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  • The mechanical data (loads, displacements) recorded during double torsion experiments on samples of 6 shale materials and a sandstone. These experiments were conducted on the I12 beamline, Diamond Light Source, Harwell as part of beamtimes EE13824-1 and EE13824-2 between 26/02/17 and 03/03/17. The data were collected using the standard double-torsion technique, with a load cell behind the actuator recording applied force. The method and results are described in detail by Chandler et al, (2018,submitted) "Correlative optical and X-ray imaging of strain evolution during Double Torsion Fracture Toughness measurements in shale" The data was collected with the aim of correlating local deformation around a progressing fracture (through X-Ray and optical imaging) with recorded mechanical data from the loading system. The data was collected by M. Chandler, A-L Fauchille, J. Mecklenburgh, H. K. Kim and L. Ma, and was processed by M. Chandler and R. Rizzo. The complete dataset is present.

  • A number of processes, both natural and anthropogenic, involve the fracture of rocks subjected to tensile stress, including vein growth and mineralization, and the extraction of hydrocarbons through hydraulic fracturing. In each case, the fundamental material property of mode-I fracture toughness must be overcome in order for a tensile fracture to nucleate and propagate. Whilst measuring this parameter is straightforward at ambient pressure, estimating the fracture toughness of rocks at depth, where they experience a confining pressure, is technically challenging. Here, we report a new analysis combining results from standard thick-walled cylinder burst tests with quantitative acoustic emission to estimate the mode-I fracture toughness (KIc) of Nash Point Shale at elevated confining pressure, simulating in-situ conditions to approximately 1km. In the most favorable orientation, the pressure required to fracture the rock shell (injection pressure, Pinj) increases from 6.1 MPa at 2.2 MPa confining pressure (PC), to 34 MPa at 20 MPa confining pressure. When fractures cross the shale bedding, the required injection pressures are 30.3 MPa (at Pc = 4.5 MPa) and 58 MPa (at Pc = 20 MPa), respectively. Applying the model of Abou-Sayed (1978) to estimate initial flaw size, we calculate that this pressure increase equates to an increase in KIc from 0.6 MPa.m1/2 at 3.2 MPa differential pressure (Pinj - PC) to 4.1 MPa.m1/2 at 22 MPa differential pressure. We conclude that the increasing pressure due to depth in the Earth will have a significant influence on fracture toughness, which is also a function of the inherent anisotropy.