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.