Microstructural Dynamics of CO2-Water Two-Phase Flow in a Spatially Heterogeneous Porous Medium: A Numerical Study

Abstract

Understanding and accurately characterizing the pore-scale distribution and flow behavior of CO2 is fundamental to assessing storage efficiency and leakage risks in geological CO2 sequestration. This study aims to elucidate the mechanisms governing CO2 invasion and two-phase displacement under varying injection rates and rock wettability in heterogeneous porous media. A heterogeneous pore-scale model was developed from three representative casting thin-section images, and a phase-field approach was employed to systematically investigate the effects of injection rate and wettability on CO2 distribution, local capillary breakthrough pressure, effective permeability, and displacement patterns. The results demonstrate that CO2 invasion is more extensive at lower injection rates within the transition zone from higher- to lower-porosity regions, whereas water reoccupation of pore space is more pronounced in the lower-porosity regions. Under different wettability conditions, increasing the injection rate reduced the residual water saturation, increasing the final CO2 saturation by 27.98% and 33.79%, respectively. Meanwhile, the effective permeability increased from 7.36 × 10−11 to 1.48 × 10−10 m2 at θ = 30°, and from 1.35 × 10−10 to 1.77 × 10−10 m2 at θ = 60°. Although higher injection rates and weaker water-wetness increase CO2 saturation and effective permeability, they concurrently reduce local capillary breakthrough pressure. These findings demonstrate that an appropriate balance between efficient CO2 injection and secure containment requires careful optimization of both storage-site selection and injection strategy.