Immiscible multiphase flow behaviours of water-oil-CO2 ternary system flooding using X-ray CT

Abstract

Carbon dioxide (CO2) injection into oil reservoirs has been widely accepted as an effective technique for enhanced oil recovery (EOR) after waterflooding. More recently, the development of carbon capture and storage (CCS) to reduce CO2 emissions has made CO2-based EOR increasingly attractive. Waterflooding is widely employed in China and, even after several years of water injection, considerable oil deposits remain in the thick, positive rhythm reservoirs in eastern China. The majority of these reservoirs may not be suitable for miscible CO2 flooding. The present work investigated immiscible CO2 flooding after waterflooding in such sites by laboratory trials. Series of large artificially-consolidated sandstone models with different levels of heterogeneity were used to simulate thick, positive rhythm oil reservoirs. Gaseous CO2 was continuously injected into these models at a constant injection pressure and an X-ray CT scanner (with a resolution of 0.7 × 0.7 mm) was used to monitor and record changes in the fluid saturation and migration. Based on the experimental results, it indicates that immiscible CO2 flooding following waterflooding is an efficient means of enhancing oil recovery. It is evident that both the reservoir heterogeneity and injection pressure differential affect oil recovery and CO2 distribution. The heterogeneity has a remarkable impact on oil recovery when the permeability differential between layers is lower than 2 millidarcy (md) or the permeability variation coefficient is less than 0.2. Fitting of the experimental results also demonstrates that there is an optimum pressure differential between inlet and outlet that maximizes oil recovery under specific inhomogeneous conditions.