Phase behavior studies of three major unconventional oil reservoirs for produced gas injection



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The injection gas mixture has an important role in the determination of phase behavior and applicability of miscible produced gas injection for unconventional oil reservoirs. This study focuses on three major unconventional reservoirs with considerable EOR potential through produced gas injection: Northern San Andres, Lower Wolfcamp and Eagle Ford. The objectives of this study, in addition to the development and tuning of the corresponding phase behavior models, are to minimize the mole fractions of the valuable intermediate components, and at the same time, accomplish favorable PVT properties for the produced gas injection process. These two objectives may conflict each other; therefore, the use of design optimization is necessary, as we discuss in the results. The separator liquid and gas samples are collected and analyzed using gas chromatography to determine the fluid composition, followed by the recombination test. Reservoir fluid characterization and the bubble point pressure analysis for reservoir fluids are performed. A comprehensive phase behavior analysis is then conducted to determine the number of separator stages and the optimum produced gas composition. A series of separator tests, minimum miscibility pressure (MMP) tests and swelling tests are performed for different injection gas compositions and the data are analyzed to tune the corresponding equations of state and phase envelops for the three reservoirs. The PVT model requires several adjustment stages to properly match the miscibility and swelling tests results. The tuned PVT models properly predict the phase behavior of these three reservoirs for the optimized produced gas injection process. The optimum separator design and injection composition results for three unconventional reservoirs are presented and discussed. The impact of different component mole fractions with suitable injection pressure for miscibility development are successfully quantified. Combining the three individual results, guidelines are proposed to specify corresponding fractions of major components (CO2, N2, C1, C2, C3) in oil and produced gas phases, resulting in potentially favorable conditions for produced gas injection. This work opens opportunities to improve technical assessments of produced gas injection in these three reservoirs as well as other unconventional reservoirs and sets a reference for future EOR operations.



Phase behavior analysis, Produced gas injection