Study of CO2 Flooding to Improve Development Effect in Conglomerate Reservoirs

Abstract

For low permeability sandstone reservoirs, CO₂ flooding has been proved to be an effective method to enhance oil recovery. Reservoir A is a typical conglomerate reservoir in Xinjiang oilfield. The reservoir has strong water sensitivity, and the injection pressure continues to rise. Furthermore the oil well pressure continues to drop. According to the screening conditions of CO₂ flooding, the reservoir A can easily achieve CO₂ miscible flooding with moderate temperature. And the reservoir has the advantage of being close to the gas source. Firstly, the relationship curve between CO₂ oil displacement efficiency and oil displacement pressure was obtained by changing the oil displacement pressure using the fine-tube experimental model, and the minimum miscible pressure of CO₂ oil displacement was determined. The minimum miscible pressure of reservoir A was 24.1 MPa. The change of oil phase state after CO₂ injection was further studied by CO₂ gas expansion experiment. The results show that the saturation pressure of formation oil increases significantly after CO₂ injection, and the more CO₂ injection, the higher the saturation pressure of formation oil. When the CO₂ content in the injected crude oil is 55.29 mol%, the saturation pressure of the CO₂ formation crude oil system reaches 31.60 MPa. Then, in order to better simulate the real reservoir conditions of conglomerate reservoir, the 2D visualization model of the outcrop was processed to analyze the microscopic oil displacement mechanism of CO₂. The experimental results show that the remaining oil after water flooding mainly exists in the form of cluster, film, column and blind end, and the remaining oil after gas flooding exists in the form of island, cluster and film, and there is an obvious gravity overlay phenomenon. CO₂ flooding after water flooding significantly improved sweep efficiency and enhanced recovery. Finally, through the analysis of CO₂ field injection effect in reservoir A, it is shown that CO₂ absorption capacity of single well is significantly higher than water absorption capacity. The equilibrium degree of production profile in gas injection stage is better than that in water flooding stage. Injecting carbon dioxide quickly restores formation pressure. The oil recovery effect after gas injection in the trial production stage is obviously improved compared with that in the water flooding stage. The research results of this paper provide a reference for the field application of CO₂ flooding enhanced oil recovery technology in Xinjiang conglomerate reservoir.