Flow through faults and fractures has been studied extensively in the context of hydrocarbon exploration and production, to understand charge and migration, hydrocarbon column heights and fault transmissibility. Learnings have typically been captured in pragmatic models, such as the Shale-Gouge-Ratio (SGR) concept, providing dimensionless or relative fault permeability definitions based on limited subsurface data.

The resulting coarse predictions are however not suitable for geoenergy applications, including CO₂ sequestration (CCS) or underground hydrogen storage (UHS), where injection into a storage reservoir requires assurance that the injected fluids or gases will not leak out of the storage complex via faults or fractured caprocks. The conventional fault seal analyses do not provide this containment assurance.

A new paradigm is required for characterizing faults and fractures in geoenergy projects, focused on derisking leakage of injected fluids and gases along faults. Such approach is not necessarily about accurately predicting the permeability of a fault or fracture, but rather about understanding what geometric properties and mechanical or chemical mechanisms would contribute to either permeable or sealing behaviour of faults. Improved insights in any of these areas would help in screening fault leakage risks in prospective subsurface geoenergy projects.

Analogue data, both from outcrops for geometric fault attributes and from the lab for mechanical and chemical properties, can help gain those fundamental insights into what controls fault leakage. Properties can be quantified and processes studied at a level of detail that cannot be matched by in-situ subsurface datasets, particularly not in the context of geoenergy systems, where operational subsurface projects are still limited. Outcrop studies can help improve our understanding of vertical connectivity, with focus on lower-permeability ductile rocks analogues to typical reservoir seals. Lab studies and in-situ experiments can provide insights into injected fluids such as CO₂ or H₂ affect the mechanical and chemical integrity of faults and subsequent flow behaviour. For geoenergy systems in particular, experiments should focus on the impact of rapid pressure or temperature cycling. Induced seismicity is another potential threat to containment integrity and requires further research to understand what fault geometries are most prone to reactivation as well as how reactivation affects the sealing behaviour of a fault.

In recent years, integrated studies such as the multi-scale, multiphysics ACT-DETECT project have started to provide some answers to these questions, resulting in novel insights and workflows that provide a first-order fault leakage risk assessment that can be used to identify ideal storage sites. However, with the envisioned increase in the number of geoenergy projects to meet carbon emission reduction targets, the need for more refined screening criteria will increase too as the flexibility in selecting ideal storage locations will decrease.

How to cite: Bisdom, K.: A new paradigm for flow through faults and fractures in the context of geoenergy , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3553, https://doi.org/10.5194/egusphere-egu22-3553, 2022.

Characterization of fracture networks, both in fault zones and in the less-fractured background, is essential for the analysis and modelling of mechanical and hydraulic properties of the rock mass (i.e. rock plus fractures). Here we present our experience in characterizing fracture networks and other structural features on large outcrops of different basement and metamorphic cover units in the Penninic, Austroalpine and Helvetic units of the Aosta Valley. These units show a variety of lithological, mechanical, and rheological characteristics and were subjected to different ductile and brittle tectonic evolution, resulting in complex combinations of compositional layering, metamorphic schistosity, and fracture networks.

Our methodology is based on a combination of traditional field surveys and remote-sensing techniques such as ground-based and UAS photogrammetric surveys, and terrestrial or helicopter laser scanning. The first task, whose importance is too often overlooked, is represented by selecting outcrops that are representative in terms of structural and lithological properties of a larger rock volume, based on a thorough knowledge of regional structural geology and tectonics. The field survey is carried out with traditional techniques, paying attention to the kinematics, relative chronology, and mineralization (e.g. veins or mineral coatings) of structures. These features, that are often overlooked in fracture studies, are fundamental to frame the evolution of a complex schistosity and fracture network, to separate tectonic fractures with respect to those related to slope dynamics, and to develop predictive models of fracturing at depth (where slope-related fracture will not be present). At the same time, remote-sensing datasets are collected. The choice of the survey technique (terrestrial vs. aerial, photogrammetry vs. laser scanning) depends on various conditions, but in all cases the output is a point cloud DOM, colorized with RGB values, that should have a density (points/area) sufficient to characterize the smallest relevant structural features. From this, also textured surface DOMs and/or DEM plus orthophotos (for almost flat outcrops) can be obtained.

