from 260 °C in wet quartz to 700 °C for dry orthopyroxene within homogeneous mineral compositions.

This study delves into the complexities of this rheological barrier in the El Teniente Mafic Complex in Chile (currently a copper mine and formerly a hydrothermal system at the BDT), unraveling its impact on the migration of magmatic fluids. Building on previous research suggesting a self-sustaining mechanism that facilitates fluid movement over the brittle-ductile transition through overpressure-permeability waves and the formation of a dense, multi-episode vein network, our focus is on understanding deformation mechanisms and the localization of deformation and permeability at the BDT. Heat transfer models propose a dual paradigm of conduction and convection, adding to the complexity.

To address these challenges, we conducted a comprehensive series of physical and mechanical measurements on 34 cylindrical samples from the El Teniente Mafic Complex. This included analyses of density, porosity, elastic wave characteristics, and electrical conductivity under variable water conductivities. Elastic wave measurements were performed using 2.25 MHz transducers on both dry and saturated samples. Permeabilities were determined by the pulse decay technique for compact rocks, complemented by triaxial tests and local strain measurement using strain gauges, along with acoustic emission measurements on dry samples subjected to confinements similar to those near the mine.

Our results highlight consistently low porosity (below 1%) in the samples, with electrical conductivity, permeability, and strength controlled by veins. Particularly, at lower salinities, the metallic particle content and the orientation of the vein with respect to the loading axis significantly influence electrical conductivity and phase. At higher water conductivities, behavior is governed by connected porosity. Furthermore, favorably oriented veins emerge as crucial controllers of both permeability and mechanical resistance.

These observations align with a convection heat flow model in a porphyry system, providing significant insights into the complex interaction between rock and vein properties. The study uniquely focuses on fossil high enthalpy systems, shedding light on their complex behavior. Additionally, the article discusses constraints on model variables, fostering a comprehensive understanding of the brittle-ductile transition in magmatic-hydrothermal systems.

How to cite: Robbiano, F., Orellana, L. F., and Violay, M.: Physical and mechanical characterization of veined rocks: Implications to a porphyry model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20381, https://doi.org/10.5194/egusphere-egu24-20381, 2024.

X4.134
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EGU24-11634
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ECS
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Fault Nucleation and Reactivation in Displaced Fault Systems: An Experimental Study 
Milad Naderloo, Jan Dirk Jansen, and Auke Barnhoorn

Understanding fault slip nucleation within the reservoir interval and its propagation beyond the reservoir is essential. Analytical and numerical studies have shown that, depending on the type of operation (injection/depletion), fault slip can nucleate at external or inner corners along the displaced fault system, driven by positive peak shear stresses. In the case of depletion, slip patches gradually start at the inner corners and grow towards the inner part of the reservoir, merging with further depletion. Conversely, injection or increased pore pressure leads to slip patches at external corners, potentially propagating beyond the reservoir into the overburden and underburden. We conducted triaxial experiments on small-scale (mm scale) cylindrical samples containing an entirely displaced vertical fault to investigate fault reactivation and slip nucleation in such settings. Two types of stress paths, monotonic and cyclic, were applied to examine the effects of stress patterns on slip nucleation. For this purpose, we utilized strain gauges to measure differential compaction along the displaced fault directly on the small-scale samples. Direct measurements with a strain gauge network adjacent to the displaced fault system during the monotonic test revealed that differential compaction intensifies from the top of the sample towards the internal corner at the center of the fault where different layers are juxtaposed vertically, indicating a variation in the stress field surrounding the fault plane. Furthermore, results from the cyclic test showed that the differential compaction increases with an increasing number of cycles. Our direct measurements near the displaced fault plane confirm/match the anomalies and peaks in stress observed in previous numerical and analytical studies.

How to cite: Naderloo, M., Jansen, J. D., and Barnhoorn, A.: Fault Nucleation and Reactivation in Displaced Fault Systems: An Experimental Study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11634, https://doi.org/10.5194/egusphere-egu24-11634, 2024.

X4.135
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EGU24-18390
 Permeability evolution of a fractured, porous and permeable sandstone at simulated georeservoir conditions 
Marco Fazio and Martin Sauter

Bentheim sandstone is regarded as a conventional georeservoir rock even at great depth, due to its mineral composition, homogeneity, micro- and macrostructure. Therefore it has been extensively tested for a variety of applications to understand its physical and mechanical properties under changing environmental conditions.

A recent study has shown how the simultaneous change of pressure, temperature and pore pressure, therefore recreating environmental conditions at selected depths, affects the evolution of permeability at depths, both when the rock is buried and when the rock is exhumed. The interaction between those variables has a complex effect on the permeability of Bentheim Sandstone, which could not have been identified by assessing individually the role of a variable. These results show that the permeability of such rock could be overestimated with classical studies and highlight the importance of investigating rock mechanical and hydraulic properties at georeservoir conditions. These experiments have been performed on intact samples.

