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Stress Effects on Solute Transport in Fractured rocks
KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Engineering Geology and Geophysics.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The effect of in-situ or redistributed stress on solute transport in fractured rocks is one of the major concerns for many subsurface engineering problems. However, it remains poorly understood due to the difficulties in experiments and numerical modeling. The main aim of this thesis is to systematically investigate the influences of stress on solute transport in fractured rocks, at scales of single fractures and fracture networks, respectively.

For a single fracture embedded in a porous rock matrix, a closed-form solution was derived for modeling the coupled stress-flow-transport processes without considering damage on the fracture surfaces. Afterwards, a retardation coefficient model was developed to consider the influences of damage of the fracture surfaces during shear processes on the solute sorption. Integrated with particle mechanics models, a numerical procedure was proposed to investigate the effects of gouge generation and microcrack development in the damaged zones of fracture on the solute retardation in single fractures. The results show that fracture aperture changes have a significant influence on the solute concentration distribution and residence time. Under compression, the decreasing matrix porosity can slightly increase the solute concentration. The shear process can increase the solute retardation coefficient by offering more sorption surfaces in the fracture due to gouge generation, microcracking and gouge crushing.

To study the stress effects on solute transport in fracture systems, a hybrid approach combing the discrete element method for stress-flow simulations and a particle tracking algorithm for solute transport was developed for two-dimensional irregular discrete fracture network models. Advection, hydrodynamic dispersion and matrix diffusion in single fractures were considered. The particle migration paths were tracked first by following the flowing fluid (advection), and then the hydrodynamic dispersion and matrix diffusion were considered using statistic methods. The numerical results show an important impact of stress on the solute transport, by changing the solute residence time, distribution and travel paths. The equivalent dispersion coefficient is scale dependent in an asymptotic or exponential form without stress applied or under isotropic compression conditions. Matrix diffusion plays a dominant role in solute transport when the hydraulic gradient is small.

Outstanding issues and main scientific achievements are also discussed.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , x, 108 p.
Series
Trita-LWR. PHD, ISSN 1650-8602
Keyword [en]
Fractured rocks, Solute transport; Stress effects, Coupled stress-flow-transport processes; Analytical solution; Particle mechanics model; Fracture surface damage; Discrete element method; Particle tracking method
National Category
Civil Engineering
Identifiers
URN: urn:nbn:se:kth:diva-42361ISBN: 978-91-7501-099-1 (print)OAI: oai:DiVA.org:kth-42361DiVA: diva2:446885
Public defence
2011-11-04, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20111011Available from: 2011-10-11 Created: 2011-10-10 Last updated: 2011-10-11Bibliographically approved
List of papers
1. Analytical solution of coupled stress-flow-transport processes in a single rock fracture
Open this publication in new window or tab >>Analytical solution of coupled stress-flow-transport processes in a single rock fracture
2011 (English)In: Computers & Geosciences, ISSN 0098-3004, E-ISSN 1873-7803, Vol. 37, no 9, 1437-1449 p.Article in journal (Refereed) Published
Abstract [en]

A closed-form solution is presented for modeling the coupled stress-flow-transport processes along a single fracture embedded in a porous rock matrix. Necessary assumptions were made to simplify the subject into a two-dimensional (2D) problem, considering the changes of fracture aperture and matrix porosity under various stress conditions. The cubic law was assumed to be valid for the fluid flow in the fracture, with an impermeable rock matrix. For transport mechanisms, advective transport along the fracture, longitudinal hydrodynamic dispersion in the flow direction, and the matrix diffusion were considered in three different transport models under constant concentration or constant flux (Danck- werts’) inlet boundary conditions. This analytical solution can be used as a constitutive model, or as an example for validation of similar constitutive models, for modeling the coupled hydro-mechanical- chemical (HMC) processes in fracture networks of crystalline rocks. The influences of stress/deformation processes on different transport mechanisms in a single fracture under different inlet boundary conditions were studied for the first time. The results show that changes of fracture, as controlled by a combination of normal closure and shear dilatancy, have a significant influence on the solute concentration distribution both along the fracture and in the rock matrix, as well as on the solute residence/breakthrough time, especially when shear-induced dilatancy occurs. Under compressions, the decreasing matrix porosity slightly increases the solute concentration along the fracture and in the rock matrix.

