Coupled supercritical CO2 dissolution and water flow in pore-scale micromodels

Dissolution trapping is one of the most important mechanisms for geological carbon storage (GCS). Recent laboratory and field experiments have shown non-equilibrium dissolution of supercritical CO2 (scCO(2)) and coupled scCO(2) dissolution and water flow, i.e., scCO(2) dissolution at local pores/pore throats creating new water-flow paths, which in turn enhance dissolution by increased advection and interfacial area. However, the impacts of pore-scale characteristics on these coupled processes have not been investigated. In this study, imbibition and dissolution experiments were conducted under 40 degrees C and 9 MPa using a homogeneous/isotropic hexagonal micromodel, two homogeneous elliptical micromodels with low or high anisotropy, and a heterogeneous sandstone-analog micromodel. The four micromodels, initially saturated with deionized (DI)-water, were drained by injecting scCO(2) to establish a stable scCO(2) saturation. DI water was then injected at different rates with logCa (the capillary number) ranging from -6.56 to -4.34. Results show that bypass of scCO(2) by displacing water is the dominant mechanism contributing to the residual CO2 trapping, triggered by heterogeneity in pore characteristics or pore-scale scCO(2)-water distribution. Bypass can be enhanced by pore heterogeneity or reduced by increasing transverse permeability, resulting in relatively low (<2% of CO2 solubility) or high (9-13% of CO2 solubility) dissolved CO2 concentration in displacing water. The overall dissolution of residual scCO(2) increases with decreasing Ca, and approaches to their solubility at low Ca value with sufficient residence time. This main trend is similar to a capillary desaturation curve that represents the relationship between the residual saturation and Ca. Spatially, dissolution initiates along the boundary of bypassed scCO(2) cluster(s) in a non-equilibrium manner, and the coupling of water flow and dissolution occurs which fragments the bypassed scCO(2) clusters and enhance scCO(2) dissolution.