Numerical Simulation Study of Near-Miscible Displacement Mechanism of Weakly Volatile Crude Oil in Low Permeability Carbonate Reservoir After CO2 Injection

The degree of miscibility of CO2 with crude oil in the oil displacement process will directly affect the displacement efficiency and the final recovery rate. Therefore, it is of great significance to study the degree of miscibility between injected CO2 and crude oil and flooding mechanism. Taking the target block of R reservoir in Kazakhstan as an example, It is planed to establish the numerical model of multi-group slim-tube experiments to calculate the minimum miscible pressure, Combined with the compositional model, the change of fluid components with production time after the injection of CO2 into the low permeability carbonate reservoir with weak volatile crude oil, so as to quantify the action stages and contribution degrees of different flooding mechanisms. The results show that the target block is always in the near-miscible flooding. By analyzing the variation characteristics of fluid composition of weakly volatile crude fluid in low permeability carbonate reservoir changes, the boundary between different displacement mechanisms is defined when the CO2 content in crude oil is 29.3%. The displacement mechanism in the early stage of reservoir development is High pressure displacement, The formation pressure increases, and the light component is dissolved in the crude oil and CO2 pushes it out of the surface, the contribution is 37.9% to the recovery; In the middle development stage, CO2 contacts with crude oil and dissolved in it, increasing the volume and reducing the viscosity, the contribution to the recovery is 26.3%;In the late development stage, CO2 extracts the light component of crude oil to the production wellhead together with the output gas, the contribution to the recovery is 35.8%. This study by tracking the changes of fluid components in different development stages in the target area, Further determined the CO2 concentration in crude oil when displacement mechanism changes and the contribution of different displacement mechanisms to oil recovery, it provides technical basis and theoretical guidance for further understanding of the mechanism of CO2 near-miscible flooding and reservoir application.