Keyword
Analytical solution, Fluid flow, Single rock fractures, Stress, Transport
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-42354 (URN)10.1016/j.cageo.2011.02.015 (DOI)000295201500024 ()2-s2.0-80051555169 (Scopus ID)
Note
QC 20111010Available from: 2011-10-10 Created: 2011-10-10 Last updated: 2017-12-08Bibliographically approved
2. Shear effects on solute retardation coefficient in rock fractures: Insights from a particle mechanics model
Open this publication in new window or tab >>Shear effects on solute retardation coefficient in rock fractures: Insights from a particle mechanics model
(English)Manuscript (preprint) (Other academic)
National Category
Geophysical Engineering
Identifiers
urn:nbn:se:kth:diva-42596 (URN)
Note
QS 2011Available from: 2011-10-11 Created: 2011-10-11 Last updated: 2011-10-11Bibliographically approved
3. Numerical modeling of stress effects on solute transport in fractured rocks
Open this publication in new window or tab >>Numerical modeling of stress effects on solute transport in fractured rocks
2011 (English)In: Computers and geotechnics, ISSN 0266-352X, E-ISSN 1873-7633, Vol. 38, no 2, 113-126 p.Article in journal (Refereed) Published
Abstract [en]

The effects of stress/deformation on fluid flow and contaminant transport in fractured rocks is one of the major concerns for performance and safety assessments of many subsurface engineering problems, especially radioactive waste disposal and oil/gas reservoir fields. However, very little progress has been made to study this issue due to difficulties in both experiments and numerical modeling. The objective of this study is to systematically investigate the influence of stress on solute transport in fractured rocks for the first time, considering different stress and hydraulic pressure conditions. A hybrid approach combining discrete element method (DEM) for stress-flow simulations and a particle tracking algorithm is developed. The impact of matrix diffusion (diffusion of molecular size solutes in and out of the rock matrix, and sorption onto the surface of micropores in rock matrix) is also included. The numerical results show that stress not only significantly changes the solute residence time through the fracture networks, but also changes the solute travel paths. Matrix diffusion plays a dominant role in solute transport when the hydraulic gradient is small, which is often encountered in practice.

Keyword
Stress effects, Solute transport, Coupled stress-flow-transport processes, Matrix diffusion, Discrete fracture network, Fractured rocks
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:kth:diva-31356 (URN)10.1016/j.compgeo.2010.10.001 (DOI)000287633700002 ()2-s2.0-79551479285 (Scopus ID)
Note
QC 20110317Available from: 2011-03-17 Created: 2011-03-14 Last updated: 2017-12-11Bibliographically approved
4. Evaluation of hydrodynamic dispersion parameters in fractured rocks
Open this publication in new window or tab >>Evaluation of hydrodynamic dispersion parameters in fractured rocks
2010 (English)In: Journal of Rock Mechanics and Geotechnical Engineering, ISSN 1674-7755, Vol. 2, 243-254 p.Article in journal (Refereed) Published
Abstract [en]

A numerical procedure to determine the equivalent hydrodynamic dispersion coefficients and Péclet number (Pe) of a fractured rock is presented using random walk particle tracking method. The geometrical effects of fracture system on hydrodynamic dispersion are studied. The results obtained from the proposed method agree well with those of empirical models, which are the scale-dependent hydrodynamic dispersion coefficients in an asymptotic or exponential form. A variance case is added to investigate the influence of longitudinal hydrodynamic dispersion in individual fractures on the macro-hydrodynamic dispersion at the fracture network scale, and its influence is demonstrated with a verification example. In addition, we investigate the influences of directional flow and stress conditions on the behavior of hydrodynamic dispersion in fracture networks. The results show that the magnitudes of the hydrodynamic dispersion coefficients are relatively smaller when the flow direction is parallel to the dip directions of fracture sets. Compressive stresses significantly reduce hydrodynamic dispersion. However, the remaining questions are: (1) whether the deformed fracture network under high stress conditions may make the scale-dependent hydrodynamic dispersion coefficients have asymptotic or exponential forms, and (2) what the conditions for existence of a welldefined equivalent hydrodynamic dispersion tensor are. They need to be further investigated.

National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-42357 (URN)10.3724/SP.J.1235.2010.00243 (DOI)
Note
QC 20111010Available from: 2011-10-10 Created: 2011-10-10 Last updated: 2012-03-22Bibliographically approved

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