Pore-scale evaluation of CO2 miscible displacement in porous rocks induced by convection and diffusion: implications for CO2 geo-sequestration

Abstract CO 2 enhanced oil recovery plays an important role in carbon storage and utilization. However, the incomplete understanding of the underlying microscopic convection–diffusion mechanisms in complex pore structures has constrained the broader industrial application of CO 2 geo-sequestration. This work develops a pore-scale numerical model considering molecular convection–diffusion to investigate CO 2 -oil miscible displacement in two- and three-dimensional porous structures of conglomerate rocks. The effects of CO 2 injection rates and pore structure properties on convection–diffusion are analyzed. By reconstructing the distribution of unexploited pores, the CO 2 sweep efficiency is quantitatively evaluated. Furthermore, a sequestration factor is proposed to evaluate the CO 2 storage capacity during miscible displacement. Convection significantly enhances the CO 2 mass fraction in fractures with high flow rates. Subsequently, CO 2 gradually diffuses into matrix pores without velocity distribution. Both convection and diffusion contribute to improving CO 2 displacement efficiency. Diffusion facilitates the dissolution of CO 2 into oil within small-diameter pores, and convection effectively mobilizes oil in large pore bodies. Developed and homogeneous pore structures enhance CO 2 displacement efficiency, whereas CO 2 flows along the main flow channels in heterogeneous pore structures, resulting in lower displacement efficiency. Diffusion plays a crucial role in CO 2 storage within porous media. At low injection rates, dissolved CO 2 is trapped in poorly connected and blind-end pores. The injection rate is negatively correlated with the sequestration factor.