Enhancing CO₂ Sweep Volume through Pressure Modulation in Heterogeneous Reservoirs: An Experimental Study

CO₂ flooding is a key technology for achieving the synergistic benefits of enhanced oil recovery (EOR) and geological CO₂ storage. However, preferential CO₂ channeling and poor sweep efficiency caused by interlayer reservoir heterogeneity severely restrict the large-scale application of this technology. Moreover, the mechanisms by which dynamic pressure modulation intervenes in the multiphase flow field remain insufficiently understood. In this study, a heterogeneous two-dimensional visual physical model consisting of high-, medium-, and low-permeability layers was constructed, and constant-pressure and stepped pressure-modulation flooding experiments were systematically conducted. Combined with a high-precision real-time monitoring system, the sweep dynamics under different pressure-control modes were quantitatively characterized, and the coupled response between displacement-front evolution and phase-behavior-related mass transfer was analyzed. The results show that, under the constant-pressure mode, increasing the displacement pressure can effectively suppress gravity override and improve the mobility ratio by reducing the oil-gas density difference and enhancing multicomponent mass transfer and extraction. Compared with the 15 MPa near-miscible flooding scheme, the 22 MPa miscible flooding scheme increased the ultimate recovery factor by 12.2 percentage points. The stepped pressure-increase strategy applies transient pressure perturbations, generates transverse pressure gradients, and increases the capillary number, thereby disrupting stable preferential flow channels formed under steady-state flow and dynamically reconstructing the flow field. Under the synergistic effects of high-pressure miscibility and staged energy release, this strategy effectively expanded the sweep volume and suppressed gas channeling to a certain extent, ultimately achieving a recovery factor of 73.8% and substantially prolonging the high-efficiency oil production period. This study clarifies the experimental observations of dynamic pressure modulation in improving CO₂ sweep performance in interlayer heterogeneous systems and provides experimental evidence and reference for optimizing CO₂-EOR injection-production schemes in interlayer heterogeneous reservoirs.