Evaluating the impacts of fine-scale capillary heterogeneity and wettability on the behavior of buoyancy-driven CO2 migration and trapping mechanisms

One of the primary concerns in CO2 sequestration is long-term immobilization of CO2. There are several storage mechanisms to immobilize CO2 in a porous medium: structural, dissolution, residual, and mineral trapping. Storage analysis with fine-scale heterogeneity requires the investigation of a newly suggested mechanism: capillary barrier trapping. This study covers a quantitative assessment of the CO2 trapping capacity with residual, dissolution, and capillary barrier trapping mechanisms in heterogeneous formations. The effects of heterogeneity on CO2 trapping capacity are studied under different wettability scenarios. The Leverett J-function is applied so that every grid block has a different drainage capillary pressure curve, which is physically consistent with its heterogeneous properties and wettability. Based on contact angle, residual saturation and relative permeability curves are modeled. Different heterogeneity and wettability causes considerable variations in leakage of CO2, ranging from 6 to 38 %. The lateral correlation length of permeability affects the spatial distribution of CO2. As the permeability variation increases, the total amount of trapped CO2 increases significantly indicating a reduced potential risk of leakage. Although the residual and dissolution trappings decreases as the wetting condition changes from strongly water-wet to intermediate gas-wet, the amount of CO2 trapped by the capillary barriers increases considerably, and the mobile phase saturation of the CO2 is reduced. The results of this study have emphasized the importance of capillary barrier trapping, which takes a large portion of the immobilized CO2 phase, for stable underground storage.