Computation of complex phase envelopes with three-phase regions for asphaltenic reservoir fluids: The long and winding road in the search for a perfect algorithm

BENELLI, FEDERICO E.; PISONI, GERARDO O.; CISMONDI-DUARTE, MARTÍN

Congreso; International Conference on Properties and Phase Equilibria for Product and Process Design; 2023

Phase equilibria is of great interest in the oil and gas industry, at either upstream or downstreamprocesses. A widely used approach for computing complex phase envelopes with multiple-phase regionsis based on the utilization of repeated multiphase flash calculations to find the boundaries betweendifferent regions, which is computationally very intensive.A better way of determining phase equilibria regions is by calculating the whole phase envelope withefficient mathematical methods like the continuation method proposed by Michelsen [1] for two-phaseenvelopes.In the case of reservoir fluids, their compositional complexity can in some cases give place to complexbehaviors, due to the high asymmetry in the system, caused by the presence of not only simplehydrocarbons, but also polar compounds like water, methanol or other additives, sometimes high contentsof CO2 and heavy compounds like asphaltenes. Any of those can cause the origination of a third phase,increasing the complexity of the studied system. A method to trace phase envelopes for asphaltenicsystems with three phase boundaries, where an incipient phase could be either a vapor or a liquid, hasbeen proposed by Cismondi [2]. The methodology is an extension of the well known methods suggestedby Michelsen in the early eighties and discussed in detail also in the book by Michelsen and Mollerup [3].This approach has two important considerations or requirements that must be taken into account:●●Provide a good initialization to start tracing the boundary line.Effectively detect critical points and intelligently readapt the algorithm to take this into account.Developing effective ways to satisfy these two points is crucial to avoid potential incomplete diagramsthat could lead to incorrect interpretations.The algorithmic strategy presented in the original paper suggests the calculation of two biphasicenvelopes, one starting from a low pressure dew point and the other starting from a low temperaturebubble point or a high-pressure liquid-liquid saturation point, depending on the outcome of the first one.If those two lines cross at some point it will provide an excellent estimation to initialize the three-phaseboundary lines, based on the K-factors of the two phases on that crossing point. On the contrary, when nocross is found, another way of initializing the three-phase boundary lines must be designed, depending onwhich phase is the incipient one.Starting from already known cases where one of the previous points is not fully satisfied, we explorepossible solutions to these two problems, beginning from an analysis on why these cases diverge and lateron how these problems can be overcome. We tackle these two scenarios separately, but that does not meanthat both can not happen at the same time in the same system.All the implementations in this work have been structured into a Modern Fortran Library, available forpublic use. Although it may sound surprising that new software is being developed in the Fortranlanguage, in recent years there have been important improvements in the language that make it stillfeasible for scientists and engineers to take advantage of the language’s well known high performance andease to implement mathematical problems [3].[1] Michelsen, M. L. Fluid Phase Equilib. 1980, 4 (1−2), 1−10.[2] Cismondi, M. Phase Envelopes for Reservoir Fluids with Asphaltene Onset Lines: An IntegralComputation Strategy for Complex Combinations of Two- and Three-Phase Behaviors. Energy and Fuels2018, 32 (3), 2742–2748.[3] Michelsen, M. L.; Møllerup, J. Thermodynamic Models: Fundamentals & Computational Aspects,2nd ed.; Stenby, E. H.,Ed.; Tie-Line Publications: Holte, Denmark, 2007.[3] Laurence Kedward et al. The State of Fortran. Computing in Science & Engineering 2022, 24, 63-72.