Prediction of waterflood performance in mixed-wet systems from pore-scale modelling and simulation

The wettability characteristics of a porous medium play a major role in a diverse range of measurements including; capillary pressure data, relative permeability curves, electrical conductivity, waterflood recovery efficiency and residual oil saturation. This study describes the development and implementation of a pore-scale simulator capable of modelling multiphase flow in porous media of nonuniform wettability. By explicitly incorporating pore wettability effects into a steady-state model, it has been possible to explain many experimental observations from a microscopic standpoint. Results are presented which show how a (the fraction of pores which are assigned oil-wet characteristics) affects resulting capillary pressure and relative permeability curves. Simulated capillary pressure data demonstrate that some standard wettability tests (such as Amott-Harvey and free imbibition) may give spurious results when the sample is fractionally-wet in nature. The corresponding relative permeability curves have been used to calculate waterflood displacement efficiencies for a range of wettability conditions, and recovery is shown to be maximum when the oil-wet pore fraction approaches 0.5. Furthermore, a novel test is proposed which could be used to determine the wettability of both fractionally-wet and mixed-wet porous media. To date, no such satisfactory test exists.