Enhancing oil recovery and carbon sequestration through alternating injection of methane-rich associated gas and CO2 in deep reservoirs

The direct emission of methane-rich associated gas from oil fields is a significant contributor to the greenhouse effect. Currently, alternative gas flooding predominantly employs CO2 and N2 as displacement phases, with limited research focusing on the alternating injection of methane-rich associated gas and CO2 tailored for oil field production. This study investigates the feasibility of enhancing oil recovery and mitigating the greenhouse effect through the alternating injection of methane-rich associated gas and CO2 into reservoirs using experimental and numerical simulation approaches. Initially, core-scale laboratory experiments were conducted on the alternating injection of methane-rich associated gas and CO2, followed by establishing a corresponding numerical simulation model. The results indicate that, compared to continuous CO2 injection, the alternating injection method results in a 2.48% reduction in oil recovery efficiency. However, this method demonstrates superior control over the mobility of oil, gas, and water. This is because there are two ways to enhance oil recovery efficiency: improve extraction efficiency and increase the swept area. CO2 has a high extraction efficiency but a relatively small swept area. Therefore, we aim to increase the swept area by using an alternating injection method of CO2 and associated gas, where mobility is a key factor in expanding the swept area. Specifically, the mobility ratio between oil and gas decreases from 9.65 to 8.24, and the ratio of oil mobility in the remobilized zone to water mobility in the unswept zone reduces from 0.770 to 0.441, suggesting significant potential for improving the sweep efficiency of heterogeneous reservoirs through alternating injection. Subsequently, numerical simulations were performed at the reservoir scale to investigate the alternating injections of methane-rich associated gas and CO2 with varying slug sizes. At the reservoir scale, compared to continuous CO2 injection, the alternating injection method increases the swept area of injected gas from 4.72 to 7.82 km2, consequently resulting in an additional production of 18 969 m3 of oil and sequestration of 9.0619 × 108 kg CO2 equivalents, representing increases of 15.6% and 460.4%, respectively. Finally, the slug ratio for alternating injection was optimized using the surrogate optimization method in MATLAB. After optimization, the cumulative oil production increased by 31.6% and the carbon sequestration capacity increased by 74.1%, compared to continuous CO2 flooding. This study offers valuable theoretical support for improving oil recovery through gas injection and facilitating carbon sequestration in reservoirs.