Multi-model hybrid compositional simulator with application to segregated flow

In cases with significant density difference between the injection mixture and the reservoir fluids, segregation can take place on short temporal scales relative to the typical time-step lengths of the simulation. A fully resolved 3D description is computationally demanding and is often difficult to achieve with buoyancy segregated flow. Vertical equilibrium (VE) is one possible technique for effective upscaling in regions of gravity segregation and has been widely considered for CO2 storage applications. VE has connections to pseudo-relative permeability models for gas injection. In this work, we use a general pseudo-relative permeability model to couple conventional 3D and upscaled models, including VE formulations with support for compositional simulations. This gives a flexible framework where different choices of coarsening and pseudoization can be used locally throughout the domain, allowing for an optimal trade-off between runtime and accuracy. The new approach is demonstrated within the framework of a fully implicit compositional flow simulator with nonlinear equation-of-state, which provides a robust and stable base for inclusion of additional physical effects. Moreover, we discuss the limitations of different possible approximations for the phase equilibrium of the compositional VE regions, and which cases they may be useful for. Possible applications for this methodology include gas injection for enhanced oil recovery, CO2 storage in aquifers, or other gas-storage scenarios. We demonstrate the approach for gas injection and migration on both conceptual unstructured grids as well as corner-point models taken from real fields and saline aquifers from the Norwegian Continental Shelf.