Study on Different Miscibility Intervals of an Impure CO₂-Crude Oil System in Offshore Low-Permeability Reservoirs

Abstract

CO2-enhanced oil recovery (EOR) technology considers the two themes of energy resource development and environmental protection. However, the effect of CO2 purity on the range of different miscible degrees of the CO2-crude oil system and the development characteristics of CO2 flooding in offshore low-permeability reservoirs. Therefore, based on the fluid samples collected from different target wells, the quantitive method of dividing the pressure range of three kinds of miscible degrees under different CO2 purities through the slim tube experiment and numerical simulation was established. In addition, the effect of the CO2 purity and the pressure ranges of near-miscible and miscible flooding in the different target wells under various development stages were also evaluated, and the lower limit of pure/impure CO2 for miscible flooding and near-miscible flooding in each well was also calculated. The results showed that the traditional slim tube experiment leads to a fluctuation pressure range, which results in errors. Besides, the near-miscibility pressure range of the pure CO2-oil system in the BZ25–1 reservoir is 27.1–31.4 MPa, and compared with the pure CO2-crude oil system, the minimum miscibility pressure (MMP) of 80% pure CO2-crude oil system and 24% pure CO2-crude oil system increased by 7.98 and 40.59%, respectively. Additionally, the CO2 purity range of near-miscible flooding is 61–100% under the condition of the BZ25–1 reservoir. The results have guiding significance for deepening the implementation of the CO2-EOR technology in offshore low-permeability reservoirs.