The experimental research for reducing the minimum miscibility pressure of carbon dioxide miscible flooding

As the minimum miscibility pressure in the research area is higher than the formation fracture pressure, it is impossible to form the miscible flooding. To address this problem, the effect of two chemical reagents (citric acid isobutyl ester, citric acid isopentyl ester) on reducing the minimum miscibility pressure (MMP) of carbon dioxide and the crude oil was determined through the long slim tube displacement experiment. The experimental results show that the minimum miscibility pressure could decrease significantly with increasing the injected slug size of the chemical reagents, but the decrease became smaller and smaller. The chemical reagents had an optimal injected slug size of 0.003 PV (pore volume). At such slug size, the minimum miscibility pressure for citric acid isobutyl ester was reduced by 6.1 MPa and for citric acid isopentyl ester the reduction was 5.5 MPa. The minimum miscibility pressure was reduced by 20.61% and 18.58%. The addition of citric acid isobutyl ester slug could greatly reduce the minimum miscibility pressure, with a better effect. The recovery of the miscible flooding experiment scheme after injection of citric acid isobutyl ester slug was higher by 6.5%–11.6% than that of the scheme without citric acid isobutyl ester injection. Therefore, the citric acid isobutyl ester was recommended as the best chemical reagent to reduce the minimum miscibility pressure. The research results are of certain guiding significance for the overall realization of miscible flooding to enhance oil recovery in the research area. •Two chemical agents were prepared by optimizing the preparation process.•The minimum miscibility pressure decreased significantly with increasing the injected slug size of the chemical reagents.•The chemical reagents had an optimal injected slug size of 0.003 PV. In this case, the MMP was reduced by 20.61% and 18.58%.•The recovery after injection of citric acid isobutyl ester was higher by 6.5% to 11.6% than that of without injection.