Study on the distribution of oil ring components and volume changes during the depletion development process of high CO₂ gas condensate reservoirs with oil ring

During the depletion development process, the fluid phase behavior of high CO2 gas condensate reservoirs with oil ring exhibits extremely complex due to the mass transfer processes between CO2 and oil. To study the phase behavior characteristics of high CO2 gas condensate reservoirs with oil ring, depletion development experiments were carried out at different depletion pressures and rates. The components of oil and gas produced in each depletion stage were analyzed by using chromatography to obtain the composition change of the remaining fluid. In addition, the reservoir phase behavior and the oil ring's volume change during the depletion development process were calculated through equation of state. The experimental results show that for high CO2 gas condensate reservoirs with oil ring, the content of heavy components in the produced oil increased as the depletion pressure decreases. Due to the change of fluid composition and pressure during depletion, the volume change of the oil ring was divided into three stages. The volume of the oil ring increased slowly at first, then increased rapidly, and finally decreased. Increasing the depletion rate would increase the content of intermediate and heavy components in the produced oil and gas. A higher depletion rate was advantageous for enhancing recovery, whereas a lower depletion rate resulted in a larger volume of the oil ring. However, experiments show that there was a critical depletion pressure, which was 25.5 MPa for the Libra Central reservoir. The pressures below this threshold resulted in a reduction in the volume of the oil ring.