Performance Evaluation of a Novel CO ₂ Flooding Thickening System for Low-Permeability Reservoirs and an Experimental Study on Enhanced Oil Recovery

To address the challenges associated with CO₂ flooding in low-permeability reservoirsparticularly inefficient gas injection and reduced displacement efficiency caused by viscous fingeringthis study synthesized the thickener PVAc-S through the copolymerization of styrene and polyvinyl acetate, selecting ethanol as a cosolvent to construct the PVAc-S thickening system. The solubility and thickening behavior of the system in CO₂ were investigated by using a PVT apparatus and a rheometer, respectively. Parallel dual-core experiments were utilized to examine the mobility control effect of the thickening system in reservoirs with permeability contrast. Online nuclear magnetic resonance (NMR) microdisplacement experiments were conducted to investigate the enhanced oil recovery effect and the degree of fluid mobilization in microscopic pores during CO₂ flooding with the thickening system. Experimental results indicate that the thickening system constructed with 5 wt % PVAc-S of 6000 molecular weight and ethanol exhibits favorable solubility in CO₂. Under a shear rate of 180 s^-1, 60 °C, and 20 MPa, the viscosity of CO₂ containing a 5 wt % thickener reached 0.552 mPa·s, representing a 24-fold viscosity increase. For heterogeneous reservoirs, the thickening system demonstrated effective mobility control capability, with a gas production rate ratio of 1.5:1 between the high-permeability and low-permeability layers. NMR imaging revealed the formation of a distinct slug-like displacement front within the core after injection of the thickening system, mitigating viscous fingering. NMR T₂ spectra indicated that thickened CO₂ flooding mobilized more fluids in micropores than pure CO₂, improving macroscopic oil recovery by 12.8%. The analysis suggests that this performance is attributed to the strong affinity of styrene/polyvinyl acetate groups to CO₂ and the enhanced solubility of PVAc-S in scCO₂ with the cosolvent, which together give rise to exceptional thickening, effectively suppress viscous fingering, improve sweep efficiency, and ultimately enhance oil recovery.