Inferences have to be made about likely structures and their effects on fluid flow in a geothermal reservoir at the pre-drilling stage. This is the case for the potential geothermal reservoir in Variscan metasedimentary rocks that are expected to occur in the subsurface at Göttingen. Simple mechanical modelling, using reasonable ranges of values for rock properties, stresses and fluid pressures, is used here to predict the range of possible structures that are likely to exist in the sub-surface and that may be generated during thermal and hydraulic stimulation. Mohr diagrams are a useful way for predicting and illustrating how rocks can go from a stable stress state (i.e., no fracturing occurs) to an unstable stress state (i.e., fracturing occurs). This transition can occur if there are changes in: (1) the failure envelope; (2) the stresses; and/or (3) fluid pressure. Mohr diagrams are used to show under what fluid pressures and tectonic stresses different types and orientations of fractures are likely to be reactivated or generated. The approach enables the effects of parameters to be modelled individually, and for the types and orientations of fractures to be considered. This modelling is useful for helping geoscientists consider, model and predict the ranges of mechanical properties of rock, stresses, fluid pressures and the resultant fractures that are likely to occur in the sub-surface.

The Mesozoic rocks of the Somerset coast, UK, are used to illustrate how Mohr diagrams can help understand the history of fracturing. Such understanding is useful for predicting which fractures are likely to occur in the subsurface, which is important for predicting reservoir behaviour.

How to cite: Peacock, D., Leiss, B., and Sanderson, D.: Use of Mohr diagrams to predict fracturing in rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10311, https://doi.org/10.5194/egusphere-egu23-10311, 2023.

Pore fluid pressure in the geological formation at depth varies spatially and temporarily. An increase in pore fluid pressure leads to a reduction in effective normal stress and thus affects the rock strength and deformation mode. Extremely high pore fluid pressure induces very low normal stress conditions, where an extension or extension-shear hybrid fractures are formed. To better quantify the stress states and fluid pressure during fracture formation, it is crucial to document mechanical strength and the transition from tensile to shear fracture at low effective stress with elevated pore fluid pressure. However, all previous experimental studies were conducted under dry conditions. Here, we investigate the effects of pore fluid pressure on tensile and hybrid fractures in Berea sandstone by conducting triaxial extension deformation experiments under pore-fluid-pressure controlled conditions at effective maximum principal stress (σ₁' = σ₁ - P_p, where σ₁ is total maximum principal stress and Pp is pore fluid pressure) ranging from 10 to 130 MPa. Fracture strength, inelastic strain, strain at failure, fracture angle to σ₁', and the amount of comminution increase with σ₁'. The transition of extension to shear fracture occurs at σ₁' = ~ 30 MPa, based on the fracture angle and the degree of comminution. All the saturated or pore fluid pressure-controlled test specimens exhibit lower fracture strength than dry samples, and the difference is distinct when the minimum principal stress is tensile (i.e., σ₃' < 0). This implies that pore fluid pressure more effectively assists the breakage of the bonds and opening of the microcracks in the extension fracture regime. A series of triaxial extension experiments at σ₁' = 20 and 50 MPa with various combinations of σ₁ and Pp indicate that the fracture angle to σ₁' is independent of σ₁ and Pp in the extension fracture regime at σ₁' = 20 MPa, and that fracture angle increases with σ₁ and Pp in the extension-shear hybrid fracture regime at σ₁' = 50 MPa. This implies that the estimation of in-situ stress and pore fluid pressure from natural or human-induced deformation at low effective pressure (such as joints, veins, and drilling-induced tensile fractures) requires careful consideration of the mode of fractures formed.

How to cite: Kitajima, H., Ruplinger, C., and Tilley, C.: Experimental investigations on effects of pore fluid pressure on extension and extension-shear mixed-mode fracture in Berea sandstone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10433, https://doi.org/10.5194/egusphere-egu23-10433, 2023.

