A fully implicit compositional model is developed following the Coats approach. Although the fully implicit formulation guarantees a high degree of computational stability, it aggravates the computational efficiency due to the large number of independent variables. To relax this deficiency, a phase-stability analysis routine is included in the model. Adopting the Gibbs free energy as a stability criterion, we can estimate the number of phase present, prior to the flash calculation. If a single phase is detected, the subsequent flash computation routine can be detoured.
The developed model is verified against an established model, an adaptive implicit compositional simulator, GEM. The computationa efficiencies with and without the stability analysis are evaluated to confirm the improvement in the iterative flash calculations. When there exists only one phase, the number of iterations is largely reduced by virtue of the stability analysis. In addition, even in two-phase situations. the flash computation is efficiently performed, because the stability analysis yields proper guesses. It is revealed that the stability analysis improves the computational efficiency regardless of the number of phases.