The first step of DOM analysis is carried out “manually”, selecting facets and traces with suitable software tools (e.g. Compass plugin in CloudCompare). This allows selecting different sets of structures, characterizing their orientation statistics, and assigning them to sets defined in the field (with kinematics, chronology, etc.). This step also allows understanding how well the structural features recognized in the field are represented in the DOM. The second step of DOM analysis consists in an automatic segmentation (in case of a point cloud) or tracing (in case of a DEM of triangulated surface textured with images) with algorithms calibrated with results of the manual interpretation. Overall, this results in a supervised semi-automatic workflow, allowing to extract huge structural datasets in a reasonable time, maintaining the connection with kinematic and chronological observations carried out in the field.

The fracture datasets can be eventually characterized with tools allowing to measure statistical distributions of different parameters of the fracture sets using virtual scanlines and/or scanareas, and these distributions can be used to model different properties of the fracture networks or generate stochastic DFN models.

How to cite: Monopoli, B., Bistacchi, A., Agliardi, F., Arienti, G., Dal Piaz, G., Bertolo, D., and Casiraghi, S.: A semi-automatic workflow for structural interpretation of large point-cloud Digital Outcrop Models on complex fractured metamorphic rocks (Aosta Valley, Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12888, https://doi.org/10.5194/egusphere-egu22-12888, 2022.

Upper Cretaceous (Turonian and Cenomanian) carbonates in the Münsterland Cretaceous Basin, NW Germany, have become a target for geothermal energy production in recent years. These carbonates are present at depths of up to ca. 1,800 m in the region of the city of Münster in the center of the basin (e.g. Münsterland-1 well) and at depths beyond 2,000 m in the so-called Vorosning Depression. They represent the shallowest calcareous strata within the sedimentary succession of the Münsterland Basin and the underlying Rhenish Massif. Previous industrial drilling campaigns mostly focused on potential hydrocarbon gas reservoirs of the Upper Carboniferous. In the context of geothermal reservoir exploration, analog studies in outcrops of the Cretaceous carbonates are a prerequisite for reservoir quality assessment since subsurface/in situ data of these stratigraphic units, and especially petrophysical properties, are very sparse, not accessible or even absent in some areas. Investigations of quarries with Cretaceous carbonates mostly focused on paleontological and facies related research in the past rather than on their petrophysical properties. Three quarries in the Lengerich and Oerlinghausen areas, all at the northern margin of the basin, were now sampled for petrophysical laboratory experiments of Cenomanian and Turonian rocks. Additionally, scanline investigations, which involve collecting information such as length and aperture and others of each fracture along a line intersecting the rock mass, capturing of Unmanned Aerial Vehicle (UAV, commonly called drones) footage and laser scanning was performed at the three Cenomanian outcrops in Lengerich and one Turonian outcrop in Oerlinghausen. Further UAV footage and laser scans were collected for other outcrops within the quarries. The facies of the investigated rocks are expected to be comparable to what can be anticipated in the center of the Münsterland Basin according to the current paleogeographical understanding. Their analysis can thus be helpful in predicting the conditions that may be encountered in the central part of the basin. However, since the data was collected at the northern margin of the basin, the influence of the Osning Fault Zone (Upper Cretaceous inversion tectonics) has to be taken under consideration when further interpreting the data. The drone footage was processed, and Virtual Outcrop Models (VOM) were created using Agisoft Metashape. The point clouds of both, the laser scanning and processed UAV footage, were analyzed using the open-source package CloudCompare with its Facets and Compass plugins. The plugins allowed the detection of differently oriented fracture sets in the point clouds. This allowed to characterize fracture distributions and the comparison between the virtual outcrop data and the scanline data. Subsequently, the parameters of the fracture distributions of these structural features together with the laboratory measurements on bulk petrophysical properties were combined in a discrete fracture network (DFN). This representation of the reservoir, and in particular the 3D distribution of permeability, will be used for reservoir analog modelling to characterize fluid flow in the subsurface.

