Abstract
Abstract Both laboratory tests and pilot wells have demonstrated the significant potential of CO2 as an enhanced oil recovery (EOR) medium. Due to the weak pressure conduction and mass transfer of the CO2-oil system, multiple zones are generated between the injection well and production well in ultra-low permeability reservoirs. This work aims to explore the characteristics of these multiple zones and the mechanisms of CO2-EOR by combining experimental results, core-scale analysis, and field-scale simulation. Long-core CO2 displacement experiments were conducted under different miscibility pressures, with production gas assayed using gas chromatography. The core-scale simulation was aligned with the experimental results, defining four CO2 fronts to distinguish different zones based on pressure, interfacial tension, and CO2 concentration along the long core. The distribution of five zones was upscaled to the field-scale model after pilot well history matching. The final step involved evaluating the miscible zone range value, CO2 injection utilization factor, oil recovery, and CO2 storage efficiency during the CO2 injection process. Results show that the boundary between the original oil zone and the oil transition zone exists at the CO2 component front, where the CO2 concentration is zero. Additionally, the location of the CO2 component front does not overlap with the contact interface of CO2-crude oil, meaning that the dissolution effect of CO2 in the oil transition zone results in the CO2 component front moving farther. Besides, when the formation pressure is higher than the minimum miscibility pressure (MMP), the distance between the CO2-effective phase front and the CO2-effective component front further expands as the pressure increases, enlarging the miscible zone range. The pressure accumulates around the injection well because of slow pressure conduction. When the average formation pressure reaches 1.1 MMP, the miscible zone range is enlarged by 2.7% higher than that of the near miscible flooding (0.92 MMP), leading to a higher rate of oil recovery by 8.6% and a utilization factor of CO2 by 0.14t/t. It is for the first time that the range of five zones and the characteristics of four CO2 fronts migration is assessed, furnishing an in-depth understanding of the complicated mechanisms and phase behavior in CO2 EOR in the ultra-low permeability oil reservoir. This work contributes to providing significant information for designing an economic and environmental CO2 flooding strategy and is significant in the improvement of oil recovery and the reduction of CO2 emission.
Published Version
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