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.
Highlights
Enhanced oil recovery (EOR) technologies have become increasingly important in the petroleum industry
Waterflooding Following the initiation of waterflooding, the model went through a water-free production period, during which no water was produced and the oil recovery increased rapidly
In contrast to the data obtained with waterflooding, oil recovery during gas flooding was found to increase with increasing heterogeneity
Summary
Enhanced oil recovery (EOR) technologies have become increasingly important in the petroleum industry. A typical primary recovery ranges from 5% to 20% of the original oil in place (OOIP) (Blunt et al, 1993) For this reason, the reservoir pressure is often increased to a level at which the reservoir fluids can be obtained at the desired rate during the secondary recovery stage. Following more than 20 years of widely applied waterflooding, the majority of reservoirs presently generate a product stream with a water content of 80%, this value can exceed 90% in mature oilfields in eastern China (Wang, 2009). This region contains numerous thick, positive rhythm reservoirs with fluvial facies, in which the permeability of the lower reservoir is much higher than that of upper part (Wang et al, 2017). As a result of the combined effect of heterogeneity and gravitational differentiation (Yang et al, 2012, 2014; Jen et al, 2017), water breakthrough can occur in the lower part of such reservoirs, leading to low sweep efficiencies and high residual oil saturation in the upper reservoir during waterflooding
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