How to cite: Slama, S., Jüstel, A., Lippert, K., and Kukla, P.: Characterizing fracture networks and petrophysical bulk properties of carbonates from the margin of the Münsterland Cretaceous Basin, NW Germany, from outcrops, virtual outcrop models and laboratory testing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2503, https://doi.org/10.5194/egusphere-egu22-2503, 2022.

In outcrop-based fracture studies, the quantification of fracture intensity is often limited by the limitations of the manual sampling technique, characterized by punctual measurements (e.g. sampling spot, scanline, scanwindow) and moderate biases (e.g. fracture length truncation, technical and personal errors). The proximal remote sensing technologies, as terrestrial or Uncrafted Aerial Vehicle (UAV)-based LiDAR and photogrammetry, can help to overcome these limitations due to the possibility to obtain high-resolution and accurate quantitative data from the digital twin of the outcrop, the so-called Digital Outcrop Model (DOM). The DOMs can be very useful in outcrop-based fracture studies because their analysis allows to obtain several quantitative information with manual and/or automatic methods and with continuity in each position of the outcrop, increasing the accuracy of the fracture intensity estimations. However, due to the novelty of DOM technology and the lack of well-defined DOM-based fracture sampling procedures, these huge fracture datasets are often difficult to study and interpret, and therefore, the benefits of the DOM cannot be fully exploited. 

For this reason we present a complete workflow based on the DICE (Discontinuity Intensity Calculator and Estimator) open-source MATLAB© application that allows to quantitatively characterize the fractures of rocky outcrops from the 3D Digital Outcrop Models (DOMs). The proposed workflow consists in the following steps: (1) fracture mapping onto the 3D DOMs; (2) calculation of the fractures dimension, position and orientation; (iii) determination by DICE algorithm of the discontinuity parameters (persistence/dimension, distribution, spacing and intensity) using different 3D sampling techniques (3D scanline, 3D circular scan window and spherical scan volume). The differences of these sampling techniques and the fracture intensity parameters that can be obtained (p10, p21, p32) are discussed, showing the advantages and limitations of each DICE method.

How to cite: Menegoni, N., Giordan, D., and Perotti, C.: 3D Digital Outcrop Model-based quantification of fracture intensity: the Discontinuity Intensity Calculator and Estimator (DICE) open-source application, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10207, https://doi.org/10.5194/egusphere-egu22-10207, 2022.

Fluid flow in low-porosity/permeability reservoir rocks such as tight carbonates is mostly restricted to structural discontinuities (e.g. faults, fractures, karstified zones). Fault zones, in particular in such rocks, offer both suitable fluid flow pathways, but may also act as impermeable barriers. The heterogeneous permeability structure of fault zones, however, impedes pre-drilling investigations of exploration targets by numerical models. A better understanding of the factors that control the fluid flow and the heterogeneity of permeability distribution along fault zones in tight reservoirs is a pre-requisite for the definition of drilling targets.

In this study, a hydraulic field laboratory with a volume of 30 m x 30 m x 20 m was set up in a quarry in SE Germany to investigate the influence of fault zones on the general permeability structure of tight carbonates. The test field contained three WNW-ESE-striking, repeatedly reactivated normal faults with offsets in the order of <1 m and two roughly perpendicularly oriented NNE-SSW-striking fracture corridors. Fault zones and fracture corridors were targeted by 62 wells. Wells that exhibited a decent hydraulic connection the to the overall conductive fracture network were logged (e.g. borehole image logs, FWS, etcs.) and in selected wells hydraulic tests were conducted. Water levels were measured both during static conditions and during testing. Due to the density of wells we were able to constrain the controlling factors for fluid flow along and across the fault zones. Damage zones were considered as conduits while the fault core was expected to be impermeable. These general assumptions could be confirmed by our tests, however we found some exceptions. While fluid flow in general is restricted to few, well-connected fractures, the majority of the fractures are dead ends, solely serving as storage for fluids. With increasing displacement and complexity of the fault zone, enhanced permeability parallel to the fault zone could be inferred. At larger offsets, where a thicker fault core develops, fhe fault core itself acts as barrier and fractures and fracture corridors do not penetrate the faults. We think that this is related to the presence of the much less competent fault core of a certain thickness which is able to accommodate the brittle deformation. Where the faults offset is less than ~0.4 m, the integrity of the fault seal is breached by fracture corridors, cross cutting the faults. This is clearly shown by the pressure distribution in static and transient conditions. Faulting, hence leads to a compartmentalization of the reservoir, where the compartments do either communicate or interact with significant delay.

