Effect of Tributyl Citrate as a Cosolvent on the Phase Behavior of Crude Oil during CO₂ Injection Process

The current research on CO₂ cosolvent primarily focuses on reducing the minimum miscibility pressure and improving oil recovery. However, investigations into the impact of additive agents on the phase behavior of crude oil during the CO₂ injection process are relatively limited. In this study, we introduced tributyl citrate as a cosolvent to the CO₂ injection process. By comparing the phase parameters of crude oil and changes in component composition in the residual oil before and after the addition of tributyl citrate, we explored the influence patterns of this cosolvent during CO₂ injection. The experiments show that the optimum concentration of tributyl citrate is 0.3%. After the addition of tributyl citrate, the bubble point pressure of crude oil is reduced from 14.28 MPa to 13.36 MPa, and the density is decreased from 1.00 g/cm³ to 0.95 g/cm³. These alterations of bubble point pressure and density indicate an enhanced solubility of CO₂ and improved miscibility with the oil, coinciding with an increased volume expansion coefficient rising from 1.12 to 1.18 under 20 MPa and a decrease in viscosity from 0.73 mPa·s to 0.64 mPa·s. Tributyl citrate primarily affects the properties of crude oil by reducing interfacial tension and the content of heavy components in the dissolution system. The addition of tributyl citrate stabilizes the deposition trend of heavy components in crude oil and promotes the transformation of heavy components into light components, thereby enhancing the efficiency of CO₂ extraction. This study provides valuable insights into a novel and simple method to further increase oil recovery in the CO₂ injection process.