Subsurface engineering, such as geothermal energy extraction, requires knowledge of the rupture of geomaterials. Of particular importance is the time- and rate-dependence of material strength, which impacts fracture architecture and thus hydraulic conductivity and system permeability. Cyclic soft stimulation (CSS) techniques have been developed to maximise the efficiency of resource extraction whilst minimising large amplitude, fluid-injection induced seismicity. Here, we explore the benefits of cyclic stimulation experimentally, utilising novel “pulsed pumping” hydraulic fracture tests in which fluid pressure is cycled within the central borehole of a suite of 20x20cm cylinders of dense granite. The response is monitored at high-resolution by fibre-optic circumferential strain measurements, fluid pressure data and acoustic emission recording. Using cyclic high-low pressure square waves, we found that breakdown pressure was reduced by up to 15% compared to the monotonic case in which pressure was increased by applying a constant flow rate. Whilst peak pressure had the primary control on the number of cycles to failure, increasing the minimum pressure in the borehole (thus increasing mean pressure) further reduced breakdown pressure, suggesting that even small pressure fluctuations during hydraulic stimulation may reduce the largest stress drops, and hence the magnitude of induced seismic events. Strain measurements detected accelerating precursory deformation a few cycles prior to failure, hinting at the opportunity for responsive stimulation practices where activity can be monitored in real-time. These novel large-scale, high-resolution experiments were complemented by indirect tensile measurements at a range of strain rates, and by cyclic fatigue Brazilian disc testing at a range of peak loads and cycle amplitudes. These results further highlight the increasing contribution of time-dependent deformation during slower and cyclic loading, resulting in lower peak loads and reducing large magnitude fracturing events. The generated S-N curves demonstrate that weakening by cyclic hydraulic pressurisation mimics relationships defined by conventional fatigue testing of geomaterials. Such experimental constraints will be of great benefit to the development of cyclic stimulation practices for subsurface engineering.

How to cite: Kendrick, J. E., Mouli-Castillo, J., Lamur, A., Fraser-Harris, A., Lightbody, A., Chandler, M., Edlmann, K., McDermott, C., and Shipton, Z.: Constraining the impact of cyclic hydraulic stimulation on granites, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14982, https://doi.org/10.5194/egusphere-egu23-14982, 2023.

During fault evolution different rock types are fractured and sheared within the fault core, producing fault gouges with heterogeneous mineralogical composition. Mineral composition exerts a primary control on fault frictional properties and hence on fault slip behaviour. Understanding the conditions that lead to seismic or aseismic fault slip is of great interest to earthquake hazard assessment both for natural and induced seismicity. Although the effect of single mineral phases is probably the most documented factor in laboratory tests, no clear link has been established to understand how systematic variation of different mineral phases in gouge mixtures influences the macroscopic frictional behaviour.

Here we present an experimental study designed to probe the control of mineral composition on fault friction and stability responses. We selected three representative mineral phases, commonly found in fault zones, that are known to have severely different frictional properties: muscovite (phyllosilicate), quartz (granular silicate) and calcite (granular carbonate). Thirty double direct shear experiments were performed using a biaxial rock deformation apparatus (BRAVA) on powders (with grain sizes < 125 µm) of pure minerals and their mixtures at normal stress of 50 and 100 MPa, at room temperature and water saturation conditions. After an initial sliding of 10 mm at 10 µm/s to develop a steady state shear fabric, slide‐hold‐slide sequences (30-1000 s) and velocity steps (0.3-300 µm/s) were employed to evaluate static healing and frictional stability, respectively.

Our experimental data indicate that the mineralogical composition of fault gouges significantly affects the frictional strength, healing, and stability with a non-trivial pattern. Increasing phyllosilicate (muscovite) content results in a decrease of the frictional strength, from 0.62 for pure calcite and 0.56 for pure quartz down to 0.33 for pure muscovite powders. This effect is more marked in calcite-rich mixtures rather than quartz-rich ones, possibly due to favourable conditions for fluid-assisted pressure-solution at grain contacts. Calcite-muscovite interaction also favours a reduction of frictional healing and a more marked velocity-strengthening behaviour (promoting stable sliding and fault creep) in comparison to quartz-muscovite mixtures.

