Experimental Study on Oxygen-Reduced Air Foam Flooding in Ultra-Low-Permeability Reservoirs: From Sensitivity Analysis and Decomposition of Oil Recovery to Pore-Scale Seepage Mechanisms

Mature domestic continental low-permeability oil reservoirs in China have generally entered the ultra-high water-cut and high-recovery stage. Complex reservoir heterogeneity, development characteristics prone to gas channeling, and unclear seepage mechanisms of air foam flooding present engineering challenges to efficient development. To address these issues, this study investigates oxygen-reduced air foam flooding in the Chang-6 ultra-low-permeability sandstone reservoir in the WY block of Changqing Oilfield. The microscopic pore network was characterized using high-pressure mercury intrusion, BET (Brunauer-Emmett-Teller) nitrogen adsorption, and scanning electron microscopy. Changes in reservoir and fluid physicochemical characteristics before and after development were evaluated using the sessile drop method, pendant drop method, and Wilhelmy plate method. Core-flooding experiments were performed to confirm preferred key injection parameters, quantitatively decompose individual enhanced oil recovery mechanisms, and verify pore-scale seepage mechanisms via online nuclear magnetic resonance. The results reveal multi-scale pore network characteristics of the target reservoir and confirm the significant reduction in oil–water interfacial tension during foam flooding. Parameter sensitivity analysis identifies preferred operating conditions within the investigated experimental range, including an injection velocity of 0.2–0.3 mL/min, a foaming-agent concentration of 0.4–0.5%, a gas-to-liquid ratio of 2:1, and a 5-cycle slug schedule. Oil recovery decomposition analysis indicates that ultimate oil recovery comprises a relative share of 77.63% from baseline waterflooding, 13.44% from surfactant-assisted oil mobilization, 6.45% from gas assistance, and 2.48% from physical foam plugging. Furthermore, online nuclear magnetic resonance T2 (Transverse relaxation time) spectrum analysis reveals distinct permeability-dependent air foam flow regimes: as permeability decreases, flow behavior transitions from smooth foam propagation with concurrent oil washing and plugging, to the formation of dynamic foam plugging, and finally to severe shear-induced foam destruction in narrow throats that causes air foam flooding to fail. This study provides preferred injection parameters and theoretical grounds for mechanism decomposition in air foam enhanced oil recovery for ultra-low-permeability reservoirs, offering key insights into air foam seepage behavior.