Effect of wettability alteration on long-term behavior of fluids in subsurface

Wettability is an important factor affecting fluid behavior in the subsurface, including oil, gas, and supercritical CO ₂ in deep geological reservoirs. For example, CO ₂ is generally assumed to behave as a non-wetting fluid, which favors safe storage. However, because of chemical heterogeneity of the reservoirs, mixed wettability conditions can exist. Furthermore, recent experiments suggest that with time, the wettability of super-critical CO ₂ may change from non-wetting to partially wetting due to changes in electrostatic interactions. These changes are caused by chemical reactions between dissolved CO ₂ and its environment. To date, the effect of wettability alteration and mixed wettability on the long-term fate of injected CO ₂ has not well been studied. Here, we use the multiphase pairwise force smoothed particle hydrodynamics model to study complex pore-scale processes involved in geological CO ₂ sequestration, including the effect of spatial and temporal wettability variations on long-term distribution of CO ₂ in porous media. Results reveal that in the absence of dissolution of supercritical CO ₂ and precipitation of carbonate minerals (mineral trapping), the amount of trapped supercritical CO ₂ significantly decreases as the wettability of the porous media changes from brine-wet to partial-wet or CO ₂-wet.