Linear Elastic Fracture Mechanics Characterization of an Anisotropic Shale

Abstract

The existence of bedding planes in natural shale formations makes the fracture characterization remarkably complicated. To achieve a further understanding of the anisotropic crack extension behaviors of shale using a linear elastic fracture mechanics approach, four groups of three-point bending tests on Longmaxi shale from southeast Chongqing were conducted in this study with different bedding plane inclination angles. The fracture propagation paths were observed using a scanning electron microscope. The results indicated that cracks initiated along the bedding plane when the bedding plane inclination angle (i.e., the angle between the loading direction and the normal direction of the bedding plane) was relatively large; in contrast, cracks penetrated into the matrix and induced higher fracture toughness in cases with lower bedding plane inclination angle. Brittle fractures occurred in the tested shale, and the fracture strength was strongly dependent on the bedding plane inclination angle. Meanwhile, the stress field around the crack tip was analyzed theoretically. The results indicated that the crack tip stress field of anisotropic shale is not only determined by the stress intensity factor but also related to the elastic constants and bedding plane inclination angle. Furthermore, a criterion for determining whether a crack extends along the bedding plane was developed by distinguishing the differences in the strengths of the shale bedding and the matrix.

Correlative Optical and X‐Ray Imaging of Strain Evolution During Double‐Torsion Fracture Toughness Measurements in Shale

“…More recently, a number of groups have addressed fracture toughness measurement in shales, with studies by Lee et al (), Chandler et al (, ), Chen et al (), Kabir et al (), Luo et al (), and Forbes Inskip et al (). Lee et al () used semicircular bend specimens to measure fracture toughness in the Divider and Arrester orientations in the Marcellus shale, as well as at an angle of 30° to the Arrester orientation.…”

“…Even at the slowest scratch speeds that they test, they found fracture toughnesses in the region of 2 Mpa.m . Luo et al () and Forbes Inskip et al () each used bend specimens to investigate the variation of K Ic in the Arrester, Short‐Transverse, and Divider orientations, as well as at angles between the Arrester and Short‐Transverse orientations. Luo et al () investigated the Chongqing shale, and found K Ic to vary in the ≃0.9–1.6 Mpa.m range.…”

“…Luo et al () and Forbes Inskip et al () each used bend specimens to investigate the variation of K Ic in the Arrester, Short‐Transverse, and Divider orientations, as well as at angles between the Arrester and Short‐Transverse orientations. Luo et al () investigated the Chongqing shale, and found K Ic to vary in the ≃0.9–1.6 Mpa.m range. Forbes Inskip et al () investigated Nash Point shale, and found K Ic to vary in the ≃0.2–0.7 Mpa.m range, with the short‐transverse orientation the weakest and the Arrester and Divider orientations comparable.…”

“…Various theoretical studies (e.g., Garagash, ; Zia et al, ) have demonstrated that K Ic becomes the dominant control on fracture propagation for conditions when the pressure gradient within a fracture has equilibrated, either due to a long timescale or a low fluid viscosity. Recent experimental studies of anisotropy and fracture deflection (Chandler et al, ; Forbes Inskip et al, ; Lee et al, ; Luo et al, ) tend to find strongly anisotropic K Ic values in the region of ≃0.2–1.5 MPa·m , with much lower K Ic values for fracture planes parallel to layering. This K Ic anisotropy is therefore thought to be a possible mechanism for the limitation of hydraulic fracture height (Zia et al, ).…”

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

“…Hydraulic fracturing in shale is important for the sealing capacity of shale layers during natural (Foschi et al, ; Petrie et al, ) and artificial (Elkhoury et al, ) fluid‐expulsion events. The mechanical anisotropy of shale is evident from its fissility in hand samples, from abundant examples of layer‐parallel natural fractures hosted in shales worldwide (Cobbold et al, ; Gale et al, ; Hooker et al, ) and from laboratory experiments indicating that shale has a lower surface energy along bedding planes than across them (Chandler et al, ; Luo et al, ). This anisotropy could therefore guide fracture propagation into a layer‐parallel orientation, improving the effective sealing capacity of shales.…”