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.

In chemical flooding, emulsification and interfacial tension (IFT) reduction are crucial for enhanced oil recovery (EOR). However, the dominant performance parameter and technical limits of surfactants for oil displacement remain underexplored. This study investigated the relationship between the emulsification capability and IFT. Accordingly, the dominant performance parameter and the technical limits of surfactants were determined using oil displacement experiments. Specifically, an analysis of 74 sets of experimental results revealed a shift in the significant correlation between EI (a quantitative measure of emulsification capability) and σ at an σ value of 8.5 × 10⁻² mN/m (i.e., critical value σ_c). For σ < σ_c, emulsification capability and IFT function as independent performance parameters. The oil displacement experiments using two surfactants with contrasting EI and σ values demonstrate that emulsification capability, rather than ultra-low IFT, is the dominant performance parameter. This study determined the technical limit of EI using oil displacement experiments via in-situ emulsification. The experimental results indicate strong correlations of EI with oil displacement and recovery efficiencies. The incremental displacement and recovery efficiencies were employed to quantify the potential of surfactants to enhance oil displacement and recovery efficiencies, respectively. The incremental displacement and recovery efficiencies versus EI curves revealed a critical EI (EI_c) value of 0.53. When EI < EI_c, the incremental displacement and recovery efficiencies increased significantly with EI. In contrast, when EI > EI_c, their increasing rates slowed down markedly. Therefore, the technical limits of the emulsification capability and IFT of surfactants used in this study are determined at EI ≥ 0.53 and σ ≤ 8.5 × 10⁻² mN/m, respectively.