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

Abstract

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.