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Under the contradiction of the fast depletion of oil production from conventional reservoirs and the increasing energy demand, unconventional oil and gas reservoirs, such as tight oil, tight gas and shale gas, have played more and more important role in the world’s energy supply. Gas injection process has contribute one of the most efficient oil recovery methods on tight oil recovery practices. To reveal the oil recovery mechanisms and the field applicability of gas injection technique, Supercritical CO2 (ScCO2) and N2 injection process in oil saturated tight cores were comparatively studied in this paper.
The Bentheimer sandstones with different permeability of 0.02mD, 0.5mD and 5.6mD were employed as the tight sample cores, and the gas injection processes were performed under two backpressure conditions of 15MPa and 25MPa at temperature of 50℃. Through quantitative comparisons of the oil recovery factor and the displacement pressure difference parameters, the oil recovery efficiency and mechanisms of ScCO2 and N2 flooding processes has been comprehensively investigated.
The experimental results show that,
1)The recovery efficiency of ScCO2 injection process increases with the increase of core permeability and system pressure. Under the system back pressure of 15MPa, the oil recovery rate increases from 59% to 91% with the increase of permeability. At the elevated system pressure of 25MPa, the recovery efficiency of CO2 displacement could reach the level of 84%~92%. Larger pore-throat sizes in the higher permeable cores as well as the higher miscibility content at higher pressures could contribute to above experimental observations.
2)The recovery efficiency of N2 injection process increases with the increase of core permeability and system pressure as well. Under the system back pressure of 15MPa, the oil recovery rate increases from 26% to 40% with the increase of permeability. At the elevated system pressure of 25MPa, the recovery efficiency of N2 displacement could approach to 21%~52%. Large pore-throat sizes in the higher permeable cores as well as the lower interfacial tension (IFT) between oil and gas phase at higher pressures could contribute to above experimental observations.
3)Under the same experimental conditions, ScCO2 injection shows unanimously and remarkably higher oil recovery rate compared with the N2 injection process, indicating the advantage and the potential application of the miscible ScCO2 injection process on tight oil reservoir development practices.
It is expected this work could help the reservoir applicability investigations of the gas injection processes based on the oil recovery effects as well as the in-depth understanding of the mechanisms of ScCO2 and N2 displacement in tight oil reservoirs.
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