A Multilevel Iterative Method to Quantify Effects of Pore-Size Distribution on Phase Equilibrium of Multicomponent Fluids in Unconventional Plays
- Baoyan Li (Baker Hughes Inc.) | Alberto Mezzatesta (Baker Hughes Inc.) | Yinghui Li (Inpetro Technologies) | Yixin Ma (Oklahoma University) | Ahmad Jamili (Oklahoma University)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- April 2016
- Document Type
- Journal Paper
- 2016. Society of Petrophysicists & Well Log Analysts
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In order to quantify the influence of pore confinement on hydrocarbon phase behavior in the nonuniform pores of organic-rich shale reservoirs, we propose a new phase-equilibrium model and a multilevel iterative method to solve the highly nonlinear system of equations governing the phase equilibrium of hydrocarbon systems subject to pore-confinement effects. Through this new method, we analyze the influence of pore-size distribution on the phase behavior of fluids in organic-rich shale reservoirs, including the Bakken and the Eagle Ford shale formations. The results show that the suppression of saturation pressures of confined hydrocarbons in shale-oil and shale-gas condensate formations can be overestimated by 10% if an average pore size is used for the estimation. Based on the proposed phase-equilibrium model, we observe that pore-confinement effects on the properties of the hydrocarbons encountered in shale-oil and shale-gas gas/condensate formations become significant only at low pressures and temperatures.
Unlike conventional reservoirs, shale oil and gas plays have micropores (<2 nm) and mesopores (2 to 50 nm) in their matrices (Kuila and Prasad, 2011). These small pores may result in capillary pressure values that are large enough to cause changes in reservoir-fluid phase behavior. The conventional flash-calculation method (Whitson et al., 2000) for bulk fluid may not be applicable to hydrocarbons in unconventional oil and gas reservoirs, due to the capillary pressure influence not being considered.Theoretical and experimental studies were performed to investigate the pore-confinement effect on the phase behavior of confined fluids. Firoozabadi (1999) developed a general phase-equilibrium model for the multicomponent fluid confined in a small pore, using fundamental chemical thermodynamic principles. Brusilovsky (1992) investigated the effect of capillary pressure on phase equilibrium of multicomponent fluid systems using an equation of state (EOS) and observed a reduced bubblepoint pressure and an increased upper dewpoint pressure. Guo et al. (1996) also developed a model to calculate such an effect. Nojabaei et al. (2013) demonstrated the expanded phase envelop under pore confinement, using the Peng-Robinson EOS. Firincioglu et al. (2013) investigated the suppression of bubblepoint pressure with core samples of shale-oil plays. Chen et al. (2012) measured the NMR responses of hexane confined in shale core samples. Shapiro and Stenby (1996) extended the Kelvin equation from a single component to a multicomponent equation to model the capillary condensation of the mixture of hydrocarbons in tight formations. Li et al. (2014) applied the thermodynamic theory to investigate the occurring condition of capillary condensation and its impact on the gas-in-place for shale-gas condensate reservoirs.
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