Active-seismic monitoring of reservoir pressure changes in an analog reservoir

ABSTRACT Geophysical monitoring of pore pressure within geologic carbon storage (GCS) reservoirs is critical for understanding reservoir containment and caprock integrity. Time-lapse seismic methods are useful for probing reservoir state at locations away from instrumented wells. Highly sensitive and stable time-lapse seismic methods are required for monitoring subtle changes induced by reservoir pressure. To test the limits of time-lapse seismic sensitivity to changes in pore pressure, we used continuous active-source seismic monitoring (CASSM) to monitor traveltime changes caused by pressure perturbations in a laboratory-scale analog reservoir. We referred to this analog reservoir as SMARTT (seismic monitoring of an analog reservoir testing tank), which is a highly instrumented tank filled with sand and bentonite clay to emulate a reservoir, caprock, and aquifer. We injected water directly into the reservoir to mimic the effects of increased pore pressure caused by far-field CO2 injection. We conducted a total of five injections over the course of two days with shut-in periods in between. CASSM successfully acquired data throughout the injections at high sensitivities, such that background changes in P-wave arrival times had an average standard deviation of 82 ns. CASSM was able to capture, at high correlations, changes in P-wave arrival (Δt) caused by changes in reservoir pore pressure (ΔPp). Using a reservoir simulator, we inverted rock property values for the reservoir and caprock layers by modeling the reservoir pressures for the entire experiment. Modeling results showed that caprock permeability evolved during the experiment. We used granular contact theory models to show that these Δt were a result of ΔPp. We used the contact theory models to upscale the stress state and pore-pressure perturbations that would be appropriate for commercial GCS sites to show that pore-pressure monitoring at this scale is feasible. These results suggested that time-lapse seismic methods, such as CASSM, can be used for probing changes in sealing integrity.