Multiscale experimental-numerical investigation of CO2 transport and phase evolution in carbonate reservoirs

Geochemical intensifications coupled with multiphase seepage in highly heterogeneous porous media are the main factors for CO₂ transport and phase evolution in carbonate reservoirs. This study establishes a mechanistic model incorporating geochemical reactions and fracture-cavern media to examine how these factors influence CO₂ transport and phase evolution. Intensified chemical reactions in carbonate reservoir promote CO₂ dissolution in aqueous and particularly in oil phases, contributing to enhanced oil recovery to some extent. Fractures parallel to the connection line between injection and production wells serve as high-permeability channels along which fluids preferentially flow, reducing the sweep efficiency of CO₂. In contrast, the vertical fracture is beneficial to lateral fluid flow. High-velocity fluid and high reactive specific surface area in fractures induce more prominently geochemical reactions. However, mineral chemical reactions occur on the fracture surfaces, resulting in a fracture plugging effect during the CO₂ sequestration phase. Mineral precipitation forms a physical barrier on the fracture surfaces, leading to hindered fluid migration and restricted reaction areas. For the fracture-cavern media, timely repositioning of CO₂ injection wells can significantly enhance oil recovery efficiency. Injection rate is a key synergistic parameter for storage and recovery in fracture-cavern carbonate reservoir. Dolomitization (CaMg(CO₃)₂) is the primary mechanism facilitating mineralization in carbonate reservoirs, which not only enhances permeability but also enables greater CO₂ mineralization compared to calcite (CaCO₃) precipitation. These findings demonstrate the potential of geochemical intensification to enhance CO₂ storage and utilization efficiency.