Investigation of fracture pressure and EOR mechanisms during fracturing flooding in low-permeability sandstone using CT and NMR

Fracturing flooding provides an effective approach to overcoming injection difficulties and enhancing oil recovery in high-water-cut, low-permeability sandstone reservoirs. Injecting fracturing-flooding fluids (FFFs) at high rates over short durations enables a coupled process of hydraulic fracturing and flooding, especially when the FFFs contain surfactants. In this study, two types of core experiments were conducted to elucidate the mechanisms of reservoir modification and residual oil mobilization during fracturing flooding. (1) Fracturing flooding combined with CT scanning: The effects of N₂ permeability (K_g) and FFF viscosity on fracture pressure (P_f) were investigated. Post-flooding CT scans of multiple cores were performed to visualize fracture propagation. (2) Fracturing flooding combined with NMR experiments: The recovery efficiencies of water-based and petroleum sulfonate (NPS)-based fracturing flooding were compared with conventional water flooding. The results showed a negative correlation between K_g and both P_f and the permeability enhancement factor (E_K), with E_K ranging from 1.61 to 0.43. Increasing FFF viscosity led to higher P_f values. The E_K first increased and then decreased, consistent with the fracture distribution observed in CT images. Moreover, NPS fracturing flooding exhibited superior enhanced oil recovery (EOR) performance compared with water fracturing flooding. The latter primarily improved recovery in small pores and mesopores, with recovery enhancement factors (E_R) of 0.44 and 0.36, respectively. In contrast, the NPS fracturing flooding achieved significant recovery improvements across all pore sizes, with E_R values in micropores and macropores approximately threefold and fifteenfold higher than those of water fracturing flooding. This study presents a novel investigation into the mechanisms of fracture propagation and enhanced oil recovery in fracturing flooding processes.