Mesoscale Oil Migration Mechanism in Fracture-Cavity Carbonate Reservoirs Based on 3D Digital Rock: Synergistic Control Effects of Temperature, Pressure, and Porous Structure

Summary Fracture-cavity carbonate reservoirs are key targets for ultradeep oil exploration but feature complex structures and highly heterogeneous fluid migration. Fractures and dissolution cavities exert cascading transport effects that significantly influence hydrocarbon movement. In this study, we develop a mesoscale phase-field model based on 3D digital rock to simulate two-phase flow and systematically evaluate the coupled effects of structure, pressure, and temperature on oil migration and accumulation. Key findings include the following: (1) Fluid pressure distribution and pore/fracture connectivity critically affect hydrocarbon migration efficiency. In fractures, pressure decreases linearly along the flow direction, enabling efficient oil transport. In contrast, narrow throats in porous media cause exponential pressure drops, reducing flow velocity and injection efficiency. (2) Increasing pressure gradients activate more reservoir space, but the effect diminishes with scale. In a fractured and porous-type model, increasing the pressure differential from 50 Pa to 200 Pa raises average flow velocity by 194.8% in fractures and 118.1% in pores. (3) Temperature rise has a greater impact in the porous domain. From 80°C to 140°C, average flow velocity and oil phase volume fraction in pores increase by 61.3% and 99.5%, respectively, while in fractures, velocity rises by only 15.2% with minimal change in saturation. These results reveal the nonlinear, synergistic influence of porous structure, pressure, and temperature on oil migration in fracture-cavity systems, providing theoretical insights to support the efficient development of ultradeep carbonate reservoirs.