Post-Injection Tracer Test to Constrain CO2 Residual Trapping and Plume Evolution

CO₂ residual trapping, post-injection plume extent, and time for plume stabilization of CO₂ geological storage highly depend on the hysteresis process which is the discrepancy between drainage and imbibition processes. CO₂ flow in the injection zone during the injection period is mainly controlled by the drainage process where the non-wetting gaseous phase replaces the wetting aqueous phase. However, during the post-injection period the viscous forces diminish allowing for imbibition process to control the CO₂-rich phase distribution. The post-injection plume extent and the amount of trapped CO₂ via residual trapping will then depend on both the imbibition and drainage processes. Therefore, conditioning the reservoir model using data collected over the injection period is insufficient in constraining the imbibition curve. Long-term data collection over the post-injection period may be required to determine the residually trapped CO₂ and to predict the CO₂ plume evolution and stabilization. Alternatively, a tracer test can be used to obtain such information in a timely manner. In this work, we introduce a tracer test at the end of the injection period to obtain information on the residual trapping parameters including imbibition/drainage discrepancy (hysteresis) and critical CO2-rich phase saturation. The sensitivity of the proposed tracer test to residual trapping parameters is evaluated based on the tracers’ peak times and concentrations at the injection well (which serves as observation well during post-injection period). The effect of residual trapping on the plume evolution is also studied considering reservoir properties representative of Citronelle CO2 injection project.