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Carbon capture and storage in geological reservoirs is essential to mitigate carbon dioxide emissions, but is only effective if long-term storage is ensured and no leakage occurs, for example through unknown faults in the caprock that seals the reservoir. To test how the integrity of a caprock can be monitored non-invasively, we conducted an injection experiment directly into a faulted caprock analog at the Mont Terri Rock Laboratory. We monitored the experiment using active seismic techniques, along with strain, fluid pressure, and microseismicity measurements. Pulse injection with pressures of up to 4.8 MPa caused a perturbation of effective stress, which we observed through a reduction of seismic P-wave velocities by around 1 %. This perturbation was not sensed by means of microseismicity, even though a sudden pressure drop indicated that the injection caused the opening of some fractures within the fault zone. Substantial rock deformation was also only detected at the injection borehole but not in surrounding monitoring boreholes. Thus, with seismic velocity monitoring, processes can be detected within a faulted caprock for which other methods may be blind. This demonstrates the value of seismic velocity monitoring as a tool to complement a monitoring system., Geophysics, 87 (4), ISSN:0016-8033, ISSN:1942-2156

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., Solid Earth, 9 (3), ISSN:1869-9510, ISSN:1869-9529