Dual Porosity, Dual Permeability Formulation for Fractured Reser- voir Simulation

Abstract

This study reviews key physical mechanisms and calculation methods for modelling of fluid flow in North Sea fractured reservoirs. The main matrixfracture fluid exchange mechanisms described are gravity drainage, capillary imbibition and molecular diffusion. Important issues such as capillary continuity between matrix blocks, reinfiltration of fluids from higher to lower blocks and effect of block shape on flow processes are also addressed. Simulation studies of water-flooding in fractured reservoirs are reported for the purpose of identifying the effects of gravity and capillary forces on oil recovery. Included are studies of effects of capillary continuity and degree of wetting. The results show that for intermediately wetted systems, such as the Ekofisk reservoir, capillary continuity has a major effect on oil recovery. Laboratory processes involving high pressure gas injection in fractured systems have been studied by compositional simulation. The results show that changes in interfacial tension caused by diffusion, may have dramatic effects on oil recovery. Computational aspects of fluid exchange processes are discussed, including conventional dual porosity formulation, use of matrix-fracture transfer functions, and detailed numerical calculation. The only solution to more representative modelling of flow in fractured reservoirs is more detailed calculations. A multiple grid concept is proposed which may drastically increase the detail of the simulation.