Pore-scale insights into CO2-water two-phase flow and implications for benefits of geological carbon storage

Abstract

The overall benefits of geological carbon storage (GCS) depend primarily on CO₂ storability and injectability, expressed as saturation and relative permeability, respectively. The effects of GCS schemes on these two properties, the macroscopic response indicators of a two-phase seepage system, are closely related to pore-scale two-phase behaviors. However, the comprehensive effects of capillary number (Ca) and wettability (θ) on saturation and relative permeability are poorly understood. Here we proposed a digital rock physics (DRP) technique workflow for the phase field method and systematically investigated how these effects control two-phase seepage at pore scale through the high-resolution visualization results obtained. We created a Ca-θ phase diagram identified by four pore-scale displacement mechanisms, including finger-like invasion, burst, cooperative filling and coexistence of concave and convex interfaces, to illustrate the comprehensive effects of Ca and θ. We found that the relative permeability of the defending phase (water in this work) is determined by the net effect of the direct driving and viscous coupling effects. We organized comprehensive Ca-θ diagrams and revealed the favorable conditions for CO₂ injectability and storability. Our results demonstrate that GCS schemes, mainly about capillary number and wettability, can significantly influence CO₂ storage performance via the two-phase flow at pore scale, which should be considered carefully. This work provides valuable insights into the selection of an optimal GCS scheme and contributes to an in-depth understanding of multiphase seepage at pore scale.