CO₂ Mobility and Transitions between Co-Current and Counter-Current Flows

In this work, we study the modeling of CO₂ migration/mobility in two-phase displacements of relevance to sequestration processes in saline aquifers. The main focus of this paper is on accurate modeling and robust numerical implementation of transitions between co-current and counter-current flows in fully implicit simulation of multiphase flow.

Several experimental and modeling studies have demonstrated that relative permeability decreases when multiphase flow transitions from co-current to counter-current settings in strongly water-wet porous materials. However, the associated implications in the context of CO₂ sequestration design and optimization have not yet been studied or implemented in commonly available reservoir simulation tools.

In this study, the relative permeability of each phase is considered to be flow dependent, and a continuous transition between co-current and counter-current relative permeability functions (including hysteresis) has been implemented and tested in a fully implicit reservoir simulator. The transition is represented as a scanning loop between the bounding co- and counter-current relative permeability functions. This allows us to gauge the impact of transitions from co-current to counter-current flow on the vertical movement of CO₂ and its impact on sweep efficiency, plume migration and entrapment in sequestration process.

Our formulation/implementation of flow dependent relative permeability functions is demonstrated to maintain the numerical stability of conventional fully implicit schemes. However, the total simulation time increases slightly because of the decrease in the vertical mobility and the associated increased travel distance prior to full phase segregation. Furthermore, the footprint of CO₂ in the top layers of a reservoir, as predicted by traditional and by our new approach, is shown to be quite different. The results and analysis presented in this paper demonstrate that it is necessary to consider/integrate this effect in numerical calculations whenever transitions between co-current and counter-current flow may occur.