The Characteristics of Open Fractures in Carbonate Reservoir Rocks

Abstract

The large heterogeneity in fractured carbonate rocks is caused by fractures superimposed on textures resulting from deposition and diagenesis. An accurate description of fractures including fracture formation and the effect of diagenesis on fracture properties is necessary to predict the fluid flow. Open dilational fractures observed in reservoir rocks are commonly partly cemented and often small parts of the fracture planes contribute to hydrocarbon storage and fluid flow. The open part of a fracture generally shows significant variation in aperture. Shear fractures may also be preserved open representing pathways to fluid flow in the subsurface, and the shape of the fracture plays an important role for the aperture variation. It is suggested that the flow in a single fracture is channelized, where most of the fluid flow takes places in a few dominating channels. The formation of flow channels in the fracture planes is caused by a combination of diagenesis and shear displacement. The exchange of flow between fractures and matrix is controlled by the degree of cementation on the fracture surfaces. The importance of channelized and anisotropic flow in single fractures should be taken into account when addressing fluid flow in fractured carbonate reservoirs.