Experimental Investigation on the Mechanism of Oxygen-Reduced Air Water Alternate Flooding for a Low Permeability and Heterogeneous Light Oil Reservoir at High Water Content Stage

Gas injection has been demonstrated as an effective method for enhancing recovery in low-permeability oil reservoirs, with its performance influenced by factors such as gas type, injection strategy, timing, and operational parameters. This study experimentally investigated the mechanism of oxygen-reduced air–water alternate flooding (WAG) in a low-permeability, heterogeneous light oil reservoir at the high water-cut stage. First, it was found that the dissolution of oxygen-reduced air slightly expands crude oil and enhances its fluidity. Nuclear magnetic resonance experiments revealed that oxygen-reduced air-heavy water alternate (WAG) flooding, following heavy water flooding, improved oil recovery in both high- and low-permeability parallel core samples. However, this method primarily displaced oil from larger pores. Furthermore, dual-core parallel flooding experiments demonstrated that water flooding followed by oxygen-reduced air (containing 5 mol % O2)-water alternate flooding significantly enhanced oil recovery. Specifically, the low-permeability core achieved a maximum oil recovery of 69.33%, representing a 6.86% increase compared to single water flooding. Notably, this recovery even slightly surpassed that of the parallel high-permeability core (64.71%), attributable to low-temperature oxidation (LTO) reactions in the low-permeability cores. The LTO effect became more pronounced with higher oxygen concentrations in the injected gas. For practical applications, it is recommended to ensure sufficient contact time between oxygen-reduced air and crude oil within the reservoir to maximize oxidation reactions and improve oil recovery efficiency.