Fully implicit two-phase VT-flash compositional flow simulation enhanced by multilayer nonlinear elimination

Abstract

VT flash calculation, with the variable specification of volume, temperature and mole numbers, is a newly-rising alternative to the conventional PT flash calculation for phase behavior modeling. Until now, VT flash is primarily used as standalone calculation to solve phase equilibria problems and few works apply it to compositional flow simulation. In this study, an improved two-phase VT-flash compositional flow algorithm is developed with the multilayer nonlinear elimination method. Compared to the preceding works, the robustness and efficiency of the new algorithm is significantly improved as the nonlinear elimination method implicitly removes locally large nonlinearities and thus restore large step length for Newton iterations. To enhance the computational efficiency, an adaptive time stepping control is used to adjust the timestep size. Moreover, unnecessary stability tests are bypassed using a modified shadow region method, which accommodates to substantial composition change under large time steps. A number of numerical examples are presented to demonstrate the robustness and efficiency of the proposed VT-flash compositional flow algorithm with multilayer nonlinear elimination. Even though the convergence issue is not fully resolved, which roots in the nondifferentiable equilibrium pressure at phase boundary, the occurrence of time refinements is significantly reduced with the help of multilayer nonlinear elimination. It is also found that multilayer nonlinear elimination generally increases the number of Newton iterations slightly but enlarges the timestep size significantly. Thus, the overall computational efficiency of the VT-flash compositional flow simulation is enhanced under the multilayer nonlinear elimination method.