The effect of CO2-enriched brine injection on the mechanical properties of calcite bearing sandstone

The mechanical and fluid-flow response of subsurface geological reservoirs due to injection of CO2 is of critical importance for the safe management and storage of anthropogenic carbon emissions. Although the time-lapse seismic method has proven to be an effective tool to remotely monitor changes in underground fluid saturations, variations in reservoir properties caused by geochemical interactions can also influence the seismic response. This can lead to ambiguity and uncertainty in monitoring the movement of injected CO2 and hence determination of reservoir seal integrity. Geochemical interactions can also modify the mechanical strength of the reservoir and therefore threaten its integrity. We conducted experiments to assess how the velocity and rock strength of a calcite-bearing sandstone are affected by flooding with CO2 saturated brine. The results indicate that both seismic velocity and rock strength are significantly reduced due to minor calcite dissolution. The implications at the reservoir scale for CO2 storage are twofold. Firstly, modifications in velocity can complicate seismic monitoring operations and lead to interpretation errors. This can be accounted for if shear wave velocity variations are used to detect fluid-rock interactions. Secondly, reduction in rock strength, caused by calcite dissolution, can threaten reservoir and wellbore integrity under stress conditions typically found in potential carbon repositories.