The hydraulic fracture – natural fracture network configuration in shale reservoirs: Geological limiting factors

Abstract

When a hydraulic fracture (HF) is propagated in a shale reservoir, it may deflect and branch via intersections with natural fractures (NF) that exist in the shale, resulting in a dispersed HF-NF network and an enlarged simulated rock volume. This article explores geomechanical and geological factors that influence the HF-NF network configuration.NF in unconventional shale reservoirs are commonly infilled with mineral cement or have extremely narrow apertures. They are poor conduits for fluid movement but can be weak surfaces in the rock body that may open during HF stimulation. A HF can dilate the NF and deflect into the plane of a NF that it intersects only if the HF fluid pressure at the intersection exceeds the in situ normal stress resolved on the NF surface plus the strength of any mineral cement. The in situ stresses, the tensile strength of the NF and the net pressure (Pnet = HF fluid pressure -Shmin) at the propagating front of the HF set limits on the range of NF orientations that can dilate and deflect the HF. A resolved stress plot facilitates assessment of the many geomechanical factors affecting the HF-NF interaction.The overall HF-NF network is composed of the main HF plus HF-NF branches comprising NF that have dilated connected by HF segments. A series of geometric models of NF populations are used to demonstrate how the orientation and size distribution of the NF in the shale, and the intensity of their development, limit the lateral spread of these branches. A broad dispersion of the HF-NF network is favored by a low in situ differential horizontal stress (ΔSh = SHmax-Shmin) and a high intensity of large and weakly cemented NF that are favorably oriented for dilation. Contrary conditions narrow the potential dispersion of the HF-NF network.