This paper describes a general formulation for phase-component partitioning that can accommodate any number of phases and components, any component existing in any phase, and requires no special ordering of phases or components. This type of formulation is desirable for flexibility in reservoir simulation but has not previously been used in commercial simulators due to its complexity and inefficiencies in both memory and speed. Here we describe an efficient natural-variable-based general formulation approach, which handles general partitioning of phase-component, consumes no extra memory, and only has a small amount of CPU overhead. This general formulation approach was developed as part of a next generation reservoir simulation project (DeBaun et al., 2005), for "next generation" is the ability to accommodate fluid models currently used in reservoir simulation as well as models that will be developed in the future. With this general formulation approach, we can model most reservoir physics with a single, unified code base, rather than custom one-off extensions of a more limited one. Currently we have tested and validated our approach on standard black-oil and compositional models, thermal steam model, and 4-phase 4-component Todd-Longstaff model.

We describe our natural variable based nonlinear system which incorporates general phase-component partitioning. We analyze the complexity of our model and explain implementation details required to minimize, and in some cases eliminate, CPU and memory overheads. We conclude with the validation of our approach and performance comparisons and analysis (all against standard implementations) for various types of field case studies.

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