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Fracture toughness anisotropy in shale

Chandler, MR; Meredith, PG; Brantut, NPL; Crawford, BR; (2016) Fracture toughness anisotropy in shale. Journal of Geophysical Research: Solid Earth , 121 (3) pp. 1706-1729. 10.1002/2015jb012756. Green open access

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The use of hydraulic fracturing to recover shale-gas has focused attention on the fundamental fracture properties of gas-bearing shales, but there remains a paucity of available experimental data on their mechanical and physical properties. Such shales are strongly anisotropic, so that their fracture propagation trajectories depend on the interaction between their anisotropic mechanical properties and the anisotropic in-situ stress field in the shallow crust. Here we report fracture toughness measurements on Mancos shale determined in all three principal fracture orientations; Divider, Short-Transverse and Arrester, using a modified Short-Rod methodology. Experimental results for a range of other sedimentary and carbonate rocks are also reported for comparison purposes. Significant anisotropy is observed in shale fracture toughness measurements at ambient conditions, with values, as high as 0.72MPam1/2 where the crack plane is normal to the bedding, and values as low as 0.21MPam1/2 where the crack plane is parallel to the bedding. For cracks propagating non-parallel to bedding, we observe a tendency for deviation towards the bedding-parallel orientation. Applying a maximum energy release rate criterion, we determined the conditions under which such deviations are more or less likely to occur under more generalized mixed-mode loading conditions. We find for Mancos shale that the fracture should deviate towards the plane with lowest toughness regardless of the loading conditions.

Type: Article
Title: Fracture toughness anisotropy in shale
Open access status: An open access version is available from UCL Discovery
DOI: 10.1002/2015jb012756
Publisher version: http://dx.doi.org/10.1002/2015jb012756
Language: English
Additional information: Copyright © 2016. American Geophysical Union. All Rights Reserved.
Keywords: shale; fracture; anisotropy; fracture mechanics; shale gas; fracture toughness
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1476712
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