The information and data received from the conducted field tests furthermore serve as input parameters and validation for a newly developed numerical approach that aims to simulate fluid flow in this type of geological settings, results of which will be presented in an additional presentation by our project partners.

How to cite: Freitag, S., Bauer, W., Stollhofen, H., and Hähnel, L.: (1)   Transmissivity of fault zones in tight carbonates – results from a reservoir-scale hydraulic field laboratory in the Franconian Alb, SE Germany, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8457, https://doi.org/10.5194/egusphere-egu22-8457, 2022.

The regionalization of hydraulic properties like specific yield/storativity or permeability in fractured crystalline rock is of utmost importance for a variety of applications, such as geothermal and other resources, waste disposal or underground construction. However, accurate predictions for these properties – particularly for undrilled sites – bear a high degree of uncertainty as already direct observations through hydraulic in-situ tests show a variance of about 2 orders of magnitude at any depth (Achtziger-Zupančič et al., 2017).

Permeability-depth relationships using multiple log-log regressions conducted on an extended version of the worldwide permeability compilation of crystalline rocks (roughly 30000 entries in Achtziger-Zupančič et al., 2017; now consisting of about 50000 single in-situ permeability measurements to depths of 2000 mbgs) indicate that depth is generally the most important geological factor, resulting in a permeability decrease of three to four orders of magnitude in the investigated depth range. Specific yield and storativity show a similar but less pronounced depth trend. Beside depth, most influential factors for permeability in crystalline rock are the long-term tectono-geological history described by geological province which locally is overprinted by current seismotectonic activity as determined by peak ground acceleration (Achtziger-Zupančič et al., 2017). Although petrography might be of local importance, only a low impact has been observed for the global dataset, besides lithologies allowing for karstification. Ongoing vertical movements – particularly resulting from glacial isostatic adjustment – alter the permeability trend with depth.

The latter shows distinct trends starting at about logK -14.5 to -14.8 m² at 100 mbgs and showing diversion of about 1.5 orders of magnitude at 1 km depth between areas without significant uplift and areas with uplift of more than 4 mm/y as determined from a probabilistic interpolation of global geodetic measurements (Husson et al., 2018). The difference is attributed either to glacial loading (normal faulting or reactivation) induced destruction preserved during glacial induced rebound and/or uplift-caused horizontal fracture growth which improved connectivity in the rock mass. Areas undergoing subsidence show similar trends like highly uplifting areas which is attributed to efficient normal faulting induced destruction of the rock mass.

References:

Achtziger-Zupančič, P, Loew, S and Mariéthoz, G (2017). A new global database to improve predictions of permeability distribution in crystalline rocks at site scale. JGR: Solid Earth 122(5): 3513-3539.

Husson, L, Bodin, Th, Spada, G, Choblet, G and Kreemer, C (2018). Bayesian surface reconstruction of geodetic uplift rates: Mapping the global fingerprint of Glacial Isostatic Adjustment. J Geodyn 122: 25-40.

How to cite: Achtziger-Zupancic, P.: The influence of glacial induced adjustment and other geological factors on the depth distribution of permeabilities in crystalline rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7157, https://doi.org/10.5194/egusphere-egu22-7157, 2022.

