Development of Robust Equilibrium Calculation Algorithms for Reservoir Fluids at Given Volume, Temperature and Composition

• Author / Creator

  Lu, Chang

• Two-phase and three-phase equilibria can be frequently observed in the petroleum industry. To accurately predict the phase behavior of complex fluid mixtures lies at the core of the analysis and design of petroleum recovery and petrochemical processes. Normally, engineers or researchers rely on isothermal-isobaric (PT) algorithms to carry out the multiphase equilibrium computations for a given fluid mixture. In such algorithms, pressure, temperature, as well as the feed composition, are specified as inputs to the algorithm and the objective is to find out the number of phases in equilibrium, and their fractions, compositions and volumes. One alternative strategy to work out the multiphase equilibrium problem is to use volume, temperature and feed composition as known information, and find out pressure, the number of equilibrating phases and their properties. Such isothermal-isochoric (VT) equilibrium calculations are found to be more convenient in some engineering calculations (e.g., multiphase equilibrium in storage vessels) than the conventional PT equilibrium calculations. Our aim of this research is to develop more, robust, accurate and efficient algorithms to simulate phase equilibria for reservoir fluids using the Peng-Robinson EOS at specified volume, temperature and mole numbers.
  Moreover, VT based phase equilibria calculation has its natural advantages to simulate the fluid phase behavior in nano-confined pores than PT-flash for that a nanopore can be considered as a volume constant system. Therefore, to give a more accurate prediction results, we develop a two-phase flash algorithm with the consideration of capillary pressure at specified mole numbers, volume and temperature. By testing it against a number of examples, and comparing the results with the ones calculated by the pressure-temperature (PT) flash algorithm with the capillarity effect, we demonstrate the correctness and robustness of our algorithm. In addition, we examine the influence of capillary pressure on the two-phase envelope in the molar-density/temperature (cT) space. Example calculations are carried out to demonstrate the superior performance of the developed algorithm.
  In addition to two-phase equilibria, three-phase equilibria are frequently encountered in a variety of petroleum engineering processes. In the second work, we develop a simple, robust VT three-phase flash calculation algorithm using a nested approach. The PT three-phase flash code is used in the inner loop, while an effective equation-solving method is applied to solve the pressure corresponding to a given volume-temperature specification. The robustness of the algorithm is safeguarded with the use of state-of-art trust-region-method-based solver in the PT three-phase flash program. By applying it to calculate the isochores of fluid mixtures, we demonstrate the robustness and efficacy of the developed algorithm in both two-phase and three-phase equilibrium calculations.
  Furthermore, similar to the inaccurate liquid-density prediction issue in the isobaric-isothermal (PT) phase equilibrium calculations, an issue of inaccurate pressure prediction can also be observed in the VT phase equilibrium calculations which involves a liquid phase. In the third work, a practical methodology is proposed to incorporate a volume-translated equation of state for more accurate pressure predictions in VT phase equilibrium calculations. By incorporating Abudour et al. (2012; 2013) volume-translated PR-EOS models into the VT-based phase equilibrium calculation algorithm, the accuracy of pressure prediction in the single liquid phase region for both pure substances and mixtures can be significantly improved. Lastly, we apply the proposed algorithm to the two-phase VT phase equilibrium calculations for the oil sample MY10. We found that more accurate pressure predictions can be obtained by applying the Abudour et al. VTPR-EOS to the VT flash results than those from the original VT-flash calculations.

• Subjects / Keywords

  • Phase split calculation
  • Isothermal-isochoric flash
  • Algorithm development
  • VT flash
  • Multiphase equilibria

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-gkgk-5v84

• License

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