For Cardium tight sandstone reservoir and Monteny liquid rich shale reservoir, horizontal well with multi-stage hydraulic fracturing is applied to develop the oil underground. However, Primary recovery of tight oil reservoirs is still low even with horizontal wells and massive hydraulic fracturing. A CO2 flooding process is regarded as a promising technique for improving tight oil recovery. However, the gas-oil relative permeability during CO2 injection is not fully understood because the nanoscale pore confinement in tight oil reservoirs alters phase behavior of the reservoir fluid.

In this paper, the effect of confinement on gas and oil phase behavior is investigated. Afterwards, the gas-oil interfacial tension and solubility parameter are analyzed, resulting in an alteration in relative permeability during CO2 injection without and with hysteresis. Results show that the relative permeability of oil and gas can be further increased by the confinement effect during CO2 flooding as the interfacial tension of gas-oil can be further decreased in a nanoscale pore. Furthermore, miscibility is enhanced by the confinement effect as the solubility parameters difference between oil and gas decreases. It is consistent with the reduction in gas-oil interfacial tension. In addition, the gas phase trapping caused by hysteresis effect is weakened by the confinement effect, resulting in a lower critical gas saturation and a better oil production during CO2 injection.

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