Pore-scale investigation of water-CO2-oil flow in shale fractures for enhanced displacement efficiency and CO2 sequestration

Abstract

CO₂ geological sequestration plays an important role in energy and environmental sustainability. However, CO₂ channeling through fracture networks in shale reservoirs reduces sequestration efficiency. Injecting CO₂ followed by water flooding, driven by capillary forces, can suppress CO₂ channeling and enhance sequestration. This work establishes pore-scale models for CO₂-oil and water-CO₂-oil flows within three-dimensional shale fractures to explain this underlying mechanism. The volume of fluid method is employed to integrate the immiscible flow between water and a mixture fluid comprising CO₂ and oil, wherein the interaction between CO₂ and oil remains miscible and is governed by molecular convection-diffusion. The effects of contact angle, injection rate, and CO₂ volume on displacement are analyzed. The results show that high injection rates of CO₂ enhance the mass transfer between CO₂ and oil components, with the injection rate positively correlating with displacement efficiency. Water flooding following CO₂ injection suppresses CO₂ channeling attributable to capillary effects, resulting in an approximately 20 % enhancement in oil recovery compared to CO₂ flooding. Although a substantial volume of CO₂ reduces displacement pressure, it leads to a premature breakthrough. An increase in contact angle results in a large amount of mixture fluid being trapped in blind-end pores, corresponding to unsatisfactory displacement effects. Nonetheless, an increased water injection rate augments the contact between the water and the mixture fluid, facilitating CO₂ dissolution. This results in a gradual decline in outlet mass flow and enhancements in oil recovery and CO₂ sequestration efficiency.