The understanding of the coupled thermo-hydro-mechanical behaviour of fault zones in naturally fractured reservoirs is essential both for fundamental and applied sciences and in particular for the safety assessment of radioactive waste disposal facilities. In this framework, an international research program callled CHENILLE was built to address key questions related to the impact of high temperatures (up to 150°C) on shear zones as well as fault reactivation processes in shale formations. The project includes a thermally controlled in situ fluid injection experiment on a strike-slip fault zone outcropping atIRSN’s Tournemire Underground Research Laboratory (URL) and a series of laboratory experiments to understand the chemical and structural evolution occurring within the fault zones during the thermal and hydraulic loading. The in situ experiment includes a heating system installed around an injection borehole will enable a precise and controlled incremental increase of the thermal load. The injection borehole will be equiped with a Step-Rate Injection Method for Fracture In-Situ Properties (SIMFIP) probe, in order to perform step pressure tests. The probe will not only measure the flow and pressure rate inside the injection borehole but also allow to monitor the borehole’s 3D deformation during the hydraulic and thermal loading steps. In addition, an array of seismicifferent sensors will be implemented around the injection area to measure the seismic and aseismic deformation induced either by thermal or by hydraulic loading. The seismic monitoring system is composed of Acoustic Emission (sensitive between 1kHz and 60kHz) enabling monitoring fracturing processes of sub-decimeter size. Furthermore, a fibre optic network will be installed in the heating boreholes to measure spatially temperature variationsvia Distributed Temperature Sensing technology in the investigation area. Active seismic surveys, using different source types, are scheduled before and after the experiment to determine the structural network but also to detect the appearance of new structures triggered from the hydro-thermal pressurization of the fault by tomography and reflection seismic methods. The overall goal of our work is to present the interaction between the different geophysical methods that we are using as well as some preliminary results. A first part is dedicated to the description of the fault zone through field and core samples observations as well as borehole to borehole correlation, whereas the second is dedicated to preliminary results on the thermal diffusion expected in the fault.

How to cite: Bonnelye, A., Dick, P., Cotton, F., Giese, R., Guglielmi, Y., Jougnot, D., Henninges, J., Kwiatek, G., and Lüth, S.: CHENILLE: Coupled beHavior undErstaNdIng of fauLts: from the Laboratory to the fiEld, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10259, https://doi.org/10.5194/egusphere-egu22-10259, 2022.

At the Äspö hard rock underground laboratory in Sweden, six in situ hydraulic fracturing experiments took place at 410 m depth. A multistage hydraulic fracturing approach is tested with a low environmental impact, e.g., induced seismicity. The idea is to mitigate induced seismicity and preserve the permeability enhancement process under safe conditions. The fractures are initiated by two different injection systems (conventional and progressive). An extensive sensor array is installed at level 410 m, including simultaneous measurements of acoustic emissions, electric self-potential, and electromagnetic radiation sensors. The monitoring catalog includes more than 4300 acoustic emission events with estimated magnitudes from the continuous monitoring setup (in-situ sensors between 1-100 kHz). The experiment borehole F1 is drilled in the direction of Shmin, perpendicular to the expected fracture plane. Two electromagnetic radiation sensors are installed and aligned to (i) Shmin and (ii) the expected fracture plane with a sampling rate of 1 Hz and a frequency range between 35-50 kHz. The self-potential sensors are installed at level 410 with a distance of 50-75 m from the borehole F1, including nine measuring probes and one base probe. A second self-potential setup is deployed at level 280 m in the far-field with a distance of 150-200 m from F1. The self-potential data were measured with a sampling rate of 1 Hz. For the first time (to our knowledge), the results of electric and electromagnetic monitoring of two hydraulic stimulation at meter-scale are presented.

How to cite: Haaf, N. and Schill, E.: Electric self-potential and electro-magnetic monitoring of hydraulic fracturing experiments in the Äspö Hard Rock Laboratoy, Sweden. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3837, https://doi.org/10.5194/egusphere-egu22-3837, 2022.

