Influence of interphase mass transfer and diffusion processes on CO₂-brine displacement in saline aquifers

The geological storage of CO₂ in saline aquifers involves various geophysical and geochemical mechanisms, in which different mechanisms play different roles at different time scales and different stages as well. At the initial stage during CO₂ continuously injected into saline aquifers, the non-equilibrium mass transfer process is one of the main factors affecting the interfacial evolution of two-phase flow. In this work, a model that couples the two-phase displacement process in saline aquifers with single-phase dissolution-diffusion is established for the initial stage of CO₂ injection, which features that the non-equilibrium mass transfer method is used to describe the interfacial evolution during the two-phase flow. The model is numerically solved and verified. The influences of mass transfer and diffusion on the distributions of CO₂ plume and CO₂ concentration in the wetting phase are investigated. In the initial stage of CO₂ injection, the Stanton number (St), representing the ratio between mass transfer process and convection process, significantly affects both the non-wetting phase at the leading edge and the CO₂ concentration distribution in the wetting phase. The Peclet number (Pe), representing the ratio between convection process and diffusion process, is a secondary factor and has limited effects on the non-wetting phase saturation and wetting phase CO₂ concentration distribution. At the initial stage, the dissolution of CO₂ during injection into saline aquifers is dominated by a large St or small Pe. The proposed model provides a useful tool and insight in analyzing the interfacial evolutions of the CO₂-brine displacement process when interfacial mass transfer is considered.