Abstract
After injection of CO2 into the subsurface formation, various storage mechanisms help immobilize CO2 in the porous medium. Injection strategies that promote the buoyant movement of CO2 after injection can increase immobilization by the mechanisms of dissolution and residual phase trapping. In a heterogeneous storage formation where capillary entry pressure of the rock is correlated with other petrophysical properties, numerous local capillary barriers exist and can trap rising CO2 below them. In this work, we study the effect of these barriers on CO2 leakage from the storage formation. We first introduce a no-risk-of-leakage scenario for storage, which sets an upper bound on the amount of CO2 that can be stored without contacting the overlying seal. Accounting for capillary heterogeneity reduces this upper bound. In practice operators are likely to seek to store more CO2 than the no-risk-of-leakage bound in any given reservoir. Thus we also examine the form of the gas cap established by the rising CO2 plume, and we simulate leakage from this gas cap through the top seal. Our leakage scenario is simulated in different cases with homogeneous and heterogeneous capillary pressure field, and CO2 trapping is quantified based on the results of the leakage. Capillary heterogeneity is introduced via Leverett scaling group. A new parameter called security index is also defined to quantify the risk of leakage. Finally, the statistics of the local capillary barriers are used to find a probability distribution of leakage amounts from the storage formation.
We conclude that ignoring heterogeneity gives the worst case estimate of the risk. Local capillary trapping reduces potential leakage through failed seal, but a range of CO2 leakage amounts can occur depending on heterogeneity and location of leak. The thickness of the sealing layer and the presence of an active open aquifer connected to the leak do not change the leakage volume of CO2. A probabilistic approach based on statistics of local capillary barriers can be used to analyze the risk associated with leakage.