Pore-Scale Intermittent Flow and Its Impact on Two-Phase Fluid Distribution in Porous Media

Abstract

Understanding two-phase flow in porous media is essential for optimizing subsurface storage efficiency. Pore-scale flow properties significantly influence macroscopic plume migration behavior and trapping performance. However, the inherent complexity of porous media impedes real-time tracking of pore-scale flow dynamics, leaving the mechanisms governing CO2-brine steady-state flow largely unexplored. In this study, CO2-brine co-injection experiments were conducted to explore fluid flow and distribution at the pore scale using X-ray computed tomography (X-ray CT) imaging. The results reveal that both the nonwetting and wetting phases, as well as the intermittent flow with CT grayscale values between those of nonwetting and wetting, are strongly affected by the capillary number. The nonwetting phase primarily occupies larger pores, while the wetting phase exhibits a bimodal distribution across small and big pores. This bimodal distribution is attributed to the core's heterogeneity and the presence of water layers along the pore walls, which also provides a unique insight into water layer identification at the pore scale. Additionally, the experiments reveal a distinct phenomenon where the morphology of the nonwetting phase transitions from clusters to singlets and then to ganglia as nonwetting phase capillary numbers vary. This transition highlights the role of the intermittent flow in modifying nonwetting phase morphology, leading to disconnections and reconnections that alter connectivity and relative permeability.