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.