Phase Behavior and Diffusion Mechanisms for CO ₂ -Enhanced Gas Recovery (CO ₂ -EGR): A Case Study of the A Gas Field, Yinggehai Basin

Carbon Capture, Utilization, and Storage (CCUS) technologies are essential for mitigating climate change while sustaining energy production, particularly in mature gas fields facing production challenges. CO2-enhanced gas recovery (CO2-EGR) offers a dual benefit by enhancing natural gas recovery and sequestering CO2 underground. This study investigates CO2-EGR in the Yinggehai Basin A Gas Field, which is in its middle to late development stage and experiencing low reservoir energy and declining recovery rates. Under deep, high-temperature, high-pressure conditions, the phase behavior and diffusion mechanisms of CO2 and natural gas are complex, and traditional phase models often fail to accurately describe nonideal miscibility and multiphase equilibrium. To address these challenges, we conducted multiscale phase experiments and mechanism analyses tailored to the phase behavior during CO2 injection in gas reservoirs. Our findings reveal layered diffusion behavior and interfacial transition zone evolution during CO2 displacement of methane, as well as the effects of varying CO2 contents and injection ratios on phase envelopes, critical properties, and fluid characteristics such as viscosity and density. Results indicate that higher CO2 concentrations and optimized injection strategies can expand miscible conditions, improve displacement efficiency, and enhance injection–production design. Additionally, CO2–natural gas–formation water phase experiments demonstrate that the water phase can mitigate increases in viscosity and density while influencing the gas–water ratio. These insights advance the understanding of phase and diffusion mechanisms in deep gas reservoirs during CO2–methane displacement and provide scientific data to support the integration of CO2 recovery and storage, thereby contributing to the broader goals of CCUS in the natural gas industry.