How to cite: Ruggieri, R., Pozzi, G., and Collettini, C.: Mineralogical control on fault friction and stability: a systematic study on quartz, calcite and muscovite ternary mixtures., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12483, https://doi.org/10.5194/egusphere-egu23-12483, 2023.

Measurements of permeability at high-pressure and high-temperature are critical to model and understand the behaviour and evolution of geothermal systems. To perform such measurements and provide constraints on the permeability of crustal rocks, we designed and tested a new apparatus.

Our high-pressure, high-temperature permeameter consists of three independent parts: the permeant gas circuit, the confining fluid circuit, and the heating element. For each measurement, a cylindrical sample is placed between the up- and downstream platens, into an annular Viton jacket which is secured within the pressure vessel. A confining pressure can be applied to the sample by filling the void space between the vessel and jacket through the inlet with kerosene. The confining pressure can be increased up to 50 MPa using a high-pressure hand pump. The temperature of the system can then be increased from room-temperature to up to 150 °C using a heating mantle wrapped around the pressure vessel and connected to a control box. After the confining pressure and temperature have been applied to the system, the permeability measurement is performed by flowing nitrogen (the permeant gas) through the sample while monitoring the pressure differential between the upstream pressure transducer and atmospheric pressure downstream of the sample at different volumetric flow rates (the steady-state method), measured using the downstream flowmeter.

Using this new experimental apparatus, the permeability of Lanhélin granite (from France) samples were measured. Cylindrical samples were prepared and thermally stressed (heated to 700 °C) to ensure that their permeabilities lie in the range that can be measured in our set-up (> 10^-18 m²). Permeability measurements were then performed under confining pressures of 2, 5, 10, 15, 20, 30, 40, and 50 MPa at room temperature, 50, and 100 °C. Our results provide the evolution of the permeability of microfractured granite in various pressure and temperature conditions, which will serve to inform numerical modelling designed to explore the influence of in-situ conditions on fluid flow within a fractured geothermal reservoir.

How to cite: Carbillet, L., Heap, M., and Baud, P.: The evolution of permeability with pressure and temperature in microfractured granite, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5623, https://doi.org/10.5194/egusphere-egu23-5623, 2023.

The Hengill region is one of the largest areas of high geothermal gradient and subsurface heat flow in Iceland, and hosts two of its largest geothermal power plants: Hellisheiði and Nesjavellir, which have a combined capacity of 423 MWe and 560 MWth. The Hengill region is located in a unique tectonic setting, characterized by the convergence of three plate boundary segments: the oblique-spreading Reykjanes Peninsula (RP), the orthogonal-spreading rift of the Western Volcanic Zone (WVZ) and the transform, South Iceland Seismic Zone (SISZ). Unlike most tectonic triple junctions, which occur on the ocean floor, the Hengill Triple Junction (HTJ) is exposed above sea level, thus providing a unique opportunity to study the interplay between three plate boundary segments and the local deformation processes occurring at their convergence site. Additionally, the injection of fluids due to on-going geothermal operations enhances the natural tendency of the region for seismic activity, and results in a significant level of induced seismic hazard. Because slip on pre-existing faults is triggered when the applied shear stress surpasses the frictional strength of the fault, regions that are naturally subjected to higher shear stresses are more prone to fault re-activation due to fluid re-injection. Therefore, a spatial variation in tectonic stresses may result in varying induced seismicity potential within a region.

