Research into the nature of fracture relative permeabilities and their effect on the performance of naturally fractured reservoirs is described. Flow of wetting phase along the fracture walls provides a means of simultaneous two-phase flow in a single fracture, under conditions where such flow is otherwise prohibited. However, fracture conductivity remains low for simultaneous flow of two phases. Asperities, locations where the fracture walls close, reduce relative permeabilities; the manner of this reduction depends on how aperture is distributed spatially along the fracture.
The effect of fracture relative permeabilities on reservoir performance is illustrated using the waterflood case of the Sixth SPE Comparative Simulation Project (SPE 6) as a basis of comparison. Reduced fracture relative permeabilities impair reservoir performance if they control the attainable flow rate at production wells; such is the case upon a roughly fivefold reduction in relative permeabilities in the SPE 6 project. Reduced fracture relative permeabilities can actually improve reservoir performance in some cases, however, by improving reservoir sweep. The sweep improvement results from an increase in lateral pressure gradient relative to gravity forces driving gravity segregation on a reservoir scale. In both cases, the effect of reduced fracture relative permeabilities is similar to a comparable reduction in absolute permeability of the fracture porespace.