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

Experimental results from the Daniudi gas field enhance our understanding of mechanisms behind CO₂ injection for enhanced recovery from tight-sand gas reservoirs. The results reveal that the diffusion coefficients of CO₂ in tight reservoirs range from 10⁻⁸ m²/s to 10⁻⁹ m²/s, correlating negatively with pore pressure and positively with pore radius. In these reservoirs, CO₂ manifests a significantly higher adsorption capability compared to CH₄, 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, CO₂ injection alleviates water locking and enhances gas-water flow, facilitating the recovery of trapped gas. The long-core CO₂ flooding experiment results in a 14.11 % increase in gas recovery efficiency. The effectiveness of CO₂-enhanced gas recovery (EGR) is primarily related to reservoir properties. Higher average permeability correlates with more effective CO₂-EGR. Although the rate and mode of injection have limited impacts on ultimate recovery efficiency, they influence CO₂ breakthrough time. Specifically, a higher injection rate leads to earlier breakthrough, and the breakthrough under pulsed CO₂ injection occurs later than that under continuous injection.