The local interplay of the three converging tectonic regimes, and their effect on the stress fields within the triple junction region, has been examined through a combination of regional structural mapping and a numerical model of the plate boundary interactions using the Boundary Element Method (BEM). Large scale structural mapping and analytical models of oblique rifting were used to estimate the degree of rift obliquity for individual fissure swarms. Our results reveal that the transition from the highly oblique rift system of the RP towards the spreading-orthogonal rift of the WVZ is smooth, and manifests as a rotation of the trend of fissures and eruptive ridges, and the strike of normal faults formed in response to the local stress field developed in the HTJ. These results were then correlated with those from the BEM model, which allows us to predict the orientation, relative magnitude, and distribution of stresses within the study area. Finally, the shear stress distribution determined from the BEM model was plotted against the location of both natural and induced seismic events detected in the region over a time span of 26 months, by the COSEISMIQ project (Grigoli et al., 2022). Our results show that seismic events cluster in either at the triple junction or SW of it, within the highly stressed regions of the RP. Furthermore, the seismicity transitions from scattered to non-existent towards the north of the region, where shear stresses also diffuse. The good correlation between the high shear stress regions predicted by the model and distribution of seismicity suggests that this approach may provide a valuable and cost-effective tool for seismic hazard prediction within regions with complex tectonic settings.

How to cite: Stanton-Yonge, A., Mitchell, T., Meredith, P., Gunnarsdóttir, S., Ósk Snæbjörnsdóttir, S., and Hjorleifsdottir, V.: Structural patterns and states of stress at the Hengill Triple Junction, SW Iceland: implications for fluid-injection induced seismic hazard, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7330, https://doi.org/10.5194/egusphere-egu23-7330, 2023.

Changes in stress regimes impact the geometry of fracture networks and affect the porosity and permeability of carbonate reservoirs. This is, predominantly, because of the complexity of the deformation phases, the poor understanding of the mechanical and diagenetic mechanisms that affect apertures, and the difficulty in precisely characterizing aperture distributions in the subsurface. Utilizing outcrop data analysis and displacement-based linear elastic finite element modelling, we study the effect of stress regime change on fracture network permeability. The model is based on fracture networks, specifically fracture sub-structures.

The Latemar, which is primarily affected by subsidence deformation and Alpine compression, is used as an outcrop analogue for isolated (Mesozoic) carbonate formations with fracture-dominated permeability. We apply a novel strategy involving two compressive boundary loading conditions constrained by the study area's NW-SE and N-S stress directions. Stress-dependent heterogeneous apertures and effective permeability were computed by: (i) using the local stress state within the fracture sub-structure and (ii) running a single-phase flow analysis considering the fracture apertures in each fracture sub-structure.

Our results show that the impact of the modelled far-field stresses at: (i) subsidence deformation (first stage loading) from the NW-SE, and (ii) Alpine deformation (second stage loading) from the N-S, increased the overall fracture aperture and permeability. In each case, increasing permeability is associated with open fractures parallel to the orientation of the loading stages and with fracture densities. The anisotropy of permeability is affected by shear dilation and is increased by the density and connectedness of the fracture network. The two far-field stresses simultaneously acting within the selected fracture sub-structure at a different magnitude and orientation do not necessarily cancel out each other in the mechanical deformation modelling. These stresses effect the overall aperture and permeability distributions. These effects, which may be ignored in simpler stress-dependent permeability, can result in significant inaccuracies in permeability estimation, especially in the subsurface carbonate reservoirs.

How to cite: Igbokwe, O. A., Timothy, J., Kumar, A., Yan, X., Mueller, M., Verdecchia, A., Meschke, G., and Immenhauser, A.: Effect of stress regime change on fractured carbonate’s permeability: A case of Latemar carbonate buildup (The Dolomites, Northern Italy) , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9317, https://doi.org/10.5194/egusphere-egu23-9317, 2023.

