Gravity Segregated Flow in Surfactant Flooding (SPE 154495)

The main recovery mechanism in surfactant flooding is improved microscopic displacement achieved by suppressing pore-scale capillary forces by approximately four orders of magnitude through reduced interfacial tension (IFT). Effects on macroscopic mechanisms like capillary trapping in presence of heterogeneities or gravity segregation are normally not considered. The influence of capillary forces on segregated flow behind the displacement front is investigated by numerical simulations in homogeneous and heterogeneous models and by steady-state upscaling. The positive effect of gravity segregation is that oil floats up, accumulates under low permeable cap rocks and thereby increases the effective horizontal oil mobility. Capillary forces act against this segregation. These mechanisms are not captured in normal coarse-gridded field models. Simulations in homogeneous layer models indicated up to 20% incremental oil production from a moderate IFT reduction (1 mN/m). More field relevant heterogeneous descriptions decreased incremental recovery down towards 5%. Gravity segregation is observed below a critical rate, depending on phase density difference, vertical permeability and layer thickness. All pertinent parameters are combined into a dimensionless viscous-gravity ratio, Rvg. The condition for gravity segregation is Rvg<1. At lower rate the oil recovery approached an upper limit obtained from upscaling under gravity-capillary equilibrium conditions. This limit was represented in terms of the dimensionless Bond number, NB. The oil production was found to be sensitive to IFT when 0.1<NB<10. The magnitude of the incremental oil production was found to increase with increasing curvature of the oil relative permeability, hence, the positive effect of decreasing IFT is larger in mixed-wet formations than in water-wet formations. The presented results and methods can be used to evaluate the positive effect that reduced IFT has on segregated flow behind the displacement front and thereby increase the potential for surfactant flooding.