Miscibility Evaluation for Reinjection of CO₂ Flooding Associated Gas in Jilin Oilfield

Abstract In view of large amounts of associated gas produced in the late stage of CO2 flooding in Jilin Oilfield, the production characteristics of CO 2 flooding associated gas are divided into three stages (i.e. before injected gas breakthrough, during injected gas breakthrough and after injected gas breakthrough). Taking the formation pressure as miscibility evaluation criteria, the miscibility effect of the produced CO 2 flooding associated gas from different stages is evaluated. According to different gas production stages, the reinjection method of mixing the associated gas with pure CO 2 under different proportions is proposed to elevate the utilization degree of the produced associated gas. The results indicate that at the very beginning of gas production process, produced gas is mainly composed of CH 4 (58%) and C2~C 3 (29%) with small amounts of N 2 and C 4~ C 6 . During this stage, the associated gas can not reach miscibility with the oil by direct reinjection. It needs to be mixed with pure CO 2 , also the blending ratio of the associated gas should not exceed 6%. In the second stage, the compositions of the produced gas are mainly CH 4 (34%) and CO 2 (40%). During this stage, the blending ratio should not exceed 11%. At the final stage, the main composition of the produced gas is CO 2 (94%). The associated gas can be directly reinjected into the reservoir during this stage.