Fluid-Driven Tensile Fracture and Fracture Toughness in Nash Point Shale at Elevated Pressure

S. Gehne, N. D. Forbes Inskip, Philip M. Benson, P. G. Meredith, N. Koor

Research output: Contribution to journalArticle

Abstract

A number of key 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 propagate. While measuring this parameter is relatively straightforward at ambient pressure, estimating fracture toughness of rocks at depth, where they experience confining pressure, is technically challenging. Here we report a new analysis that combines results from thick-walled cylinder burst tests with quantitative acoustic emission to estimate the mode-I fracture toughness (KIc) of Nash Point Shale at confining pressure simulating in situ conditions to approximately 1-km depth. 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 are forced to cross the shale bedding, the required injection pressures are 30.3 MPa (at Pc = 4.5 MPa) and 58 MPa (Pc = 20 MPa), respectively. Applying the model of Abou-Sayed et al. (1978, https://doi.org/10.1029/JB083iB06p02851) to estimate the initial flaw size, we calculate that this pressure increase equates to an increase in KIc from 0.36 to 4.05 MPa·m1/2 as differential fluid pressure (Pinj − Pc) increases from 3.2 to 22.0 MPa. 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.

Original languageEnglish
Article numbere2019JB018971
JournalJournal of Geophysical Research: Solid Earth
Volume125
Issue number2
Early online date29 Jan 2020
DOIs
Publication statusPublished - 15 Feb 2020

Keywords

  • Fracture Toughness
  • Shale
  • Tensile Fracture

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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