Small pore sizes on the order of nanometers in the shale gas and tight oil reservoir formations can lead to a large capillary pressure. The presence of capillary pressure significantly affects both the thermodynamic behavior of fluid mixtures and the fluid flow process. Although there have been some attempts to study the effect of capillary pressure on phase behavior, it has not been clearly understood in the application considering multiple components in tight oil reservoirs. In this work, we present a methodology to calculate the phase behavior of CO2/hydrocarbon systems in the presence of capillary pressure. We modify the Peng-Robinson equation of state considering inequalities of hydrocarbon liquid and vapor pressures. The criterion of Gibbs free energy minimization and Rachford-Rice flash calculation are applied in the phase equilibrium calculation. The Young-Laplace equation is utilized to calculate capillary pressure. The Newton-Raphson method is used to solve the nonlinear phase equilibrium equations. We validate the methodology against two experimental measurements and a published numerical model. Subsequently, binary mixture and one typical fluid from the Bakken Formation are used to study the influence of capillarity in the unconventional reservoir. The simulation results indicate that capillary pressure plays an important role in the phase equilibrium calculation when pore size is less than 50 nm. Additionally, the bubble-point pressure of Bakken oil reduces nearby 500 psi when the nano-pore size is 10 nm. The developed method can address the thermodynamics governing unconventional reservoirs and provide better understanding of the phase behavior of CO2/hydrocarbon systems in the case of CO2 injection into unconventional reservoirs.

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