Dynamic behavior of miscible binary fluid mixtures in nanopores: Implications for CO2-enhanced oil flow in shale reservoirs

Abstract

Dynamic behavior of miscible CO2-oil mixtures is the heart of CO2 enhanced oil recovery (EOR) and CO2 storage in shale reservoirs. However, the omnipresent nanopores not only raise the difference in CO2-reduced oil viscosity through thermodynamic effect from that of bulk condition but also add additional kinetic resistance resulting from a pore surface. In this work, a novel thermo-kinetic model is proposed to characterize the miscible mixtures flow in nanopores with involving changes in activation free energy induced by thermodynamic effect and kinetic effect, where the former is correlated with Helmholtz free energy by a modified Soave-Redlich-Kwong equation of state, while the latter is determined by a solid–fluid potential model with assuming a miscible mixture as a single pseudo-fluid. Subsequently, CO2-enhanced oil flow in nanopores and its deviation from bulk results are demonstrated. Moreover, the roles of different microscopic interactions are clarified, and the effects of a pore size, CO2 content as well as the strength of fluid-wall interactions are discussed. It is found that CO2 can simultaneously weaken the fluid–fluid interactions and fluid-wall interactions, jointly leading to a reduction in local fluid viscosity and an enhancement in oil flow. The increase in wall-oil affinity and CO2 content can strengthen the enhancement effect. Moreover, our results indicate that the relationship between enhancement effect and pore size strongly depends on the strength of interfacial interactions. For a nanopore with low wall energy (εs/kb < 10 K), a reduction in the enhancement factor is obtained with a decrease in pore size, while the opposite phenomenon is observed for a pore wall with a strong affinity to the oil. Our study lay a theoretical foundation for the reservoir simulation of CO2-EOR and CO2 storage in shale reservoirs.