Multiphase Flow Coupling Characteristic and Oil Enhancement Mechanism in Fractured Oil-Bearing Tight Sandstone

Summary The multiphase flow behavior in fractured tight sandstone fundamentally governs the production of tight oil and gas reservoirs. For this study, we used triaxial stress-based low-field (LF) nuclear magnetic resonance (NMR) (LF-NMR) displacement experiments to analyze gasflooding, waterflooding, and water-alternating-gas (WAG) processes in fractured tight sandstones. Real-time monitoring of multiphase fluid migration patterns and spatial distribution characteristics of residual oil was conducted during displacement processes. Furthermore, the mass transfer mechanisms between the matrix and fractures in gas-liquid displacement systems were systematically characterized. The experimental results demonstrated final oil recovery factors of 6% (gasflooding), 7.13% (waterflooding), and 9.87% (WAG). Pore structure analysis revealed that fractured tight sandstone comprises three distinct pore types, which are adsorption pores (APs, T2 ≤ 2.5 ms), percolation pores (PPs, 2.5 ms < T2 < 100 ms), and migration pores (MPs, T2 > 100 ms). Notably, APs served as the primary storage space for crude oil and water, whereas MPs functioned as the dominant gas-containing pores. Fracture was observed to control multiphase fluid migration, with MPs and PPs acting as principal conduits for oil, gas, and water transport. The matrix was mainly controlled by capillary force, while the fractures were mainly displaced by pressure difference. The alternating injection of water and gas formed a positive recovery cycle. During the gas injection stage, the oil saturation inside the fractures decreased, providing space for crude oil migration. During the water injection stage, capillary force played an important role in displacing crude oil from the matrix pores into the fractures, thereby increasing the oil saturation of MPs. The cyclic injection of gas avoided the capillary end effect during the water injection stage and improved the efficiency of crude oil migration. The mass transfer between fractures and matrix affected the recovery efficiency of fractured tight oil reservoirs, and the research results provided important theoretical support for increasing production in tight oil reservoirs.