Carbon capture and storage in geologic reservoirs is essential to mitigate carbon dioxide emissions, but it is only effective if long-term storage is ensured and no leakage occurs, for exam-ple, through unknown faults in the caprock that seals the reser-voir. To test how the integrity of a caprock can be monitored noninvasively, the authors conducted an injection experiment directly into a faulted caprock analog at the Mont Terri Rock Laboratory. The authors monitored the experiment using active seismic techniques, along with strain, fluid pressure, and micro -seismicity 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 approximately 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 frac-tures within the fault zone. Substantial rock deformation also was only detected at the injection borehole but not in surround-ing monitoring boreholes. Thus, with seismic velocity moni-toring, 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.
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