Full Length Article
A comparative study of stress influence on fracture apertures in fragmented rocks

https://doi.org/10.1016/j.jrmge.2018.05.003Get rights and content
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Highlights

  • Fracture aperture change due to applied stresses is calculated using two methods.

  • The 1st method projects far-field stresses to fractures in a 2D fracture network.

  • The 2nd method considers 3D stress distribution and layer and block interactions.

  • Using far-field stresses to predict apertures can cause errors in the results.

Abstract

This study compares the calculated fracture apertures in a fragmented rock layer under different stress scenarios using two different approaches. Approach 1 is a simplified method using a two-dimensional (2D) mapping of the fracture network and projects the far-field stresses to individual fractures, and calculates the dilation, normal and shear displacements using experimental stiffnesses available in the literature. Approach 2 employs a three-dimensional (3D) finite element method (FEM) for the mechanical analysis of the fragmented rock layer considering the interaction with the neighbouring rock layers, frictional interfaces between the rock blocks, stress variations within the fragmented rock layer, and displacements, rotations and deformations of rock blocks. After calculating the fracture apertures using either of the approaches, the permeability of the fragmented rock layer is calculated by running flow simulations using the updated fracture apertures. The comparison between the results demonstrates an example of the inaccuracies that may exist in methods that use simplified assumptions such as 2D modelling, ignoring the block rotations and displacements, projected far-field stresses on fractures, and the stress variations within the rock layer. It is found that for the cases considered here, the permeability results based on apertures obtained from the simplified approach could be 40 times different from the results from apertures calculated using a full mechanical approach. Hence, 3D mechanical modelling implementing realistic boundary conditions, while considering the displacements and rotations of rock blocks, is suggested for the calculation of apertures in fragmented rocks.

Keywords

Fracture aperture change
In situ stresses
Frictional interfaces
Ensemble permeability

Cited by (0)

Dr. Hossein Agheshlui received his BSc degree in Civil Engineering and MSc degree in Marine Structures Engineering from KNT University of Technology, Tehran, Iran. He completed his PhD in Structural Engineering in 2014 from The University of Melbourne, Australia during which he focused on the development of a novel structural connection for use in concrete structures, focusing on studying concrete behaviour under complex stress states involving damaged plasticity constitutive models and concrete cracking and crushing. He has several years of industry experience in oil and gas, infrastructure and building sectors. His interest in continuum mechanics motivated him to investigate more complex problems of subsurface engineering involving fractured rocks including fluid flow through fractures as a research fellow at The University of Melbourne. He also has lectured structural engineering subjects at that university for a few years. His main research areas include continuum mechanics, nonlinear finite element modelling, subsurface geomechanics, and steel-concrete composite structures. He has published several papers in the top of the field journals and has presented in international conferences in these areas.

Peer review under responsibility of Institute of Rock and Soil Mechanics, Chinese Academy of Sciences.