| Reservoir processes generally requiring compositional modeling are depletion and/or cycling of volatile oil and retrograde gas condensate reservoirs and miscible flooding with multiple-contact-miscibility generated in-situ. Most compositional models described in the literature use explicit transmissibilities. Because of the complexity of the equations and the use of a large number of components, the explicit formulation has been the only feasible approach to field-scale simulation. The drawback of the explicit formulation is the severe time step size limitation which excludes its application in single well models that are used to study productivity impairment as a result of condensate dropout around the well. Some attempts have been made to develop a fully-implicit equation of state model. However, the application of such a model has been restricted to very small problems because of the prohibitive cost associated with it.; We present a novel highly efficient approach to the discretization and solution of the flow and thermodynamic equations describing multicomponent fluid flow in a porous medium. Comparisons with a standard fully-implicit compositional simulator, which utilizes Newton's method, indicates that typical execution times for our simulator are five to seven times faster than the fully-implicit model, and that these speeds are achieved with no degradation in accuracy, stability or robustness. In fact, our simulator does not experience convergence difficulties (oscillations) during phase transitions (crossing phase envelopes) that have been routinely observed in other simulators. The remarkable speed achieved by our simulator (with time step sizes controlled by accuracy requirements rather than stability requirements) arises from our radically novel approach to the linearization and solution of the discretized flow equations and thermodynamic equilibrium equations. An incremental gain in efficiency is also achieved by solving the minimum number of equations necessary for accurate flow description at each time step. For an {dollar}Nsb c{dollar}-component system, our model solves a maximum of {dollar}Nsb c{dollar} equations at each gridblock containing two phases (e.g., oil and gas). This reduction in equations is achieved by rigorously and explicitly incorporating the thermodynamic equilibrium equations in the component mass balance equations at the beginning of each time step. |