Between 2018 and 2021, the STIMTEC and STIMTEC-X hydraulic stimulation experiments were conducted at 130 m depth in the Reiche Zeche underground research laboratory in Freiberg/Germany. The STIMTEC experiment was designed to investigate the rock damage resulting from hydraulic stimulation and to link seismic activity and enhancement of hydraulic properties in anisotropic metamorphic gneiss. The following STIMTEC-X experiment aimed at better constraining the stress field in the rock volume to investigate the mechanisms leading to induced acoustic emission (AE) activity. Here, we present results from focal mechanism analysis of high-frequency (>1 kHz) AE events, associated with brittle deformation at the cm- to dm-scale induced by hydraulic stimulations. Focal mechanisms are calculated using full moment tensor inversion of first P-wave amplitudes using the hybridMT package. We use polarity and amplitude data from a (near) real-time seismic monitoring network, consisting of AE sensors, AE-hydrophones, accelerometers, and one broadband sensor. We observe changes in the predominant type of faulting from reverse faulting focal mechanisms during the frac and refrac cycles to oblique strike-slip focal mechanisms observed during subsequent high-volume fluid-injections performed during periodic pumping test. The observed differences in dominant focal mechanisms are consistent with the activation of less favourably oriented faults at increased pore fluid pressure during extended periodic pumping. We observe a reverse-faulting stress regime from focal mechanism inversion of low-volume injection stages for different boreholes, representative for the rock volume (typically ~5 m radially) surrounding the injection intervals. In contrast, stress field estimates obtained from analysing the instantaneous shut-in pressures of hydraulic stimulations in different boreholes indicate a regime change from thrust to strike-slip faulting in the investigated rock volume. The reservoir complexity seen at the scale of the experiment (30m x 30m x 20m) is large and is reflected by the significant variations in AE event activity in response to stimulation as well as small-scale rock, stress and structural heterogeneities.

How to cite: Boese, C., Kwiatek, G., Renner, J., and Dresen, G.: Stress field observations from hydraulic fracturing and focal mechanism inversion at the STIMTEC underground research lab, Reiche Zeche mine, Germany, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7986, https://doi.org/10.5194/egusphere-egu22-7986, 2022.

Fractures can significantly impact fluid flow and pore pressure distribution in the subsurface. Understanding the mechanisms and conditions influencing their ability to transport fluids and to promote pore pressure diffusion is key for many activities relying on fracture-controlled flow such as, for example, enhanced geothermal systems. In situ characterization of these properties is typically done by performing hydraulic tests in selected intervals of a borehole and their interpretation relies on the solution of a linear pressure diffusion equation. However, it has been shown that the hydraulic behavior of fractures as well as the associated near borehole flow regimes can be largely affected by the coupling between the solid deformation and fluid pressure upon injection/production. In this work, we explore these effects by performing a series of harmonic injection tests (HIT) as well as non-harmonic production tests (NHPT) in a packed-off interval of a borehole containing multiple natural fractures. The borehole is located in the Bedretto Underground Laboratory for Geosciences and Geoenergies in Switzerland and penetrates granitic rock mass. The two kinds of tests consist of a periodic repetition of the same injection or production protocol. Flow rates, interval pressures as well as pressures above and below the double-packer probe are recorded at the surface. An important advantage of periodic testing is that it permits a continuous tracking of hydraulic changes during the test. For our study, we conducted a so-called injectivity analysis, in which the phase-shift (time delay) and amplitude ratio between flow rate and interval pressure are used to infer effective hydraulic properties. We performed over 200 periodic tests including both HIT and NHPT with a large range of periods (7.5 s to 1800 s) as well as varying mean interval pressures (~1300 kPa to 2100 kPa) and flow oscillation amplitudes. As a result, we obtained a robust constraint of the radial flow regime prevailing in the fractures. Overall, we found that results from HIT and NHPT are in very good agreement despite the remarkably different injection protocols. For all cases, a prominent and consistent period dependence of phase shifts and amplitude ratios of flow rates and interval pressure was observed, in which both increase as the oscillatory period decreases. Amplitude ratios showed almost no variation with mean interval pressure regardless of the injection protocol. In contrast, a prominent pressure dependence of the phase shifts is captured by the HIT but not the NHPT data. Using the pressure-independent NHPT results, we reconstruct the general hydraulic response of the tested fractured section, which can be well represented by an analytical solution of the pressure-diffusion equation. This general trend explains the HIT data as well, although evidence of significant variations that are correlated with the amplitude of the pressure oscillations points to the predominant role of hydromechanical coupling effects on the fluid pressure diffusion process.

