Hi Dmitry,
thank you for your report, this is very valuable! It would be even more valuable and highly appreciated if you decide to add your model to DuMux. As far as I can see, this would in essence add the Michelsen test and your flash calculation. And the completeFluidState method would branch depending on whether the traditional "PVS" or the new "flash" model is used. Right? Or do you see potential complications? I consider your statement "The primary variables. Those must be the pressure and n-1 (overall) mole fractions. I do not think that it is possible at all to implement a two-phase compositional model with the usual primary variables of the 2pnc model" very bold. The 2pnc PVS model is standard which has been used successfully not only in DuMux to simulate two-phase compositional flow for many years. Nevertheless, I certainly acknowledge the fact that a flash-based model might be superior for particular applications. With the drawback on computational performance due to the more expensive flash calculations for one Newton step. In total, I would heartily encourage you to contribute your model! Kind regards Bernd -- _________________________________________________________________ Bernd Flemisch IWS, Universität Stuttgart phone: +49 711 685 69162 Pfaffenwaldring 61 email: be...@iws.uni-stuttgart.de D-70569 Stuttgart url: www.iws.uni-stuttgart.de/en/lh2/<http://www.iws.uni-stuttgart.de/en/lh2/> _________________________________________________________________ ________________________________ Von: DuMux <dumux-boun...@listserv.uni-stuttgart.de> im Auftrag von Dmitry Pavlov <dmitry.pav...@outlook.com> Gesendet: Donnerstag, 17. März 2022 23:20:09 An: dumux@listserv.uni-stuttgart.de Betreff: [DuMux] Oil-gas compositional simulation in DuMux Hello, This is for anybody who will be wondering someday whether the two-phase oil-gas compositional model is doable in DuMux. The short answer is yes. The numerical algorithms required for such a model are described in many sources (e.g. [1]) and implemented in ECLIPSE and a lot of other programs, including the open-source MRST [2]. Now, what is required for implementing the two-phase compositional model in DuMux: 1. The primary variables. Those must be the pressure and n-1 (overall) mole fractions. I do not think that it is possible at all to implement a two-phase compositional model with the usual primary variables of the 2pnc model [3] (pressure, saturation, n-2 mole fractions). But the good news is that DuMux, for the most part, does not care about what is stored in the primary variables. So it is possible to just treat 2pnc's primary variables as the pressure and n-1 (overall) mole fractions. The only thing that must be redesigned for that to work is VolumeVariables and, in particular, completeFluidState() method that builds the FluidState from the primary variables. 2. In order to build the FluidState from the primary variables, one needs two algorithms: the Michelsen stability test (it decides whether we have one-phase or two-phase composition with this pressure and temperature) and the two-phase flash algorithm (it calculates the mole fractions for each phase from overall mole fractions). Those algorithms are not present in DuMux. DuMux has constraint solvers called MiscibleMultiPhaseComposition and NcpFlash, but they will not work for this kind of problem. Only the "classic" flash algorithm works. One has to implement the Michelsen test and flash in the aforementioned completeFluidState() replacement. These algorithms, in turn, require an implementation of equation of state (EOS), usually Peng-Robinson (PR). 3. Good news: DuMux has a completely suitable implementation PR EOS! One pitfall is that the default behavior of PR in DuMux is "too correct" in its handling of single-root case, which causes Michelsen test to fail. A flag "handleUnphysicalPhase" was added to computeMolarVolume() method to allow to simplify the calculation and make it suitable for Michelsen test. 4. Switching of primary variables. Actually, no, this one is not needed at all. The built-in TwoPNCPrimaryVariableSwitch is not suitable for this problem; it is possible to implement a custom PrimaryVariableSwitch which would be based of the Michelsen test, but it creates more problems than it solves. To be specific, it reduces performance and worsens Newton convergence and gives nothing in return. Phase presence in FluidState is updated in completeFluidState() and it is fine. One must be cautious about the performance because DuMux tends to call the computationally expensive completeFluidState() more often than it is needed to do the job [5]. At least EnableGridVolumeVariablesCache and EnableGridFluxVariablesCache should be turned on to avoid unnecessary recalculations. Also, PrimaryVariableSwitch should be disabled, as said above. Finally, if the box method is used, DuMux calls completeFluidState() on every scv connected to every node, which on a 2D grid is 4x the number of nodes, while in principle, it is possible to complete the task by making 1x the number of nodes calls. Maybe a rewrite of GridVolumeVariables will help to achieve that. For now, oil-gas simulation in DuMux is inferior to MRST in performance. While MRST works on a much slower engine (Matlab), it does not waste time on 4x recalculations, and also uses automatic differentiation, while DuMux uses slow numerical differentiation. One the other hand, DuMux, unlike MRST, can be parallelized via both MPI and threads [6]. This is it, thank you for attention. Regards, Dmitry [1] https://store.spe.org/Phase-Behavior-P46.aspx [2] https://www.sintef.no/contentassets/2551f5f85547478590ceca14bc13ad51/compositional.html [3] https://dumux.org/docs/doxygen/master/a18414.html [4] https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/merge_requests/2831 [5] https://listserv.uni-stuttgart.de/pipermail/dumux/2021q4/002811.html [6] https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/merge_requests/2640
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