Impact of SC-CO ₂ Prefracturing Fluid Intrusion on the Physical Properties of Offshore Tight Reservoirs

The Cenozoic continental reservoir of the Bozhong 25-1 Oilfield in the Bohai Bay Basin is a typical low-porosity and low-permeability tight sandstone, which is prone to causing severe water lock after hydraulic fracturing. Supercritical CO2 (SC-CO2) prepad fracturing can mitigate water blocking through mineral dissolution and replacement effects, but its influence mechanism on the physical properties of offshore low-permeability tight reservoirs after fracturing fluid invasion remains unclear. Core-scale microscopic characterization and physical simulation experiments were conducted to investigate the dynamic evolution characteristics of SC-CO2 invasion, the impact of SC-CO2 on rock properties after fracturing fluid interaction, and the effect of SC-CO2 prepad fracturing on oil-phase permeability. The results indicate that SC-CO2 exhibits a nonuniform frontal advance during core invasion, with the invasion velocity increasing with pressure. The recovery rate of water-blocking damage caused by the fracturing fluid reached 21.61%. NMR analysis revealed improved pore-throat structure and a 16.46% increase in movable water content postreaction. Oil-phase permeability increased with higher initial core permeability, flowback velocity, temperature, and SC-CO2 injection volume. Compared with conventional hydraulic fracturing, SC-CO2 fracturing enhanced the oil-phase permeability by 14.97%.