Numerical simulation of CO2 miscible front migration by taking Block G of Jidong oilfield as an example

The mechanisms of CO2 Miscible Front Migration is a key factor for enhanced oil recovery in CO2 flooding, it is difficult to be described due to complex interactions between fluids, therefore, the CO2 Miscible Front Migration characteristics are unclear. In this paper, we established an injection-production pair model for coupled Gas-Oil Two-Phase Flow in Miscible Flooding based on the theory of Fluid Interactions in Miscible Flooding and gas-oil two-phase fluid flow in porous media, considering the heterogeneity, reservoir dip angle and miscibility degree. The results show that with the increase in gas viscosity, dip angle and decrease of intralayer heterogeneity, the gas breakthrough time is later and the distance of miscible front migration is longer; With the increase of interlayer heterogeneity and reservoir pressure, the gas breakthrough time is earlier and the distance of miscible front migration is shorter. At the same time, due to the negligence effect between CO2 and oil, and well pattern in the analytical model, an ideal model based on reservoir parameters from Block G66 × 1 fault-block reservoir in Jidong Oilfield is established. The model identified the CO2-crude oil miscible zone by using oil saturation and CO2 molar fraction in oil and characterized the migration patterns of the CO2 miscible front through bottom-hole pressure, gas-oil ratio, and dynamic front-position monitoring. Simulation results show when areal heterogeneity increases from 1 to 9, CO2 breakthrough time is earlier, the contribution of the pure oil flow zone to recovery factor decreased from 72.62% to 26.31%, the contribution of CO2 mass transfer zone to recovery factor increased from 14.06% to 47.11%. When the formation dip angle increased from 0° to 35°, CO2 breakthrough time is earlier, the contribution of the pure oil flow zone to recovery factor increased from 59.32% to 65.62%, the contribution of CO2 mass transfer zone to recovery factor decreased from 17.54% to 16.32%. With initial water saturation increasing from 50% to 90%, displacement efficiency continued decreasing, CO2 flooding breakthrough time is later. The contribution of the pure oil flow zone to recovery factor increased from 55.68% to 82.58%, and the contribution of CO2 mass transfer zone to recovery factor decreased from 27.42% to 10.20%. The results provide the basis for front prediction and control in CO2 miscible flooding development.