How to cite: Barbosa, N. D., Gholizadeh Doonechaly, N., and Renner, J.: Fluid pressure diffusion in fractured media: insights from harmonic and non-harmonic periodic pumping tests, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3414, https://doi.org/10.5194/egusphere-egu22-3414, 2022.

The Alpine Fault is the principal component of the plate boundary through the South Island of New Zealand, separating the Pacific and Indo-Australian Plates. It is recognised internationally as an important site for studying earthquake physics and tectonic deformation, as it produces large (M7-8) earthquakes approximately every 330 years and last ruptured in 1717. Therefore, the fault is considered to be late in its seismic cycle. It accommodates dextral-slip at a rate of 26 mm/yr with reverse slip at a maximum rate of 10 mm/yr in its central part, thus exhumed a fossil ductile shear zone, that was damaged brittlely during its exhumation.

The central Alpine Fault is the focus of the Deep Fault Drilling Project (DFDP), sponsored by the International Continental Drilling Project, which takes advantage of its globally rare tectonic situation to determine what temperatures, fluid pressures, and stresses exist within a plate-boundary fault in advance of an expected large earthquake. During DFDP phase II in 2014, an ~ 900 m drilled well that encountered an exceptionally high geothermal gradient (120 °C/km was measured in the borehole), was extensively characterized by repeated electric and sonic logs. These logs enable detailed study of fracture patterns near a major fault. The more than 19 km of logs run within the borehole gathered datasets covering, among others, thermal resistivity, sonic velocities, acoustic borehole imaging, and electrical resistivity. They show that the hanging wall is extensively fractured, explaining the high geothermal gradient measured in the borehole by lateral flow of hot water deep seated in the mountains.

We particularly focus on seven dual laterolog logs that provide a robust and reproducible dataset from which to determine the positions and orientations of conductive fractures. From these, different patterns of damage could be identified within the well. A first pattern consists of an extensive and dense pattern of isolated fractures that could be identified throughout the borehole. A second pattern suggests that decametric  zones of low resistivity localize damage and focus thermal anomalies. This suggests hierarchy of damage zone evolution of the damage zone of the Alpine Fault. A possible explanation is an initial phase of diffuse fracturing (pattern 1) that is followed by subsequent alteration of the major shear zone, which focuses fluid and heat flow (pattern 2).

How to cite: Doan, M.-L., Toy, V., Sutherland, R., and Townend, J.: Assessing damage pattern at depth near the Alpine Fault, New Zealand, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6166, https://doi.org/10.5194/egusphere-egu22-6166, 2022.

Rocksalt caverns are considered or already used as storage for nuclear waste, petroleum, hydrogen, CO2, and compressed air energy because of the low permeability and potential of fracture healing of salt. An important concern is the sealing capacity. Undisturbed rocksalt deposits in nature generally have very low permeability. However, as a result of excavation stress, a network of fractures will be induced within the rocksalt formation, increasing the permeability. At low deviatoric stresses and/or at low effective stresses, a fracture network filled with brine is expected to heal, and the connectivity of the brine-filled network, consisting of grain boundaries, pores, and microcracks, is expected to decrease over time. The driving force for such a healing process is the tendency to reduce the interfacial energy by reducing the total interfacial area. In order to assess the rate of pore reconfiguration and permeability evolution in damaged salt and to capture the key process of crack network evolution during healing, we employ time-resolved 3D microtomography to study the long-term evolution of the fracture network of small-scale polycrystalline rocksalt samples. We found that precipitation prefers to occur in open spaces in the early stage of healing, such as new cracks. As a result, flat cracks evolve into zigzag cracks, which create narrow throats, thereby reducing the permeability of the crack network. Our study also offers a way to testify the thermodynamic models quantitatively.

How to cite: Ji, Y., Spiers, C., Hangx, S., de Bresser, H., and Drury, M.: Crack healing in salt: time-resolved 3D microtomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12748, https://doi.org/10.5194/egusphere-egu22-12748, 2022.

