Relative permeability of two-phase flow through rough-walled fractures: effect of fracture morphology and flow dynamics

This paper considers the influence of geometry and flow conditions on the relative permeability of wetting and non-wetting components of multiphase flows through fractures. Using an explicit immiscible two-phase fracture-flow model, 945 multiphase flow simulations were conducted on artificially-generated fracture geometries. These simulations accounted for the effects of initial fracture roughness, confining stress, pressure gradient, surface tension and volume fraction on the fracture flow. Time averaged relative permeabilities were recorded for both the wetting and non-wetting phases in each of the simulations. The simulation results demonstrate that relative permeability curves are strongly influenced by contact area, fracture morphology and competition between viscous and capillary forces. Increasing the normal stress creates more contact in the fracture plane, which increases interference between the fluid phases. The effect is more pronounced in fractures with strongly correlated apertures where the chance of fluid trapping is increased. The relative importance of viscous forces compared to capillary forces was investigated by applying different pressure gradients to the fluids. Increasing the pressure gradient tends to cause the flow paths to segregate and the relative permeability curves approach the linear model, in agreement with experimental results. From these observations, two new relative permeability models were introduced to describe the behavior of wetting and non-wetting phases in fractures. The performance of the new models were compared to other published relative permeability models for fracture geometries. The new models were found to more accurately predict the relative permeability when compared to the other available models for both the wetting and non-wetting phases.