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Solid Earth An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/se-2020-114
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2020-114
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  14 Jul 2020

14 Jul 2020

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A revised version of this preprint is currently under review for the journal SE.

The competition between fracture nucleation, propagation and coalescence in the crystalline continental upper crust

Jessica A. McBeck1, Wenlu Zhu2, and François Renard1,3 Jessica A. McBeck et al.
  • 1Njord Centre, Department of Geosciences, University of Oslo, Norway
  • 2Department of Geology, University of Maryland, College Park, USA
  • 3University Grenoble Alpes, University Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, France

Abstract. Different modes of fracture growth produce fracture networks with distinctive geometric attributes that exert important controls on the extent of fluid-rock interactions. We perform in situ X-ray tomography triaxial compression experiments on monzonite to investigate the influence of fracture nucleation, preexisting fracture propagation, and coalescence on fracture network development in crystalline rocks under crustal conditions. We impose a confining pressure of 20–35 MPa and then increase the differential stress in steps until the rock fails macroscopically. After each stress step we acquire a three-dimensional (3D) X-ray adsorption coefficient field from which we extract the 3D fracture network. To examine the influence of pore fluid on fracture network development, we perform two experiments under nominally-dry conditions and one under water-saturated conditions with 5 MPa pore fluid pressure. We develop a method of tracking individual fractures between subsequent tomographic scans that identifies whether fractures grow from the coalescence and linkage of several fractures or from the propagation of a single fracture. Throughout loading until shortly before failure in all of the experiments, the volume of coalescing fractures is smaller than the volume of propagating fractures, indicating that fracture propagation dominates coalescence. Immediately preceding failure, however, the volume of coalescing fractures is at least double the volume of propagating fractures in the experiments deformed at nominally dry conditions. In the water-saturated sample, although the volume of coalescing fractures increases during this stage, the volume of propagating fractures remains dominant. The influence of stress corrosion cracking associated with hydration reactions at fracture tips and/or dilatant hardening may explain the observed difference in fracture development under dry and water-saturated conditions. Our experimental data on fracture growth at different conditions provide new constraints in assessing fluid flow in subsurface fracture networks that are central to energy and environmental engineering practices.

Jessica A. McBeck et al.

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Jessica A. McBeck et al.

Data sets

Critical evolution of damage towards system size failure in a crystalline rock Francois Renard https://doi.org/10.11582/2017.00025

Volumetric and shear processes in crystalline rock during the approach to faulting Francois Renard https://doi.org/10.11582/2018.00023

Jessica A. McBeck et al.

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Short summary
The competing modes of fault network development, including nucleation, propagation and coalescence, influence the localization and connectivity of fracture networks, and thus are critical influences on permeability. We distinguish between these modes of fracture development using in situ X-ray tomography triaxial compression experiments on crystalline rocks. The results underscore the importance of confining stress (burial depth) and fluids on fault network development.
The competing modes of fault network development, including nucleation, propagation and...
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