A critical review of carbon dioxide enhanced oil recovery in carbonate reservoirs

Abstract

Carbon dioxide (CO2) enhanced oil recovery (EOR) in carbonate reservoirs is regarded as a promising EOR technology. However, the oil recovery factor of CO2 EOR is low due to the CO2 EOR mechanisms in carbonate reservoirs are not clear. In this study, the CO2 EOR in carbonate reservoirs is reviewed in detail. First, the carbonate reservoirs are characterized by strong heterogeneity, fractures and vugs with poor connectivity, complex fluid properties, leading to a variety of difficulties in drilling, acid fracturing and CO2 EOR. Second, CO2 EOR in carbonate reservoirs includes CO2 continuous injection, water-alternating-CO2 (WAG), carbonated water injection (CWI), CO2 foam and CO2 huff-n-puff. But the screening criterion for different CO2 injection schemes in carbonate reservoirs has not been established. Third, the mechanisms of the CO2 diffusion, CO2-oil interaction, and CO2-fluid-rock interaction are discussed. (1) The main experimental test method for CO2 diffusion coefficient is pressure decay, and the temperature of the CO2 diffusion coefficient test is 25–130 ℃, and the pressure is 0.772–34 MPa. But the matrix-fracture-vug system and CO2-fluid-rock interaction effects in the CO2 diffusion coefficient experiments, and the impure CO2 diffusion behavior and CO2 diffusion behavior in the oil–water mixture have not been studied. (2) The main mechanism of pure CO2 flooding for EOR has been studied, but the development of impure CO2 oil interaction and complex the matrix-fracture-vug system are unclear. (3) The CO2-fluid-rock interactions have great influences to CO2 EOR in the carbonate reservoirs. But CO2-fluid-rock interactions in-situ formation state is still unclear. Fourth, pilot test studies in carbonate reservoirsare discussed, and the challenges of CO2 EOR in carbonate reservoirs are pointed out. The findings of this study can help for better understanding of CO2 EOR mechanism in carbonate reservoirs, and it also proposes some future clues of CO2 EOR in carbonate reservoirs.