A new four-phase adaptive implicit method for compositional reservoir simulation

Abstract

Estimating oil and gas production from assets is essential to carry oil recovery processes. From the many techniques and tools used for such a task, the compositional simulation model is important for miscible displacement and problems with complex phase behavior. In this model, the fluid flow in porous media is described by a set of conservation equations. The solution of these problems involves spatial and time discretization schemes and approaches for handling the coupling of fluid flow and phase behavior. As a consequence, several solution algorithms arise from combining different selection of primary variables and equations, phase behavior decoupling techniques, and spatial/time discretization, which define the computational performance of these algorithms. In this work, a new Adaptive Implicit Method (AIM) is proposed by combining a global intensive variables Fully Implicit (FI) formulation and an IMPEC (IMplicit Pressure, Explicit Compositions) approach. In this approach, gridblocks are dynamically selected as FI or IMPEC based on a stability analysis algorithm. The fully implicit part considers pressure, water saturation, and overall compositions as primary variables and the IMPEC approach considers pressure and total number of moles for each component as primary variables. A new stability analysis algorithm is proposed and used for up to four-phases, but it is general enough to be used for any number of phases. The eigenvalues of the amplification matrix are used for the stability analysis and are computed using the Power's iteration method. The new approach is implemented in the in-house simulator called UTCOMPRS. The AIM formulation is compared to the fully implicit and IMPEC versions. We observed considerable improvement in the computational performance of UTCOMPRS with the new AIM when compared to the pure FI and IMPEC formulations.