Shear reactivation of pre-existing fractures can play a crucial role in hydraulic stimulation to enable production from unconventional shale reservoirs. However, the mechanisms of permeability enhancement contributed by fracture shear slip are still poorly understood, and the laboratory experiments on shale fractures are insufficient. In this work, injection-induced shear tests were conducted on two reservoir shale samples (from the depth >10,000 ft.) each having a single rough fracture to characterize permeability evolution during fracture shear slip. In the tests, remarkable enhancement of flow rate/permeability have been achieved on both samples through fracture shear slip induced by elevating injection pressure. It is shown that ∼100 times increase in flow rate was induced by a ∼0.1 mm scale of shear slip on the two cylindrical shale samples (with ∼38 mm diameter). The permeability enhancement was retained on the sheared samples even with the decline of fluid pressure. This means that the permeability increase by fracture shear slip may be permanent. In addition, significant stress drops were induced by fracture shear slip in both tests, resulting in further fracture aperture/permeability enhancement. In one sample, a new fracture plane was formed during the shear slip of the original pre-existing fracture, which may help generate an interconnected fracture network. Therefore, dilatant shear slip, stress drop, and the new fractures formation are important and often integral mechanisms of permeability enhancement contributed by pre-existing fractures during shale reservoir stimulation. The results improve the understanding of shear slip in enhancing permeability of shale fractures, and would help engineer solutions for maintaining these fractures open, reducing costs (proppant/water and additive cost savings).
Shear slip of pre-existing fractures has been recognized as a major permeability creation mechanism of reservoir stimulation for a long time (e.g. Mayerhofer et al., 1997; Pine & Batchelor, 1984; Rutledge et al., 2004; Zoback et al., 2012). Most reservoir rocks contain abundant pre-existing fractures, some of which may be sealed with calcite or other infill minerals. Usually, these fractures are inactive and without sufficient permeability before stimulation. A number of modeling and field studies have shown that when the pre-existing natural fractures are favorably oriented with respect to the in-situ stresses, an injection or the leak-off from a hydraulic fracture (even with low fluid pressure) can cause the fractures sliding and propping due to asperities. Also, the interpretations of induced microseismic events during hydraulic injection have indicated that shear slip of pre-existing fractures around hydraulic fractures can help generate a large stimulated reservoir volume and benefit the production performance (e.g. Fisher et al., 2004; Mayerhofer et al., 2010). However, the fundamental mechanisms of permeability creation by fracture shear slip underlying in shale reservoir stimulation are still poorly understood.
