A study on the miscibility mechanisms and patterns of high CO₂ content associated gas reinjection

To investigate the enhanced oil recovery mechanisms during the reinjection of CO₂-rich associated gas, analyse the miscibility behaviour between associated gas and crude oil, and provide guidance for increasing oil recovery in field development, in this study, gas injection expansion experiments, solubility measurements of various gases in crude oil, and slim tube experiments were conducted. The experimental results demonstrated that CH₄ and N₂ could reduce the solubility of associated gas in crude oil. The solubility of associated gas without CH₄ and N₂ in crude oil was 1.05 to 3.22 times greater than that of CO₂, whereas their removal enabled the solubility of associated gas in crude oil to surpass that of CO₂. Both CO₂ and associated gas could cause crude oil to swell and reduce its viscosity, and the absence of CH₄ and N₂ amplified these effects. The minimum miscibility pressure (MMP) for CO₂ flooding is 24.29 MPa, while a reservoir pressure of 21 MPa is insufficient to achieve miscible flooding. Removing CH₄ and N₂ from the associated gas can reduce the MMP by up to 48%, resulting in a 25.59% increase in the oil recovery efficiency.