In this work, we are investigating the effect of flow reversals from co-current to counter-current flow on the displacement performance of simultaneous water and gas (SWAG) injection processes. In simultaneous injection processes, gas migrates towards the top of the formation while water tends to move towards the bottom of the formation. The gravity segregation between gas, oil and water will result in counter-current flow occurring in the vertical direction in the reservoir. Experimental and theoretical studies have reported that relative permeability to each phase is less during counter-current flow as compared to co-current flow. The significance of this reduction depends on the gravity number. A reduction of relative permeability in the vertical direction results in anisotropy in relative permeability. This effect has to the best of our knowledge not previously been considered in the simulation of EOR processes.
To investigate the impact of flow reversals on displacement performance, we have implemented a new model that accounts for mobility reductions during counter-current flow in a three-phase compositional, IMPES simulator. A range of displacement settings, covering relevant gravity numbers and miscibility conditions (near-miscible and multicontact miscible), have been investigated to gauge the impact of mobility reductions due to flow reversals. The conventional mobility model (ignoring the impact of counter-current flow) is compared to the proposed mobility model and we demonstrate significant differences in the predicted displacement performance in terms of saturation distributions and recovery estimates. Accordingly, we recommend that an explicit representation of flow transitions between co-current and counter-current flow should be included to ensure accurate optimal design of SWAG processes.