Time‐lapse monitoring carbon sequestrated brine aquifers‐ a feasibility study

Abstract

Substantial research efforts are now underway on injecting (sequestrating) carbon dioxide (CO2) into deep saline aquifers. These sequestration efforts require remote monitoring using available geophysical tools to ensure that the sequestrated CO2 is in place and does not disturb the geological integrity of the surrounding rocks. Since seismic method is the only accepted geophysical tool that can potentially image detailed subsurface information to large depths, here we develop a monitoring strategy using seismic data alone. Fluid substitution at different saturations of CO2 in a brine filled aquifer and comparing its elastic properties with the original indicates that the formation density will play the key role in successful monitoring of carbon-sequestrated aquifers. As multicomponent seismic data are more sensitive to subsurface density variations than vertical (P-wave) component data, we believe that multicomponent seismic data are necessary for obtaining an accurate subsurface presequestration model. Multicomponent data are however more expensive than conventional (P-wave) data. Therefore, acquiring multicomponent data both for baseline and for successive monitoring surveys is not cost-effective. Since above-normal pore pressure due to sequestration may fracture the overlying formations, we investigate if microseismic events generated from these fractures could be used for monitoring. Inducing microseismic events with different fault-plane source mechanisms and computing passive seismic responses from them, we find that these computed responses are sensitive to the fracture fault plane geometry, and passive seismic data could be a potential monitoring tool. We conclude that if multicomponent seismic data are acquired prior to sequestration as a baseline survey and inverted for an accurate presequestration elastic earth model, we can then use passive seismic data for subsequent monitoring. This strategy, in turn, may provide a cost-effective way to monitor carbon sequestrated deep saline aquifers.