Pore-Scale Simulations of Residual Trapping in Homogeneous and Heterogeneous Porous Media

Abstract

Summary Trapping of the non-wetting carbon dioxide (CO₂) phase in interstitial rock pore spaces by capillary forces (residual trapping) is critical to CO₂ storage operations and is strongly influenced by the structural heterogeneity and wettability of the reservoir rock. Whilst much time and effort has been devoted to studying residual trapping in water-wet systems, very few studies have focused on residual trapping under intermediate-conditions. This work provides detailed insight regarding fluid displacement process under such conditions. Pore-scale, immiscible two-phase flow simulations were conducted using the Finite Volume discretization method (FVM) in OpenFOAM® to investigate the effect of structural heterogeneity on residual trapping in porous media. Two pore-network patterns were implored in the simulations; a homogeneous pore-network analogous to that of a reservoir rock known as Oolitic limestone and a heterogeneous pore-network pattern similar to that of Berea sandstone. Simulation results showed that homogeneity in porous structure results in cooperative pore-filling mechanisms dominating over snap-off mechanisms, encouraging compact, stable front invasion and high fluid sweep efficiency. Higher pore to throat size aspect ratios were found to promote snap-off mechanisms which in-turn results in enhanced residual trapping. The presence of dead-end type pores was also found to promote residual trapping.