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Stress effects on permeability in fractured rock mass with correlated fracture length and aperture
KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Engineering Geology and Geophysics.
KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Engineering Geology and Geophysics.
2008 (English)In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, Vol. 45, no 8, 1320-1334 p.Article in journal (Refereed) Published
Abstract [en]

The effect of stress on permeability and fluid flow patterns in fractured rock masses is studied when distributed fracture aperture is correlated with fracture trace length, using a discrete element method (DEM). The basic assumptions are that the rock matrix is impermeable and linearly elastic, and that the fluid flows only in fractures. A new nonlinear algorithm is developed for prediction of normal stress-normal displacement behavior of fractures based on the Bandis model and the correlation between aperture and length. The results show that when small stress ratios (K = horizontal/vertical stress) are applied at the model boundaries, the overall permeability of the fracture network is generally decreased. However, contribution from a few large fractures of higher hydraulic conductivity prevents drastic reduction of the overall permeability, compared with models that assume uniform fracture apertures. With large values of the stress ratio, both the overall permeability and flow patterns are controlled by a combination of highly conductive larger fractures and fractures with shear slipping and dilation, with much increased overall permeability and shear-induced flow channeling. With increasing stress ratios, it becomes more and more difficult to establish an equivalent permeability tensor and representative elementary volume (REV) of a fractured rock, compared with the unstressed model. These results show significant difference between correlated and non-correlated aperture and fracture length distributions, and highlight more significant scale and stress dependence of hydro-mechanical behavior of fractures rocks when geometric parameters of rock fractures are correlated.

Place, publisher, year, edition, pages
2008. Vol. 45, no 8, 1320-1334 p.
Keyword [en]
Stress effect; Scale effect; Equivalent permeability tensor; Aperture-trace length correlation; Fracture rocks; Discrete fracture network; Deformability of fractures; Discrete element method; HYDRAULIC-PROPERTIES; FLUID-FLOW; FIELD DATA; NETWORKS; TENSOR; MODEL; DEFORMATION; STIFFNESS; JOINT; CALIBRATION
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:kth:diva-8520DOI: 10.1016/j.ijrmms.2008.01.015ISI: 000260926500009Scopus ID: 2-s2.0-55749098480OAI: oai:DiVA.org:kth-8520DiVA: diva2:13867
Note
QC 20100701Available from: 2008-05-27 Created: 2008-05-27 Last updated: 2010-07-02Bibliographically approved
In thesis
1. Scale and Stress Effects on Hydro-Mechanical Properties of Fractured Rock Masses
Open this publication in new window or tab >>Scale and Stress Effects on Hydro-Mechanical Properties of Fractured Rock Masses
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

In this thesis, the effects of size and stress on permeability, deformability and strength of fractured rock masses are investigated. A comparison study was carried out to examine the effects of considering, or not considering, the correlation between distributions of fracture apertures and fracture trace lengths on the hydro-mechanical behavior of fractured rocks. The basic concepts used are the fundamental principles of the general theory of elasticity, Representative Elementary Volume (REV), the tensor of equivalent permeability, and the strength criteria of the fractured rocks.

Due to the size and stress dependence of the hydro-mechanical properties of rock fractures, the overall effective (or equivalent) hydro-mechanical properties of the fractured rocks are also size and stress-dependent. However, such dependence cannot be readily investigated in laboratory using small samples, and so numerical modeling becomes a necessary tool for estimating their impacts. In this study, a closed-form relation is established for representing the correlation between a truncated lognormal distribution of fracture apertures and a truncated power law distribution of trace lengths, as obtained from field mapping. Furthermore, a new nonlinear algorithm is developed for predicting the relationship between normal stress and normal displacement of fractures, based on the Bandis model and the correlation between aperture and length.

A large number of stochastic Discrete Fracture Network (DFN) models of varying sizes were extracted from some generated large-sized parent realizations based on a realistic fracture system description from a site investigation programme at Sellafield, UK, for calculating the REV of hydro-mechanical properties of fractured rocks. Rotated DFN models were also generated and used for evaluation of the distributions of directional permeabilities, such that tensors of equivalent permeability could be established based on stochastically established REVs. The stress-dependence of the permeability and the stress-displacement behaviour were then investigated using models of REV sizes. The Discrete Element Method (DEM) was used for numerical simulation of the fluid flow, deformability properties and mechanical strength behavior of fractured rocks.

The results show significant scale-dependency of rock permeability, deformability and strength, and its variation when the correlation between aperture and trace length of fractures are concerned, with the overall permeability and deformability more controlled by dominating fractures with larger apertures and higher transmissivity and deformability, compared with fracture network models having uniform aperture. As the second moment of aperture distribution increases, a fractured rock mass shows more discrete behavior and an REV is established in smaller value of second moment with much larger model size, compared with the models with uniform fracture aperture. When the fracture aperture pattern is more scattered, the overall permeability, Young’s modulus and mechanical strength change significantly.

The effect of stress on permeability and fluid flow patterns in fractured rock is significant and can lead to the existence or non-existence of a permeability tensor. Stress changes the fluid flow patterns and can cause significant channeling and the permeability tensor, and REV may be destroyed or re-established at different applied stress conditions. With an increase in the confining stress on the DEM models, the strength is increased. Compared with the Hoek-Brown criterion, the Mohr-Coulomb strength envelope provides a better fit to the results of numerical biaxial compression tests, with significant changes of the strength characteristic parameters occurring when the second moment of the aperture distribution is increased.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. xiv, 76 p.
Series
Trita-LWR. PHD, ISSN 1650-8602 ; 1040
Keyword
Aperture-trace length correlation; Discrete Fracture Network (DFN); Representative Elementary Volume (REV); Strength; Stress-Strain curve; Discrete Element Method (DEM); Stress effect; Deformability of fractures
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:kth:diva-4772 (URN)978-91-7415-003-2 (ISBN)
Public defence
2008-06-04, V3, Mark och Vatenteknik, Teknikringen 72, Mark och Vatenteknik, 10:00
Opponent
Supervisors
Note
QC 20100702Available from: 2008-05-27 Created: 2008-05-27 Last updated: 2010-07-02Bibliographically approved

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