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

Abstract

Understanding how fracture networks develop in shale formations is important when exploiting unconventional hydrocarbon reservoirs and analyzing the integrity of the seals of conventional and carbon capture and storage reservoirs. Despite this importance, experimentally derived fracture data for shale remains sparse. Here we characterize shale from Nash Point in South Wales, United Kingdom, in terms of ultrasonic wave velocities, tensile strength, and fracture toughness (KIc). We measure these properties in multiple orientations, including angles oblique to the three principal fracture orientations—Short‐transverse, Arrester, and Divider. We find that the Nash Point shale is mechanically highly anisotropic, with tensile strength and KIc values lowest in the Short‐transverse orientation and highest in the Arrester and Divider orientations. Fractures that propagate in a direction oblique or normal to bedding commonly deflect toward the weaker Short‐transverse orientation. Such deflected fractures can no longer be considered to propagate in pure mode‐I. We therefore present a method to correct measured KIc values to account for deflection by calculating mode‐I and mode‐II deflection stress intensities (KId and KIId, respectively). Because of the mixed‐mode nature of deflected fractures, we adopt a fracture (Gc) energy‐based approach that allows analysis of critical fracture propagation conditions for both deflected and undeflected fractures in all orientations. We find that Gc increases as the angle from the Short‐transverse plane increases. We conclude that a modified elliptical function, previously applied to tensile strength and KIc, can be used to estimate values of Gc at angles between the Short‐transverse and Arrester orientations.

Seismo‐Mechanical Response of Anisotropic Rocks Under Hydraulic Fracture Conditions: New Experimental Insights

“…This particular (mature) shale was selected partly for its ability to be easily prepared and machined (Forbes Inskip et al, ; Text S1); a selection of samples were scanned and micro‐CT images taken before the hydraulic fracturing experiment to check for preexisting fractures within the sample (Figure SF1). The entire sample assembly is separated from the confining medium using an engineered nitrile jacket fitted with ports for 11 AE sensors evenly distributed over the sample (Figure c; Sammonds, ).…”

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.…”

“…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, ).…”

Fluid Injection Experiments in Shale at Elevated Confining Pressures: Determination of Flaw Sizes From Mechanical Experiments

“…Figure shows the tensile strength, σ T , of a wide range of rock types all plotted as a function of their mode‐I fracture toughness, K Ic . These data were compiled from Zhang (), Schmidt and Huddle (), Chandler et al (), Forbes Inskip et al (), and Chandler et al (). The relationship between σ T and K Ic should be independent of the method used to determine each parameter, provided the samples were of sufficient size in each case.…”

“…Tensile strength, σ T , as a function of mode‐I fracture toughness, K Ic , for a range of rock materials. The nonshale materials are from Zhang () and Chandler et al (), while the shale materials are from Schmidt and Huddle (), Chandler et al (), Forbes Inskip et al (), and Chandler et al (). The dashed and solid lines are least squares fits made to the nonshale and shale data sets, respectively (and forced to intercept the origin).…”