Advances in horizontal drilling and multistage hydraulic fracturing have unlocked tight oil resources, such as the Montney Formation in Western Canadian Sedimentary Basin. However, the average oil recovery factor after primary production is 5-10 % of the original oil in place. The aim of this study is to investigate phase behavior and estimate minimum miscibility pressure (MMP) of the Montney oil-natural gas systems. The gas samples used in this study are methane (C1) and mixtures of methane and ethane (C1/C2).

To achieve these objectives, we first measure MMP of the oil-gas systems using vanishing interfacial tension (VIT) technique. Second, we perform constant composition expansion (CCE) tests to study phase behavior of the oil-gas systems using a pressure-volume-temperature cell. Finally, we use the measured CCE data to calibrate Peng-Robinson equation of state (EOS) and estimate MMPs for the oil-gas systems using ternary diagrams. The results suggest that the dominant mechanism for developing miscibility conditions for oil-C1 and oil-C1/C2 systems is vaporizing and condensing gas drive, respectively. According to the results of VIT tests, increasing C2 mol% in the gas mixtures significantly reduces the MMP of the oil-gas system (from 4366 psi for oil-C1 to 1467 psi for oil-C1/C2 with 71.3 mol% C2), and increases the oil swelling factor. The MMP values predicted by plotting two-phase equilibrium data on ternary diagrams appear to be in good agreement with the measured ones.

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