A Comparative Study of Reduced-Variables-Based Flash and Conventional Flash
- Michael Michelsen (Technical University of Denmark) | Wei Yan (Technical University of Denmark) | Erling H. Stenby (Technical University of Denmark)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- January 2013
- Document Type
- Journal Paper
- 952 - 959
- 2013. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.2.2 Fluid Modeling, Equations of State
- 2 in the last 30 days
- 286 since 2007
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For compositional transient simulations including compositional reservoirsimulations, phase-equilibrium calculation, often formulated as a flashproblem, can be time consuming. It is therefore important to speed up thecalculation of phase equilibrium to improve the efficiency of the simulator.The reduced-variables methods, or the reduction methods, reformulate theoriginal phase equilibrium problem with a smaller set of independent variables.Various versions of the reduced-variables methods have been proposed since themid-1980s. The methods were first proposed for cubic equations of state (EOSs)with zero binary interaction parameters (BIPs) and later generalized tosituations with nonzero-BIP matrices. Most of the studies in the last decadesuggest that the reduced-variables methods are much more efficient than theconventional flash method. However, Haugen and Beckner (2011) questioned theadvantages of the reduced-variables methods in their recent paper. A faircomparison between the reduced-variables-based flash and the conventional flashis not straightforward because it is difficult to formulate the former asunconstrained minimization problems, and the flash calculation time is alsorelated to the implementation quality. With the recent formulations by Nichitaand Graciaa (2011), it is possible to code the reduced-variables methodswithout extensive modifications of Michelsen's conventional flash algorithm. Aminimization-based reduced-variables algorithm was coded and compared with theconventional minimization-based flash. A test with the use of the SPE 3 example(Kenyon and Behie 1987) showed that the best reduction in time was less than20% for the extreme situation of 25 components and just one row/column withnonzero BIPs. A better performance can be achieved by a simpler implementationdirectly using the sparsity of the BIP matrix.
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