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Solute transport through fractured rocks: the influence of geological heterogeneities and stagnant water zones
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.ORCID iD: 0000-0003-2353-6505
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

To describe reactive solute transport and retardation through fractured rocks, three models are developed in the study with different focuses on the physical processes involved and different simplifications of the basic building block of the heterogeneous rock domain. The first model evaluates the effects of the heterogeneity of the rock matrix and the stagnant water zones in part of the fracture plane. The second and the third models are dedicated to different simplifications of the flow channels. Both account for radioactive decay chain, but consider either a rectangular channel with linear matrix diffusion or a cylindrical channel with radial matrix diffusion. Not only an arbitrary-length decay chain, but also as many rock matrix layers with different geological properties as observed in the field experiments can be handled.

The analytical solutions thus obtained from these three models for the Laplace-transformed concentration in the flow channel can all be conveniently transformed back to the time domain by use of e.g. de Hoog algorithm. This allows one to readily include the results into a fracture network model or a channel network model to predict nuclide transport through flow channels in heterogeneous fractured media consisting of an arbitrary number of rock units with piecewise constant properties. The relative impacts and contributions of different processes in retarding solute transport in fractured rocks can easily be evaluated by simulating several cases of varying complexity.

Additionally, a model is developed to study the evolution of fracture aperture in crystalline rocks mediated by pressure dissolution and precipitation. It accounts for not only advective flow that can carry in or away dissolved minerals but also the fact that dissolved minerals in the fracture plane, in both the flow channel and the stagnant water zone, can diffuse into the adjacent porous rocks. The analytical solution obtained in the Laplace space is then used to evaluate the evolution of the fracture aperture under combined influence of stress and flow, in a pseudo-steady-state procedure. The simulation results give insights into the most important processes and mechanisms that dominate the fracture closure or opening under different circumstances.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , x, 53 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:15
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-183592ISBN: 978-91-7595-906-1 (print)OAI: oai:DiVA.org:kth-183592DiVA: diva2:912774
Public defence
2016-04-26, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160318

Available from: 2016-03-18 Created: 2016-03-17 Last updated: 2016-03-18Bibliographically approved
List of papers
1. Solute transport through fractured rock: Radial diffusion into the rock matrix with several geological layers for an arbitrary length decay chain
Open this publication in new window or tab >>Solute transport through fractured rock: Radial diffusion into the rock matrix with several geological layers for an arbitrary length decay chain
2016 (English)In: Journal of Hydrology, ISSN 0022-1694, E-ISSN 1879-2707, Vol. 536, 133-146 p.Article in journal (Refereed) Published
Abstract [sv]

The paper presents a model development to derive a semi-analytical solution to describe reactive solute transport through a single channel in a fracture with cylindrical geometry. The model accounts for advection through the channel, radial diffusion into the adjacent heterogeneous rock matrix comprising different geological layers, adsorption on both the channel surface, and the geological layers of the rock matrix and radioactive decay chain. Not only an arbitrary-length decay chain, but also as many number of the rock matrix layers with different properties as observed in the field can be handled. The solution, which is analytical in the Laplace domain, is transformed back to the time domain numerically e.g. by use of de Hoog algorithm. The solution is verified against experimental data and analytical solutions of limiting cases of solute transport through porous media. More importantly, the relative importance and contribution of different processes on solute transport retardation in fractured rocks are investigated by simulating several cases of varying complexity. The simulation results are compared with those obtained from rectangular model with linear matrix diffusion. It is found that the impact of channel geometry on breakthrough curves increases markedly as the transport distance along the flow channel and away into the rock matrix increase. The effect of geometry is more pronounced for transport of a decay chain when the rock matrix consists of a porous altered layer.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Analytical solution, Radial matrix diffusion, Geological rock layers, Fractured rock, Radionuclide decay chain
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-183591 (URN)10.1016/j.jhydrol.2016.02.046 (DOI)000374811200011 ()2-s2.0-84959564763 (Scopus ID)
Note

QC 20160318

Available from: 2016-03-17 Created: 2016-03-17 Last updated: 2017-11-30Bibliographically approved
2. Solute transport in a single fracture involving an arbitrary length decay chain with rock matrix comprising different geological layers
Open this publication in new window or tab >>Solute transport in a single fracture involving an arbitrary length decay chain with rock matrix comprising different geological layers
2014 (English)In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009, Vol. 164, 59-71 p.Article in journal (Refereed) Published
Abstract [en]

