The interaction of propagating opening mode fractures with preexisting discontinuities in shale

Abstract

Field observations show that hydraulic fracture growth in naturally fractured formations like shale is complex. Preexisting discontinuities in shale, including natural fractures and bedding, act as planes of weakness that divert fracture propagation. To investigate the influence of weak planes on hydraulic fracture propagation, we performed Semicircular Bend tests on Marcellus Shale core samples containing calcite-filled natural fractures (veins). The approach angle of the induced fracture to the veins and the thickness of the veins have a strong influence on propagation. As the approach angle becomes more oblique to the induced fracture plane, and as the vein gets thicker, the induced fracture is more likely to divert into the vein. Microstructural analysis of tested samples shows that the induced fracture propagates in the middle of the vein but not at the interface between the vein and the rock matrix. Cleavage planes and fluid inclusion trails in the vein cements exert some control on the fracture path. Combining the experimental results with theoretical fracture mechanics arguments, the fracture toughness of the calcite veins was estimated to range from 0.24 MPa m 1/2 to 0.83 MPa m 1/2 , depending on the value used for the Young's modulus of the calcite vein material. Measured fracture toughness of unfractured Marcellus Shale was 0.47 MPa m 1/2 .

Fracture Properties of Nash Point Shale as a Function of Orientation to Bedding

“…Most sedimentary rocks exhibit some form of structural anisotropy, which is either the result of their depositional environment and the alignment of mineral grains, or is caused by the alignment of pores or microfractures. Such structural anisotropy commonly results in physical and mechanical anisotropy (Chandler et al, ; Chong et al, ; Kabir et al, ; Lee et al, ; Schmidt, ; Young et al, ). Therefore, many sedimentary rocks, including shales, exhibit transverse isotropy parallel to their bedding planes.…”

Shale Anisotropy and Natural Hydraulic Fracture Propagation: An Example from the Jurassic (Toarcian) Posidonienschiefer, Germany

“…Laboratory studies of fracture propagation along bedding in shales suggest that the fracture toughness, which is proportional to the square root of surface energy, may be reduced to roughly half the value across bedding (Chandler et al, 2017; Luo et al, ). Preexisting fractures are also planes of weakness that can deflect fracture paths (Lee et al, ). Sedimentary planes of weakness appear to be the reason for the stepwise propagation of the present veins.…”

Recovery rates of fracturing fluids and provenance of produced water from hydraulic fracturing of Silurian Qusaiba hot shale, northern Saudi Arabia, with implications on fracture network

“…The resulting hydraulic fracture network geometry for Barnett Shale wells is thought to be largely dictated by the interaction of hydraulic fractures with natural fractures (Gale et al, 2007). In the case of the Marcellus Shale, hydraulic fracture growth in shale and naturally fractured formations is more like a complex propagation of multifracture networks than the extension of a single planar fracture (Lee et al, 2015).…”