Forbes Inskip, ND;
Meredith, PG;
Chandler, MR;
Gudmundsson, A;
(2018)
Fracture Properties of Nash Point Shale as a Function of Orientation to Bedding.
Journal of Geophysical Research: Solid Earth
, 123
(10)
pp. 8428-8444.
10.1029/2018JB015943.
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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 (K_{Ic}). 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 K_{Ic} 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 K_{Ic} values to account for deflection by calculating mode‐I and mode‐II deflection stress intensities (K_{Id} and K_{IId}, respectively). Because of the mixed‐mode nature of deflected fractures, we adopt a fracture (G_{c}) 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 K_{Ic}, can be used to estimate values of G_{c} at angles between the Short‐transverse and Arrester orientations.
| Type: | Article |
|---|---|
| Title: | Fracture Properties of Nash Point Shale as a Function of Orientation to Bedding |
| Open access status: | An open access version is available from UCL Discovery |
| DOI: | 10.1029/2018JB015943 |
| Publisher version: | https://doi.org/10.1029/2018JB015943 |
| Language: | English |
| Additional information: | © 2018 The Authors. This is an open access article under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/). |
| Keywords: | shale, anisotropy, fracture toughness, fracture energy |
| 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/10060114 |
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