Effect of Volume Translation on Two-Phase Equilibrium Calculations for Hydrocarbon Mixtures in a Confined Space

• Author / Creator

  Wenran Zhao

• Oil production from tight/shale reservoirs has drawn an increasing attention over the past decades. One of the essential mechanisms affecting the oil/gas production is the phase behavior of the reservoir fluids in the confined nanopores inside the tight/shale reservoirs. Many theoretical works couple the conventional multiphase flash calculation procedure with capillary pressure to study the phase equilibrium of reservoir fluids in nanopores. However, the most widely used Peng-Robinson equation of state (PR-EOS) (Peng and Robinson, 1976), which is applied in the conventional multiphase flash calculations, does not provide an accurate prediction on liquid-phase density. This inherent flaw of PR-EOS weakens the reliability of the calculation algorithm developed to study the phase equilibrium in nanopores because the incorrect phase density prediction leads to incorrect vapor-liquid interfacial tension predictions. Capillary pressure, which relies on vapor-liquid interfacial tension, is thereby calculated with poor accuracy. This work aims to address this issue by developing a new two-phase equilibrium calculation algorithm that couples capillarity effect and volume translation in the conventional multiphase flash calculation. In this work, PR-EOS together with the volume translation method proposed by Abudour et al. (2012) is applied to more accurately predict liquid-phase density. Vapor-liquid interfacial tension is calculated using the Weinaug-Katz model (Weinaug and Katz, 1943). The Young-Laplace equation (Young, 1805) is used to calculate capillary pressure with assumptions of zero contact angle and equal principle radii. Phase behavior of two mixtures in confined nanopores is studied to examine the accuracy and robustness of the two-phase flash algorithm developed in this work. For each mixture, two-phase envelope is calculated using the algorithm developed in this work and the calculated results are then compared with experimental data collected from the literature. It is observed that, at a fixed temperature, the bubble point and dew point pressures of the two mixtures in nanopores are reduced due to capillary pressure compared to those under bulk condition. After the phase densities are corrected by the volume translation method, the bubble point and dew point pressures are reduced to a larger extent at a fixed temperature. Compared to the two-phase flash algorithm without the use of volume translation, the two-phase flash algorithm with the use of volume translation gives a better match to the measured dew point pressures.

• Subjects / Keywords

  • Phase Behavior

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-15s0-2b72

• License

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