A model is developed to describe solute transport and retention in fractured rocks. It accounts for advection along the fracture, molecular diffusion from the fracture to the rock matrix composed of several geological layers, adsorption on the fracture surface, adsorption in the rock matrix layers and radioactive decay-chains. The analytical solution, obtained for the Laplace-transformed concentration at the outlet of the flowing channel, can conveniently be transformed back to the time domain by the use of the de Hoog algorithm. This allows one to readily include it into a fracture network model or a channel network model to predict nuclide transport through channels in heterogeneous fractured media consisting of an arbitrary number of rock units with piecewise constant properties. More importantly, the simulations made in this study recommend that it is necessary to account for decay-chains and also rock matrix comprising at least two different geological layers, if justified, in safety and performance assessment of the repositories for spent nuclear fuel.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Radionuclide decay chain, Solute transport model, Rock matrix diffusion, Laplace transform, Simulation
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-141786 (URN)10.1016/j.jconhyd.2014.05.011 (DOI)000340989100006 ()2-s2.0-84903220683 (Scopus ID)
Note

QC 20140919. Updated from manuscript to article in journal.

Available from: 2014-02-24 Created: 2014-02-24 Last updated: 2017-12-05Bibliographically approved
3. Solute transport in fractured rocks with stagnant water zone and rock matrix composed of different geological layers-Model development and simulations
Open this publication in new window or tab >>Solute transport in fractured rocks with stagnant water zone and rock matrix composed of different geological layers-Model development and simulations
2013 (English)In: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 49, no 3, 1709-1727 p.Article in journal (Refereed) Published
Abstract [en]

A model is developed to describe solute transport and retention in fractured rocks. It accounts for the fact that solutes can not only diffuse directly from the flowing channel into the adjacent rock matrix composed of different geological layers but also at first diffuse into the stagnant water zone occupied in part of the fracture and then from there into the rock matrix adjacent to it. In spite of the complexities of the system, it is shown that the analytical solution to the Laplace-transformed concentration at the outlet of the flowing channel is a product of two exponential functions, and it can be easily extended to describe solute transport through channels in heterogeneous fractured media consisting of an arbitrary number of rock units with piecewise constant geological properties. More importantly, by numerical inversion of the Laplace-transformed solution, the simulations made in this study help to gain insights into the relative significance and the different contributions of the rock matrix and the stagnant water zone in retarding solute transport in fractured rocks. It is found that, in addition to the intact wall rock adjacent to the flowing channel, the stagnant water zone and the rock matrix adjacent to it may also lead to a considerable retardation of solute in cases with a narrow channel.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2013
Keyword
Tracer Tests, Radionuclide Migration, Performance Assessment, Numerical Inversion, Laplace Transforms, Fluid-Flow, Media, Diffusion, Network
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-123442 (URN)10.1002/wrcr.20132 (DOI)000317829900034 ()2-s2.0-84876729488 (Scopus ID)
Note

QC 20130617

Available from: 2013-06-17 Created: 2013-06-10 Last updated: 2017-12-06Bibliographically approved
4. Evolution of fracture aperture mediated by dissolution in a coupled flow channel–rock matrix–stagnant zone system
Open this publication in new window or tab >>Evolution of fracture aperture mediated by dissolution in a coupled flow channel–rock matrix–stagnant zone system
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Fracture aperture is an important entity controlling fluid flow in natural fractures in rocks. The aperture of fractures in crystalline rocks may decrease or increase by different mechanical and chemical mechanisms. A model to describe the evolution of fracture aperture mediated by dissolution and precipitation is presented in this study. It includes advective flow in the fracture that can carry in or away dissolved minerals. The model also accounts for the fact that dissolved minerals in the fracture plane, in both flow channel and stagnant water zone, can diffuse into the adjacent porous rock matrix. The analytical solution obtained in the Laplace space is then used to study evolution of the fracture aperture under combined influence of stress and flow, in a pseudo-steady-state procedure. The simulation results give insights into the most important processes and mechanisms that dominate the fracture closure or opening under different circumstances. It is found that the times involved for any changes in fracture aperture are very much larger than the times needed for concentrations of dissolved minerals to reach steady state in the rock matrix, the stagnant water zone and the flow channel. This suggests that the steady state model can be used to assess the evolution of concentration of dissolved minerals in the rock fracture. Moreover, it is shown that diffusion into the rock matrix, which acts as a strong sink or source for dissolved minerals, clearly dominates the rate of concentration change and consequently the rate of evolution of the fracture aperture.

Keyword
Fracture aperture, Dissolution, Rock matrix diffusion, Fluid flow, Channeling, Modeling
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-183773 (URN)
Note

QS 2017

Available from: 2016-03-18 Created: 2016-03-18 Last updated: 2016-03-18Bibliographically approved

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