Effect of CO2-Thermal Synergy on Nonlinear Percolation Behavior of Heavy Oil in Porous Media

As a greenhouse gas, CO2 can dissolve into heavy-oil systems through molecular diffusion and phase interactions, altering their rheological properties. It reduces steam consumption in the traditional thermal recovery of heavy oil. Clarifying the control laws of CO2-heavy oil interactions on flow characteristics in porous media has important significance for formulating enhanced oil recovery (EOR) schemes in high-viscosity reservoirs and achieving CCUS goals. This study focuses on ordinary heavy oil (OHO) and extra-heavy oil (EHO) from the J230 block of the Xinjiang Oilfield. Through systematic experiments and theoretical modeling, it comprehensively reveals the regulatory mechanisms of CO2 dissolution on the non-Darcy percolation characteristics of heavy oil. The results demonstrate that CO2 dissolution significantly reduces the viscosity, anomalous temperature, and yield stress of the heavy oil. The viscosity–temperature relationship follows an Arrhenius-type exponential model, based on which a viscosity prediction equation incorporating CO2 solubility was established. Percolation experiments show that CO2 can reduce the immobile area of OHO/EHO by 67.11–96.25%. On the basis of non-Darcy percolation theory, a power-law relationship between PTPG (pseudothreshold pressure gradient) and CO2 dissolution amount in double-logarithmic coordinates was derived, and a percolation capacity evaluation chart was constructed. On the basis of the quantitative analysis of the CO2-heavy oil seepage dynamic response, this study derived a theoretical drainage radius prediction equation after CO2 dissolution. Under idealized flow models and experimental conditions, the equation indicates that increasing the level of CO2 dissolution can significantly enhance drainage capacity, providing a theoretical reference for well spacing optimization. This study establishes, for the first time, a quantitative relationship model among CO2 solubility, PTPG, and drainage radius, providing theoretical guidance for CCUS and CO2-assisted steam flooding EOR technologies in high-viscosity reservoirs.