Experimental Investigation of Oxygen-Reduced Air Injection Mechanisms for Enhanced Oil Recovery

This study investigates the mechanisms and performance of oxygen-reduced air flooding (ORAF) and oxygen-reduced air gravity drainage (ORAGF) through laboratory experiments on crude oil and cores from the Kunbei Oilfield. PVT experiments show that injecting N2 or oxygen-reduced air (5% and 10% O2) increases saturation pressure and reduces oil viscosity comparably. Low-temperature oxidation (LTO) tests reveal that oxygen consumption rate declines exponentially with decreasing initial O2 concentration; at 5% O2, oxidation products are nearly indistinguishable from the original crude oil. Long-core displacement experiments demonstrate that vertical (gravity-assisted) injection significantly outperforms horizontal injection, with oil recovery reaching 39.8% (10% O2) versus 26.2% horizontally, owing to gravity segregation suppressing gas fingering and enhancing oil–gas contact. Among injection strategies, gas-assisted gravity drainage (GAGD) and water-alternating-gas (WAG) improve recovery by 7.7% and 7.2% over continuous waterflooding, respectively, with GAGD being more suitable for high water cut reservoirs. Reservoir rhythm and permeability contrast affect performance and gravity-driven injection mobilizes low-permeability layers more effectively than horizontal injection, exhibiting good adaptability to heterogeneous reservoirs. Fracture orientation relative to injection direction plays a critical role—horizontal fractures achieve the highest recovery (~50%), whereas through-going fractures impair performance. These findings provide experimental guidance for optimizing oxygen-reduced air gravity flooding in tight and heterogeneous oil reservoirs.