The study of fault damage zones is key to the understanding of fault-related fluid flow in the upper crust with many applications, including groundwater and hydrocarbon exploration, and underground storage of CO₂ and H. Many studies reveal that a relationship exists between fault damage zone width and net fault displacement. Despite this positive relationship, several factors such as the tectonic setting, the depth of deformation, the deformation mechanisms, and the evolving mechanical properties of fault rocks affect damage zone characteristics (e.g., width, asymmetry, fracture attitude, deformation intensity). Furthermore, recent studies show that the overall along-strike fault geometry may play a pivotal role in controlling damage zone characteristics. In particular, areas such as tip regions, linkage sectors, relay ramps and step-overs can be characterised by fault damage zone parameters markedly different from sectors away from these structural complexities. In this contribution, we present new structural data of fault damage zone parameters acquired along the 8 km long extensional Kornos-Aghios Ioannis Fault (KAIF) on Lemnos Island, North Aegean Sea, Greece. The KAIF deforms lower Miocene effusive and hypabyssal magmatic rocks and middle Eocene to lower Miocene turbidites. Deformed rock volumes along the KAIF are locally strongly altered by hydrothermal fluids (e.g., hydrothermal silicification). We provide a detailed characterization of the KAIF in terms of 2D fault geometry (mapped at 1:1000 scale) and kinematics and we present a characterization of fault damage zone parameters, including frequency and attitude of subsidiary fault-related fractures, in different fault sectors. The acquired data allowed us to define the boundaries of fault damage zones in the different sectors and to discuss the differences in terms of fracture attributes in linking- and tip-damage zones compared to damage zones away from these structural complexities. Our results show that fault damage zones in linkage and tip sectors are wider and that fault-related fractures are more clustered around several subsidiary faults with centimetre- to metre-offsets. Also, secondary fractures in linkage and tip sectors are less systematically oriented, thus increasing fracture network connectivity and, consequently, facilitating fluid mobility in structurally complex fault sectors.

How to cite: Berio, L. R., Balsamo, F., Pizzati, M., Storti, F., Curzi, M., and Viola, G.: Along-strike fault geometry controls damage zone parameters: the case of the Kornos-Aghios Ioannis Extensional Fault (Lemnos Island, NE Greece), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5675, https://doi.org/10.5194/egusphere-egu23-5675, 2023.

A number of six in situ hydraulic fracturing experiments were carried out at the Äspö Hard Rock Laboratory (Sweden) in 2017 in a depth of 410 m. Here we present electric self-potential monitoring during the conventional and the step-wise cyclic injection experiments HF2 and HF3. Electric self-potential data were acquired through a two-sensor array, each including nine measuring probes and one base probe, that were installed at the 410 m and 280 m levels. The experimental borehole F1 is drilled in the direction of S_hmin, perpendicular to the expected fracture plane. The self-potential sensors are installed sub-parallel to S_hmin at level 410 at a distance of 50-75 m to the borehole F1 and sub-perpendicular to S_hmin at level 280 m at a distance of 150-200 m to F1. The self-potential data were measured with a sampling rate of 1 Hz. Here, we propose a 1-D modelling of the streaming potential that approximates the measured self-potential data. These streaming potential gradients ∆V are estimated from the simulated pressure signals and the coupling coefficient.

How to cite: Haaf, N., Guarracino, L., Jougnot, D., and Schill, E.: Electric self-potential monitoring of hydraulic fracturing experiments in the Äspö Hard Rock Laboratory, Sweden., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13820, https://doi.org/10.5194/egusphere-egu23-13820, 2023.

The characterization and modelling of fractured reservoirs of geofluids are becoming increasingly important in the ongoing energy transition and climate crisis. Fractured reservoirs are fundamental for critical applications such as CO₂ sequestration, H₂ and natural gas storage, exploitation of geothermal fluids and hydrothermal ore deposits, and management and safeguard of groundwater resources (deep aquifers are considered more resilient in drought scenarios). In addition, the characterization of fracture networks is relevant in earthquake mechanics, slope stability and engineering geology.

Characterization of natural fracture systems and fracture networks is often based on characterization of outcrop analogues, with measurement of large structural datasets with a combination of field and remote sensing techniques (e.g. Digital Outcrop Models - DOMs), leading to statistical and topological analysis. Numerous studies provide significant amounts of data from a broad variety of methodologies and protocols used in the field. These methodologies aim at characterizing fracture systems by a large number of parameters. Individual “fracture” sets are characterized by genetic features (e.g. joint vs. stylolite), relative chronology, spatial distribution (regular, random, clustered...), density and intensity (e.g. P₂₀ and P₂₁), and by statistical distributions of spacing, orientation, length, height, and aperture (the latter being a dynamical property that varies with fluid pressure and confining stress). Fracture networks composed by several sets are also characterized by topology and connectivity (characterized for instance in terms of fracture terminations or with graphs).

