Wettability-aware phase-field micro-continuum modeling of two-phase carbonate acidizing with residual CO₂

Following pre-CO₂ fracturing of carbonate reservoirs, residual CO₂ remaining in and remobilized from the wellbore-fracture system may introduce multiphase-flow and interfacial effects that are often neglected in conventional acidizing models. In this study, we develop a pore-scale micro-continuum framework that couples the Darcy-Brinkman-Stokes equations with a wettability-aware phase-field formulation. A key feature of the model is an implicit wetting scheme that allows the contact angle to evolve consistently with the receding fluid–solid interface during mineral dissolution, thereby addressing the moving contact-line problem. The simulations show that the CO₂ phase fundamentally reshapes acid migration and dissolution patterns through two competing mechanisms: flow diversion, whereby CO₂ clusters redirect acid into non-preferential pores, and bubble shielding, whereby adhered CO₂ shields reactive mineral surfaces. As a result, CO₂-bearing systems exhibit compact dissolution patterns and fluctuating permeability evolution driven by the competition between capillary trapping and pore enlargement. Surface wettability is further identified as a primary control factor: strongly water-wet conditions facilitate CO₂ mobilization, whereas wettability reversal promotes inlet blockage. In addition, parametric analysis reveals a nonlinear trade-off between acid concentration and injection rate. Specifically, increasing acid concentration under high-velocity injection aggravates live-acid bypass and reduces acid-utilization efficiency. These results provide mechanistic insight into gas-liquid-rock interactions and offer a physically grounded basis for optimizing acidizing strategies in carbonate reservoirs after pre-CO₂ fracturing.