A Reinterpretation of Fracture Toughness from Fluid Injection Testing

Yue Gao, Robin Eve, Robert Heller, Ion Ispas, John McLennan, Emmanuel Detournay

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

There is an long-standing debate in the rock mechanics community about the dependence of fracture toughness KIc on the confining stress. One of the tests used to assess this dependence relies on injecting fluid into a slotted borehole drilled along the axis of a cylindrical sample and interpreting the toughness from the breakdown pressure (Abou-Sayed et al. 1978). However, the interpretation of the observed breakdown pressure relies critically on assuming that (i) the fluid pressure in the slots and the cracks ahead is uniform and (ii) the peak (breakdown) pressure is the fracture initiation pressure. The model described in this paper challenges these assumptions by considering a fluid lag at the tip of the hydraulic fracture growing from the pre-existing notches and by incorporating the hydraulic compliance of the injection system. This model indicates that the peak pressure generally differs from the fracture initiation pressure due to an episode of stable hydraulic fracture propagation following initiation. The difference between the peak pressure and the fracture initiation pressure increases with the fluid viscosity and the confining pressure, which leads to an artificial dependence of the toughness on the confining pressure if the peak pressure is interpreted as the initiation pressure. Comparison between the predicted and experimental peak pressure for a series of tests on cement samples indicates similar trends between experiments and numerical simulations. However, the predicted peak pressure generally underestimates the experimental one.

Original languageEnglish (US)
Title of host publication55th U.S. Rock Mechanics / Geomechanics Symposium 2021
PublisherAmerican Rock Mechanics Association (ARMA)
Pages375-381
Number of pages7
ISBN (Electronic)9781713839125
StatePublished - 2021
Event55th U.S. Rock Mechanics / Geomechanics Symposium 2021 - Houston, Virtual, United States
Duration: Jun 18 2021Jun 25 2021

Publication series

Name55th U.S. Rock Mechanics / Geomechanics Symposium 2021
Volume3

Conference

Conference55th U.S. Rock Mechanics / Geomechanics Symposium 2021
Country/TerritoryUnited States
CityHouston, Virtual
Period6/18/216/25/21

Bibliographical note

Funding Information:
The authors acknowledge the support of bp, in sponsoring the research that laid the foundation for this work, as well as for providing the experimental data and permission to publish it. Support of YG and ED to conduct the analysis reported in this paper was provided by the T.W. Bennett Chair in Mining Engineering and Rock Mechanics.

Publisher Copyright:
© 2021 ARMA, American Rock Mechanics Association

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