Here we propose a thorough review, supported by rich case studies, of quantitative methods for fracture network characterization and analysis on DOMs. This review aims at determining the most relevant and efficient methods for field and remote-sensing measurement, and best-practice statistical analysis techniques, in order to accurately characterize outcrop analogues that can be used to model fractured reservoirs.

How to cite: Bistacchi, A., Mayolle, S., Casiraghi, S., De Paolo, E., Martinelli, M., Agliardi, F., and La Valle, F.: Quantitative structural analysis of fracture networks in outcrop analogues of fractured reservoirs: a review of measurement methodologies and statistical techniques, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9661, https://doi.org/10.5194/egusphere-egu23-9661, 2023.

Fluid flow within low permeable reservoirs such as carbonates is primarily controlled by faults, fractures and other structural networks which can be defined through properties such as intensity, connectivity and aperture. These properties can vary not only among individual fractures but also between scales which can influence uncertainties within permeability calculations and fluid flow simulations. Therefore, understanding the interactions and variations within these networks is fundamental to deriving properties such as permeability and characterising fluid flow through naturally fractured reservoirs.

Determining fracture network properties of reservoirs can be undertaken using several methods across different scales from both surface and subsurface sources. However, where subsurface data is limited (e.g., within the geothermal reservoirs of Northern Bavaria), outcrop analogues become vital for obtaining the important information required for characterising fracture networks. Outcrops such as quarry sections can be imaged and scanned using both 2D and 3D photogrammetry techniques, from which fault and fracture networks can be detected and analysed. Previous work has presented a method to upscale fracture networks to 2D permeability tensors from outcrop sections through independently assigning properties to individual fractures within the networks. However, upscaling the networks to larger scales can lead to uncertainties due to variations within the modelled fracture networks. It its therefore important to understand the how the permeability tensor varies between scales and dimensions to reduce upscaling uncertainties.

Using examples from multiple outcrops within the Franconian Basin, Germany, we present an improved workflow to derive the tensors between dimensions and an investigation of the relationships among fracture networks at different scales. This will show the effect on permeability within geothermal reservoirs in the region and how to reduce the uncertainty in upscaling outcrops to subsurface reservoir scale.

How to cite: Smith, R., Prabhakaran, R., and Koehn, D.: Investigating scale variations in outcrop derived permeability tensors and the effect on geothermal fluid in upscaling naturally fractured reservoirs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6756, https://doi.org/10.5194/egusphere-egu23-6756, 2023.

Characterizing fluid transport and pore pressure diffusion is key for understanding and monitoring many natural (e.g., seismically active zones and volcanic systems) and engineered environments (e.g., enhanced geothermal reservoirs and CO₂ underground storage). Borehole hydraulic testing allows to infer relevant properties of the probed sub-surface volume, such as, for example, its transmissivity and diffusivity, for assessing the governing flow regime as well as for detecting the presence of hydraulic boundaries. Periodic hydraulic tests (PHT) achieve these objectives using a time-harmonic fluid injection procedure while measuring the fluid pressure response in monitoring boreholes. The relevant information on the pressure diffusion process occurring in the probed formation is retrieved from the phase shifts and amplitude ratios between the injected flow rate and the interval pressure. In general, the interpretation of PHT data relies on the assumption that the pressure diffusion process is uncoupled from the solid deformation of the probed rock volume. We present a poroelastic numerical approach to investigate the role played by hydromechanical coupling (HMC) effects during PHT and to assess whether and to what extent additional mechanical information can be extracted from these tests. We focus on (i) the influence of the borehole wall deformation on the wellbore storage coefficient S_w, which quantifies the difference between the injected flow rates and those actually entering the porous formation; and on (ii) the HMC effects associated with the presence of fractures in the formation. Following the commonly taken approach, we also interpret the synthetic data from the numerical poroelastic approach using the uncoupled diffusion solution. For different rock physical properties, we demonstrate that, in homogeneous formations, the uncoupled diffusion solution reproduces the poroelastic results. In this scenario, neglecting the effect of the deformation of the borehole wall on S_w upon injection can lead to an underestimation of both the transmissivity and diffusivity, which becomes worse for shorter oscillation periods. We also show that the effective values of S_w depend on the shear modulus of the formation and do not change with the oscillatory period. Based on this evidence, we present a methodology to obtain the effective S_w along with the hydraulic properties using observations at various oscillatory periods. Next, we consider formations containing hydraulically open and compliant fractures intersecting the borehole perpendicularly. Here, a single uncoupled diffusion model is not able to fully describe the poroelastic response of the medium at different periods. Furthermore, the presence of fractures significantly affects the effective value of S_w: it increases with respect to the one associated with the intact homogeneous rock, and the HMC effects associated with the compressibility contrast in the formation result in a period dependence of S_w. The characteristic period of the latter is primarily related to the diffusivity and size of the fractures. This result is particularly relevant for the planning and interpretation of monitoring experiments, in which the mechanical properties of the formation are expected to evolve, such as, for example, hydraulic stimulation procedures, seismic and/or volcanic regions, and injection of wastewater and CO₂ for subsurface storage.

