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Abstract. The success of geological carbon storage depends on the assurance of permanent containment for injected carbon dioxide (CO2) in the storage formation at depth. One of the critical elements of the safekeeping of CO2 is the sealing capacity of the caprock overlying the storage formation despite faults and/or fractures, which may occur in it. In this work, we present an ongoing injection experiment performed in a fault hosted in clay at the Mont Terri underground rock laboratory (NW Switzerland). The experiment aims to improve our understanding of the main physical and chemical mechanisms controlling (i) the migration of CO2 through a fault damage zone, (ii) the interaction of the CO2 with the neighboring intact rock, and (iii) the impact of the injection on the transmissivity in the fault. To this end, we inject CO2-saturated saline water in the top of a 3 m thick fault in the Opalinus Clay, a clay formation that is a good analog of common caprock for CO2 storage at depth. The mobility of the CO2 within the fault is studied at the decameter scale by using a comprehensive monitoring system. Our experiment aims to close the knowledge gap between laboratory and reservoir scales. Therefore, an important aspect of the experiment is the decameter scale and the prolonged duration of observations over many months. We collect observations and data from a wide range of monitoring systems, such as a seismic network, pressure temperature and electrical conductivity sensors, fiber optics, extensometers, and an in situ mass spectrometer for dissolved gas monitoring. The observations are complemented by laboratory data on collected fluids and rock samples. Here we show the details of the experimental concept and installed instrumentation, as well as the first results of the preliminary characterization. An analysis of borehole logging allows for identifying potential hydraulic transmissive structures within the fault zone. A preliminary analysis of the injection tests helped estimate the transmissivity of such structures within the fault zone and the pressure required to mechanically open such features. The preliminary tests did not record any induced microseismic events. Active seismic tomography enabled sharp imaging the fault zone.

Abstract. This study characterizes the elastic and fluid flow properties systematically across a ductile–brittle fault zone in crystalline rock at the Grimsel Test Site underground research laboratory. Anisotropic seismic velocities and permeability measured every 0.1 m in the 0.7 m across the transition zone from the host Grimsel granodiorite to the mylonitic core show that foliation-parallel P- and S-wave velocities systematically increase from the host rock towards the mylonitic core, while permeability is reduced nearest to the mylonitic core. The results suggest that although brittle deformation has persisted in the recent evolution, antecedent ductile fabric continues to control the matrix elastic and fluid flow properties outside the mylonitic core. The juxtaposition of the ductile strain zone next to the brittle zone, which is bounded inside the two mylonitic cores, causes a significant elastic, mechanical, and fluid flow heterogeneity, which has important implications for crustal deformation and fluid flow and for the exploitation and use of geothermal energy and geologic waste storage. The results illustrate how physical characteristics of faults in crystalline rocks change in fault zones during the ductile to brittle transitions.

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Abstract. This study characterizes the elastic and fluid flow properties systematically across a ductile-brittle fault zone in crystalline rock at the Grimsel Test Site underground research laboratory. Anisotropic seismic velocities and permeability measured every 0.1 m in the 0.7 m across the transition zone from the host Grimsel granodiorite to the fault core show that foliation-parallel p-and s-wave velocities systematically increase from the host rock towards the fault core, while permeability is reduced nearest to the fault core. The results suggest that although brittle deformation has persisted in the recent evolution, 5 antecedent ductile fabric continues to control the matrix elastic and fluid flow properties outside the fault core. The juxtaposition of the ductile strain zone next to the brittle zone, which is bounded inside the two mylonitic fault cores, causes a significant elastic, mechanical, and fluid flow heterogeneity, which has important implications for crustal deformation and fluid flow, and exploitation and use of geothermal energy and geologic waste storage. The results illustrate how physical characteristics of faults in crystalline rocks change in fault zones during the ductile to brittle transitions.

Stress‐induced borehole deformation analysis in the Collisional Orogeny in the Scandinavian Caledonide deep scientific borehole establishes in situ stress orientation in a poorly characterized region in central Sweden. Two acoustic televiewer logging campaigns, with more than 1 year between campaigns, provide detailed images along the full length of the 2.5 km deep borehole for breakout, drilling‐induced tensile fracture (DITF), and natural occurring structural analysis. Borehole breakouts occur in 13 distinct zones along total length of 22 m, indicating an average maximum horizontal stress, SHmax, orientation of 127° ± 12°. Infrequent DITFs are constrained within one zone from 786 to 787 m depth (SHmax orientation: 121° ± 07°). These SHmax orientations are in agreement with the general trend in Scandinavia and are in accordance with many mechanisms that generate crustal stress (e.g., ridge push, topographic loading, and mantel driven stresses). The unique acquisition of image logs in two successions allows for analysis of time‐dependent borehole deformation, indicating that six breakout zones have crept, both along the borehole axis and radially around the borehole. Strong dynamic moduli measured on core samples and an inferred weak in situ stress anisotropy inhibit the formation of breakouts and DITFs. Natural fracture orientation below 800 m is congruent to extensional or hybrid brittle shear failure along the same trend as the current SHmax. Analysis of foliation in the image logs reinforces the interpretation that the discontinuous seismic reflectors with fluctuating dip observed in seismic profiles are due to recumbent folding and boudinage.

The shearing of fractures can be a significant source of permeability change by altering the distribution of void space within the fracture itself. Common methods to estimate the effects of shearing on properties, such as aperture, roughness, and connectivity, are incapable of providing these observations in situ. Laboratory protocols are needed that enable measurements of the spatial structure of the fracture aperture field in the medium, noninvasively. Here, we investigate changes in rough-walled Brazilian-induced tensile fracture aperture distribution with progressive shear displacement in Westerly granite and Carrara marble using a novel X-ray transparent core holder. The so-called calibration-free missing attenuation method is applied to reconstruct highly resolved (submillimeter) fracture aperture maps as a function of displacement (0 to 5.75 mm) in induced fractures. We observe that shearing increases the core-averaged fracture aperture and significantly broadens the distribution of local values, mostly toward higher apertures. These effects are particularly strong in Westerly granite and may be the result of the higher initial roughness of its fracture surfaces. Also, while the correlation length of the aperture field increases in both parallel and perpendicular directions, significant anisotropy is developed in both samples with the progression of shearing. The results on Westerly granite provide a direct indication that fracture aperture remains largely unaffected until 1 mm of displacement is achieved, which is important when estimating permeability enhancement due to natural and induced shear displacement in faults.

The transport of chemically reactive fluids through fractured clay‐rich rocks is fundamental to many subsurface engineering technologies. Here, we present results of direct‐shear laboratory experiments with simultaneous imaging by X‐ray Computed Tomography in Opalinus claystone with subsequent fluid injection to unravel the interplay between mechanical fracture deformation, fluid sorption, and flow. Under constant radial stress (σc = 1.5 MPa), the average mechanical aperture trued¯CT increases with shear displacement. Upon brine injection, trued¯CT is reduced by 40% relative to initial conditions (trued¯CT0=140−250 μm) and fluid‐sorption induces a divergent displacement of the two sample halves (Δh = ±50 − 170 μm) quantified by digital image correlation. None of these changes are observed in a control experiment with decane, indicating that creep is subordinate to swelling in sealing the fracture. Swelling‐induced changes in permeability within the fracture are heterogeneous and largely affect the fracture flow field, as computed using numerical simulations.