Synergistic Mechanisms of CO2 Injection-Assisted Depressurization for CH4 Recovery and CO2 Sequestration under Coupled Operational Conditions

Abstract

Against the backdrop of the “dual carbon” goals, the synergistic “depressurization + CO2 displacement” extraction method offers a new pathway for the green and efficient development of offshore natural gas hydrates. This approach combines the engineering feasibility of the depressurization method with the carbon sequestration advantages of the displacement method, delivering both energy production and carbon emission reduction benefits. However, there is currently a lack of systematic understanding regarding the dynamic coupling mechanisms and parameter influence patterns between in-reservoir displacement and pressure reduction, which has hindered the transition of this technology from theory to practical engineering application. Therefore, this study focused on the silty sand reservoir in the LS36–1 block of the South China Sea. Using a high-pressure hydrate production experimental reactor, experiments were conducted under various operating conditions─including different initial hydrate saturations, pressure reduction points, depressurization magnitude, CO2 injection locations, and injection modes─under the “liquid CO2 injection followed by pressure reduction” production scheme. The results indicate that the displacement volume and displacement efficiency are primarily controlled by the initial saturation and injection location, peaking at 45% saturation (245.456 L, 46.14%); gas production and recovery rate are influenced by multiple factors, with the effect of pressure reduction exhibiting a “stable-then-increasing” pattern, resulting in a maximum increase in gas production of 85.85 L and a 15.58% improvement in recovery rate; CO2 storage volume and storage efficiency increase with rising saturation, peaking at 45% saturation (23.631 L, 78.71%); injection into the underlying formation yields higher storage efficiency, and the magnitude of pressure reduction exhibits an “n-shaped” influence on storage performance. The sensitivity ranking for methane recovery is magnitude of pressure reduction > CO2 injection location > injection mode > injection point; the sensitivity ranking for CO2 sequestration performance is pressure reduction magnitude > injection mode > injection location > pressure reduction point. The study reveals the synergistic regulatory mechanism of key parameters on CH4 recovery and CO2 sequestration under the “inject-then-recover” mode, providing experimental evidence for the engineering practice and parameter optimization of “inject-then-recover” synergistic technology.