However, rocks at depth contain fractures and faults, which may alter the interconnectivity of the pore space, hence the permeability of the rock itself. The deformation and failure of Bentheim Sandstone at high strain resulted in permeability loss due to the formation of comminuted material and grain crushing which lowered the pore space interconnectivity. No fractured sample has been tested under simultaneously changing environmental conditions.

To fill this gap, we replicate the experimental procedure used to test intact samples of Bentheim sandstone, both under simultaneously changing conditions and under a sequential variation of different variables, after the sample has been brought to failure. Our goal is to understand the importance of fractures on the permeability evolution at different simulated depths.

How to cite: Fazio, M. and Sauter, M.:  Permeability evolution of a fractured, porous and permeable sandstone at simulated georeservoir conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18390, https://doi.org/10.5194/egusphere-egu24-18390, 2024.

X4.136
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EGU24-20658
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ECS
Impact of Capillary Number, Fluid Viscosity Ratio, and Fracture Closure on Two-Phase Flow Regimes in Geological Fractures: An Experimental Study 
Amin Rezaei, Francesco Gomez, Oshri Borgman, Insa Neuweiler, and Yves Meheust

Accurately predicting fluid-fluid interface displacement in fractured reservoirs is paramount for optimizing subsurface operations, particularly in the context of enhanced oil recovery and geological carbon sequestration (GCS). However, a comprehensive understanding of two-phase flow behavior in fractures, including the impact of fracture closure, fluid viscosity ratio, and capillary number, is yet to be achieved. To address this challenge, we have developed an analog experimental setup to investigate the intricate relationship between fracture surface roughness and fluid-fluid interface displacement. Our experimental setup features a transparent fracture flow cell with self-affine rough-walled surfaces that are matched to each other above a chosen length scale (denoted below as the correlation length) and a precisely controlled mean aperture. Realistic synthetic fracture geometries were generated numerically. They are characterized by their Hurst exponent, fracture closure, and correlation length. High-speed imaging captures the dynamic spatial distribution of fluid phases within the fracture plane during drainage processes in a given fracture geometry. The mean aperture can be varied between experiments for a given geometry of the fracture walls. We investigate a comprehensive range of capillary numbers, spanning both viscous and capillary-dominated regimes, vary viscosity ratios, and characterize the resulting displacement regimes. Our results reveal a profound impact of fracture closure and correlation length on trapping efficacy, particularly in the capillary-dominated regime. These findings can be interpreted in terms of residual trapping of CO2 during GCS in fractured reservoirs.

How to cite: Rezaei, A., Gomez, F., Borgman, O., Neuweiler, I., and Meheust, Y.: Impact of Capillary Number, Fluid Viscosity Ratio, and Fracture Closure on Two-Phase Flow Regimes in Geological Fractures: An Experimental Study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20658, https://doi.org/10.5194/egusphere-egu24-20658, 2024.

X4.137
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EGU24-22156
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ECS
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FracAbility: A python toolbox for survival analysis in fractured rock systems 
Gabriele Benedetti, Stefano Casiraghi, Andrea Bistacchi, and Daniela Bertacchi

When analysing fractured rock outcrops, the fracture network's topology and length statistics are of fundamental importance. Past literature focused on adopting a non-parametric approach for the unbiased estimation of fracture length data mean, and with some additional steps, the variance of a population. However, technology improved, and necessities shifted. Now it is possible to quickly obtain dense length datasets with thousands of measurements and the emergence of stochastic DFNs increased the demand for parametric solutions to correctly fit several types of distributions. These conditions highlighted an absence of works on these topics. Of particular interest is the right censoring bias effect of the interpretational boundary on the fracture length statistics. We tackle this problem by applying survival analysis techniques, a branch of statistics that includes methods for modelling time to event data and correctly estimating the model’s parameters with data affected by censoring. Synthetic testing has been carried out, showing a reliable estimate of the distribution parameters with up to 80% of the total measurements being censored. Moreover, it is shown that the correction is independent from the orientation of the fracture set or boundary geometry. We propose FracAbility, a new open-source Python package capable to both analyse the topology of fracture networks and, by using the latest SciPy version, correctly fit different parametrical distributions on length data with right censored measurements. The library and the proposed approach have been applied to real world data, successfully correcting length distributions affected by censoring.

How to cite: Benedetti, G., Casiraghi, S., Bistacchi, A., and Bertacchi, D.: FracAbility: A python toolbox for survival analysis in fractured rock systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22156, https://doi.org/10.5194/egusphere-egu24-22156, 2024.