Pore-scale investigation of interfacial mass transfer and asphaltene evolution during cyclic CO2–crude oil interactions

Cyclic CO 2 huff-and-puff involves transient interfacial mass transfer, phase behavior, and multiphase transport in fracture-matrix porous media, yet the pore-scale mechanisms controlling these coupled processes remain insufficiently resolved. In this study, multi-cycle CO 2 huff-and-puff experiments were conducted using a high-temperature, high-pressure microfluidic visualization platform with fracture-matrix micromodels reconstructed from tight-conglomerate pore structures. A Swin Vision Transformer-based image-analysis workflow was used to quantify residual-oil morphologies across cycles. The results show that asphaltenes undergo a reversible four-stage evolution during depressurization, including rapid precipitation, oscillatory decay, in situ re-dissolution, and final dissipation. Fracture-tip corners induce quasi-stationary vortices that renew CO 2 -oil interfaces, strip wall-bound oil films, and intensify local mass transfer. Residual water substantially modifies transport pathways: continuous water films impose a serial dissolution-diffusion-partitioning resistance that reduces CO 2 -oil mass-transfer efficiency by approximately 55%, whereas dispersed water ganglia trigger capillary snap-off and redistribute CO 2 into bubble-scale transport pathways. Across successive cycles, residual oil evolves from connected clusters to isolated droplets and wall-bound films, with the dominant mobilization mechanism shifting from swelling-assisted displacement to fragmentation and finally diffusion-limited recovery. Oil saturation decreases from 100% to 49.34%, 16.35%, and 12.50% after the first, second, and third cycles, respectively. These findings provide pore-scale mechanistic evidence for optimizing soaking, depressurization, residual-water management, and cycle design in cyclic CO 2 processes.