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

Abstract 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 CO 2 (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°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 logC a (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 CO 2 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 solubility) or high (9 to 13% of CO 2 solubility) dissolved CO 2 concentration in displacing water. The overall dissolution of residual scCO 2 increases with decreasing C a , and approaches to their solubility at low C a value with sufficient residence time. This main trend is similar to a capillary desaturation curve that represents the relationship between the residual saturation and C a . 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.