How to cite: Barbosa, N., Müller, T., Favino, M., and Holliger, K.: Poroelastic modeling of borehole-based periodic hydraulic tests in non-fractured and fractured porous rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4952, https://doi.org/10.5194/egusphere-egu23-4952, 2023.

Subsurface storage of CO₂ and retrievable green energy (i.e. compressed air and hydrogen) in depleted gas reservoirs, deep saline aquifers and salt caverns, are considered as key transitions within the energy sector to enable many countries to meet their net zero carbon emissions targets. One of the main challenges in underground storage is the presence of potential leakage pathways (such as caprock fractures/faults) which pose a threat to economic feasibility and to the environment. Seeking an efficient technique to remediate leakage pathways is crucial to guarantee sealing efficacy. Microbially induced carbonate precipitation (MICP) is regarded as a promising bio-grouting technique for leakage remediation due to its advantages such as the low water-like viscosity and micron-size microbes, which enables permeation far from the injection point and excellent penetration into fine aperture fractures.

MICP utilizes ureolytic bacteria, commonly Sporosarcina pasteurii (S. pasteurii) to produce highly active urease enzyme, which can catalyze urea hydrolysis and result in the production of carbonate ions. Carbonate precipitation occurs if a calcium source is supplied, which can effectively remediate leakage by filling void space and improve strength by bonding solid grains.

We propose a micro-continuum model which accounts for the coupled hydro-bio-chemical MICP processes in fractured porous rock. In this micro-continuum model, the flow is controlled by the Darcy-Brinkman-Stokes equation over a number of control volumes (i.e. voxels based on X-ray CT scan). The kinetic parameters of urea hydrolysis and calcium carbonate precipitation are calibrated based on batch experiments. The bacteria deposition parameters, which feature the bacteria deposition in fractured rock, are calibrated based on experimentally measured bacteria breakthrough curves in a 1-dimensional column filled with fracture gouge materials. The proposed micro-continuum model can well represent the decreases in porosity and permeability of an artificially-cut anhydrite fracture filled with anhydrite gouges under MICP treatment cycles. Our modelling results also suggest that when the CaCl₂ concentration is equal to or higher than the urea concentration in the injected cementing solution, the rate of microbially induced carbonate precipitation is predominantly limited by the kinetics of urea hydrolysis rather than the kinetics of calcium carbonate precipitation. The proposed micro-continuum model serves as a useful tool for evaluating MICP treatment strategies and may be upscaled to predict and optimise field-scale leakage remediation.

How to cite: Sang, G., Lunn, R., El Mountassir, G., and Minto, J.: Micro-Continuum Modelling of Coupled Hydro-Bio-Chemical MICP Processes in Fractured Rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17201, https://doi.org/10.5194/egusphere-egu23-17201, 2023.