The "Half-Life" of Injected CO2: The Immobilization of Injected CO2 by Capillary Trapping

Leakage of injected CO2 is a major concern in saline aquifer storage of anthropogenic CO2. Therefore the time over which CO2 is still available for leakage is an important timescale in aquifer CO2 storage. Capillary trapping in the wake of the CO2 plume is the major mechanism reducing the amount of CO2 that is available for leakage. We use a semi analytical model to assess the time scales over which the CO2 is immobilized. Assuming homogeneous system and sharp interface the problem reduces to a non-linear heat equation with a discontinuous coefficient. A scaling law with an non-rational exponent governs the reduction of the mobile CO2. The anomalous exponent is determined by a non-linear eigenvalue problem. The analysis of Kochina et al.. (1983) has been extended to a sloping aquifer. The eigenvalue problem has been solved numerically. And results are compared to numerical solutions of the governing one dimensional heat equation, as well as results obtained from full reservoir simulations. The time necessary to reduce the mobile CO2 by half decreases with increasing permeability, density difference and dip of the aquifer. Results are uesful in assesing suitability of saline aquifers and designing the monitoring strategy.