Determination of minimum near miscible pressure region during CO2 and associated gas injection for tight oil reservoir in Ordos Basin, China

CO2 flooding is believed as an effective and promising method to enhance oil recovery. However, for tight oil reservoirs of Long Dong (LD) region in Ordos Basin, CO2 flooding cannot achieve miscible condition since reservoir pressure is lower than minimum miscibility pressure (MMP). Meanwhile, this region has abundant associated gas production. Impure CO2 with associated gas re-injection at the near-miscible condition is a promising enhanced oil recovery (EOR) method which may solve the problem above. This study aims to investigate the oil recovery potential of this method by conducting experiments and simulations. Firstly, based on the reservoir condition (64 ℃), slim-tube experiments and simulations were performed to acquire correlations of pressure, interfacial tension (IFT) and oil recovery of pure CO2 flooding. Then the MMP, the minimum near-miscible pressure (MNMP), and near-miscible region were determined. The pressure interval of the near-miscible region ranges from 17.82 MPa to 22.75 MPa. Secondly, the minimum CO2 concentration for conducting impure CO2 with associated gas re-injection at the near-miscible condition was investiaged, which is 73.3% for the target formation. Furthermore, by studying the composition of injected gas, it is found that the presence of methane and nitrogen impedes oil and gas to achieve the miscible or near-miscible phase. With the decrease of CO2 concentration of injected gas by 10% or the concentration of impurities in the injected gas increases by 7% each time, MMP and NMMP increase by 1.08 MPa and 0.89 MPa, respectively. This study explores the feasible application of using impure CO2 with associated gas re-injection to improve oil recovery for tight oil reservoirs.