--> All fractures are not equal: The effect of fracture type on reservoir permeability

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

Datapages, Inc.Print this page

All fractures are not equal: The effect of fracture type on reservoir permeability

Abstract

The default image of natural fractures in reservoirs, i.e., that of easily-counted, high-permeability planes, is not universally nor even commonly valid. Moreover, the two basic, most common fracture types, shear and extension fractures, create significantly different fracture permeabilities, form significantly different permeability networks, and have different susceptibilities to damage during production. Fracture type must be correctly recognized in order to correctly predict and model the effects of the fractures on a hydrocarbon reservoir.

Unless mineralized, extension fractures have relatively uniform apertures that provide good permeability pathways. They form sets of parallel planes that create highly anisotropic drainage in a reservoir. Most extension fractures terminate at relatively minor mechanical boundaries and therefore do not typically enhance vertical permeability across bedding. During production, extension fracture apertures will not close significantly unless the maximum in situ compressive stress has changed orientation since fracturing. Hydraulic stimulation fractures in the same stress setting will propagate parallel to the natural fractures, connecting few of them and in fact potentially damaging them. Multiphase stress systems can create superimposed sets of extension fractures with more radial drainage patterns and more complex interactions with stimulation fractures. In contrast, shear fractures offer more irregular apertures that provide less efficient conduits for fluid flow. However they commonly form as conjugate sets that are better interconnected than extension fractures, creating more isotropic drainage areas and offering better connectivity to a wellbore. Shear fractures also more commonly cut across bedding, offering better vertical connectivity within a reservoir. Shear fractures form oblique to the maximum in situ compressive stress and are therefore subject to shear during production. Shear initially enhances but ultimately damages permeability along a fracture. Shear fractures have more complex interactions with hydraulic stimulation fractures, with more opportunities for damaging high treatment pressures and sand screen-outs.