Phase-state identification bypass method for three-phase thermal compositional simulation

In this paper, we present a novel strategy for phase-state identification that can be used to bypass the need for full Equation-of-State Computations in multicomponent, multiphase thermal-compositional displacement processes. Analysis based on the Method of Characteristics (MOC) indicates that the displacement path in compositional space is determined by a limited number of tie-simplexes. Our "bypass" method uses information from the parameterized extensions of these "key" tie-simplexes. The parameterization is performed in the discrete phase-fraction space. In this space, the phase fractions can be negative, or greater than unity; this allows for parameterization of the hyperplane that corresponds to a tiesimplex extension. Once the tie-simplexes extension is discretized, a conventional three-phase flash is used adaptively to compute the phase states at the discrete points in compositional space. If all discretization nodes for a given discrete cell that lie on the hyperplane have the same phase state, this state is assigned to the entire computational cell, and flash calculations are bypassed for the compositions of that cell. Here, we make use of the continuity of the tie-simplex parameterization, which was proven in earlier work. We demonstrate the efficiency and robustness of our "bypass'" strategy for the simulation of flow and transport in thermal, three-phase compositional models of heterogeneous reservoirs.