Economically viable geothermal systems rely on the efficiency of fluid circulation and heat transfer. Permeable fault zones are therefore excellent candidates for geothermal exploitation. In crustal fault zones, hot fluids from depths that correspond to the brittle-ductile transition are brought to the surface via crustal-scale permeable fault zones and may therefore constitute a new kind of geothermal system. To assess their geothermal potential, we measured the permeability of reservoir rock during deformation to large strains (up to an axial strain of about 0.1) in the brittle regime - fault formation and sliding on the fault - by performing triaxial experiments on samples of well-characterised Lanhélin granite (France). Prior to deformation, samples were thermally-stressed to 700°C to ensure that their permeability was sufficiently high to measure on reasonable laboratory timescales. All experiments were conducted on water-saturated samples under drained conditions, at a constant pore pressure of 10 MPa and confining pressures of 20, 40, and 60 MPa (corresponding to a maximum depth of about 2 km), and at room temperature. Our data show that permeability decreases by about an order of magnitude prior to macroscopic shear failure. This decrease can be attributed to the closure of pre-existing microcracks which outweigh the formation of new microcracks during loading up to the peak stress. As the macroscopic shear fracture is formed, sample permeability increases by about a factor of two. The permeability of the sample remains almost constant during sliding on the fracture to large strains (corresponding to a fault displacement of ~7 mm), suggesting that the permeability of the fracture does not fall below the permeability of the host-rock. The permeability of the sample at the frictional sliding stress is lower at higher confining pressure (by about an order of magnitude between 20 and 60 MPa) but, overall, the evolution of sample permeability as a function of strain is qualitatively similar for confining pressures of 20−60 MPa. These experimental results will serve to inform numerical modelling designed to explore the influence of macroscopic fractures on fluid flow within a fractured geothermal reservoir.

How to cite: Carbillet, L., Heap, M. J., Duwiquet, H., Griffiths, L., Guillou-Frottier, L., Baud, P., and Violay, M.: Influence of brittle deformation on the permeability of granite: assessing the geothermal potential of crustal fault zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-66, https://doi.org/10.5194/egusphere-egu22-66, 2022.

Enhanced temperature gradients related to locally elevated heat production in granitic plutons offer the potential for low carbon geothermal energy production. Cornwall in SW England hosts several granitic plutons that are the subject of current geothermal projects (United Downs Deep Geothermal Power [UDDGP] Project and Eden Project). These projects target fault zones in crystalline rock that provide pre-existing pathways for fluid flow. Reinjection of cooler fluids into the reservoir after heat extraction may result in chemical disequilibrium with the host rock, potentially driving precipitation or chemical alteration. Such changes could influence the frictional properties of the fault zones, and hence require modifications to numerical risk-based calculations of the likelihood, or not, of induced seismicity.

In order to study the effects of such alterations, we have conducted a series of direct shear experiments under representative in-situ conditions on Cornish Carnmenellis granite samples which have undergone varying degrees of natural chemical alteration. The direct shear experiments were conducted on gouges (grain size < 125 μm) and at effective normal stresses of 80-105 MPa, pore fluid pressures of 25-50 MPa and temperatures of 16-180 °C. These conditions are relevant for the depths where the UDDGP project injection and production boreholes intercept the Porthtowan Fault zone, the assumed main conduit for fluid flow. In each test, load point velocity was stepped between 0.3 μm/s, 1 μm/s and 3 μm/s, and shear resistance of the sample was measured to determine the stability of sliding and thus the likelihood of induced seismicity as a function of degree of alteration. Initial shear tests at room temperature suggest little difference in the frictional response of altered and unaltered samples.

How to cite: Harpers, N., Forbes Inskip, N., Allen, M. J., Faulkner, D., Claes, H., Busch, A., and den Hartog, S.: Direct shear experiments to investigate the effect of chemical alteration on fault frictional behaviour in granitic geothermal systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12855, https://doi.org/10.5194/egusphere-egu22-12855, 2022.