Mechanisms of CO2 enhanced gas recovery in tight-sand gas reservoirs

Experimental results from the Daniudi gas field enhance our understanding of mechanisms behind CO 2 injection for enhanced recovery from tight-sand gas reservoirs. The results reveal that the diffusion coefficients of CO 2 in tight reservoirs range from 10 -8 m 2 /s to 10 -9 m 2 /s, correlating negatively with pore pressure and positively with pore radius. In these reservoirs, CO 2 manifests a significantly higher adsorption capability compared to CH 4 , suggesting a competitive adsorption advantage. Further, the amount of adsorbed gas correlates negatively with core permeability and positively with pore pressure. In the late-stage depletion-drive development of tight-sand gas reservoirs, CO 2 injection alleviates water locking and enhances gas-water flow, facilitating the recovery of trapped gas. The long-core CO 2 flooding experiment results in a 14.11 % increase in gas recovery efficiency. The effectiveness of CO 2 -enhanced gas recovery (EGR) is primarily related to reservoir properties. Higher average permeability correlates with more effective CO 2 -EGR. Although the rate and mode of injection have limited impacts on ultimate recovery efficiency, they influence CO 2 breakthrough time. Specifically, a higher injection rate leads to earlier breakthrough, and the breakthrough under pulsed CO 2 injection occurs later than that under continuous injection. • CO 2 diffusion coefficient and adsorption content in tight-sand gas reservoirs have been measured. • Mechanism of water locking alleviated by CO 2 injection was revealed based on online CT scanning. • Effectiveness of CO 2 -enhanced gas recovery (EGR) was assessed by long core displacement experiments.