Pore-scale relative permeability and saturation analysis under wide-ranging injection velocity and wettability during primary CO2 injection for geological carbon sequestration

We performed two-dimensional (2D) pore-scale simulations of primary CO2 injection using a weakly compressible scheme for geological carbon sequestration (GCS) applications. The aim was to analyze pore-scale relative permeability and saturation of CO2 under wide-ranging injection velocities and wettabilities. The results show that saturation is highest for viscous fingering, lowest for crossover (- 5.82 < logCa < - 4.86; theta < 60 degrees), and remains high in the capillary fingering regime even though the relative permeability of CO2 is minimum. This trend occurs because saturation is influenced not only by the value of relative permeability but also by the frequency of relative permeability fluctuations. At a low injection velocity and contact angle, frequent permeability fluctuations due to Haines jumps result in high saturation despite the low relative permeability. At intermediate injection velocity and low contact angle, both the relative permeability and its fluctuations are moderate, leading to lower CO2 saturation. The present work bridges the understanding of displacement-front advancement at the pore-network scale with relative permeability, which links the pore-scale meniscus dynamics with the large-scale Darcy-flow parameters. As the CO2 flows away from the injection site in large-scale GCS applications, the displacement pattern exhibits crossover regime, resulting in minimal displacement efficiency. In a strongly wetting porous medium, this condition is severe because crossover regime spans a wide range of capillary numbers.