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Geier, J. E., Lindgren, G. A. & Tsang, C.-F. (2019). Simplified representative models for long-term flow and advective transport in fractured crystalline bedrock. Hydrogeology Journal, 27(2), 595-614
Open this publication in new window or tab >>Simplified representative models for long-term flow and advective transport in fractured crystalline bedrock
2019 (English)In: Hydrogeology Journal, ISSN 1431-2174, E-ISSN 1435-0157, Vol. 27, no 2, p. 595-614Article in journal (Refereed) Published
Abstract [en]

Simplified representative models (SRMs) of the hydrogeological system at radioactive-waste repository sites are presented and demonstrated to give useful predictions of the key hydrogeological factors affecting long-term safety. The SRM is constructed from complex site-descriptive models, which have been developed to be consistent with detailed site information and data from short-term (with duration of days, weeks, up to months) field experiments, by keeping elements that are important for long-term predictions into thousands of years and simplifying features of less importance. The simplified approach relies only on fundamental hydrogeological principles and the mathematics can be kept relatively simple. The purpose of SRM is to provide a means of verifying predictions from complex numerical models, with an approach that is easy to evaluate and allows transparent evaluation of factors influencing long-term results. The approach is applied to evaluations of sites for two repositories in fractured crystalline bedrock in Sweden: one for spent nuclear fuel rod assemblies and one for waste with lower levels of radioactivity. The results indicate that the SRMs are able to yield results similar to those of calculations based on much more complex models. Further, the approach allows an evaluation of additional sources of uncertainty that are difficult or expensive to conduct with the complex models. These capabilities make SRMs a very useful and transparent tool for regulatory review.

Abstract [sv]

Förenklade representativa modeller (SRM) av det hydrogeologiska systemet kring slutförvar för kärnavfall presenteras och visas ge meningsfulla förutsägelser av centrala hydrogeologiska faktorer som påverkar säkerheten efter förslutning. SRM upprättas utifrån komplexa platsbeskrivande modeller som har utvecklats för att vara i överenstämmelse med detaljerad platsinformation och data från korttidsmätningar (dagar, veckor och upp till månader) i fält genom att bibehålla delar i modellen som är betydelsefulla för förutsägelser på lång tid upp till tusentals år, medan delar av mindre betydelse förenklas. Den förenklade ansatsen utgår från grundläggande hydrogeologiska principer och matematiken kan hållas förhållandevis enkel. Syftet med SRM är att ge möjligheter till att styrka förutsägelser från komplexa numeriska modeller med en ansats som är enkel att utvärdera och som möjliggör en transparent granskning av faktorer som påverkar långtidsresultaten. Ansatsen tillämpas på utvärderingen av plaster för två slutförvar i sprickigt kristallint berg i Sverige. Ett för använt kärnbränsle och ett för avfall med lägre nivåer av radioaktivitet. Resultaten pekar på att SRM kan ge resultat som överensstämmer med resultaten från beräkningar som baseras på väsentligt komplexare modeller. Därutöver ger ansatsen möjligheten att utvärdera ytterligare osäkerheter som är svåra eller mödosamma att undersöka med de komplexa modellerna. Detta gör SRM till ett användbart och transparent verktyg för myndighetsgranskningar av slutförvar för kärnavfall.

Keywords
Radioactive waste, Crystalline rocks, Fractured rocks, Waste disposal, Sweden
National Category
Oceanography, Hydrology and Water Resources Geosciences, Multidisciplinary
Identifiers
urn:nbn:se:uu:diva-381832 (URN)10.1007/s10040-018-1875-2 (DOI)000462179900013 ()
Funder
Swedish Radiation Safety Authority
Available from: 2019-04-16 Created: 2019-04-16 Last updated: 2019-04-16Bibliographically approved
Dessirier, B., Tsang, C.-F. & Niemi, A. (2018). A new scripting library for modeling flow and transport in fractured rock with channel networks. Computers & Geosciences, 111, 181-189
Open this publication in new window or tab >>A new scripting library for modeling flow and transport in fractured rock with channel networks
2018 (English)In: Computers & Geosciences, ISSN 0098-3004, E-ISSN 1873-7803, Vol. 111, p. 181-189Article in journal (Refereed) Published
Abstract [en]

Deep crystalline bedrock formations are targeted to host spent nuclear fuel owing to their overall low permeability. They are however highly heterogeneous and only a few preferential paths pertaining to a small set of dominant rock fractures usually carry most of the flow or mass fluxes, a behavior known as channeling that needs to be accounted for in the performance assessment of repositories. Channel network models have been developed and used to investigate the effect of channeling. They are usually simpler than discrete fracture networks based on rock fracture mappings and rely on idealized full or sparsely populated lattices of channels. This study reexamines the minimal requirements to describe a channel network in terms of groundwater flow and solute transport, leading to an extended description suitable for unstructured arbitrary networks of channels. An implementation of this formalism in a Python scripting library is presented and released along with this article. A new algebraic multigrid preconditioner delivers a significant speedup in the flow solution step compared to previous channel network codes. 3D visualization is readily available for verification and interpretation of the results by exporting the results to an open and free dedicated software. The new code is applied to three example cases to verify its results on full uncorrelated lattices of channels, sparsely populated percolation lattices and to exemplify the use of unstructured networks to accommodate knowledge on local rock fractures.

Keywords
channel network, groundwater flow, solute transport, graph theory
National Category
Oceanography, Hydrology and Water Resources
Research subject
Hydrology
Identifiers
urn:nbn:se:uu:diva-321572 (URN)10.1016/j.cageo.2017.11.013 (DOI)000423005900018 ()
Funder
Swedish Radiation Safety Authority, SSM2016-763
Available from: 2017-05-08 Created: 2017-05-08 Last updated: 2018-03-06Bibliographically approved
Dessirier, B., Tsang, C.-F. & Niemi, A. (2018). A new scripting library for modeling flow and transport in fractured rock with channel networks. Computers & Geosciences, 111, 181-189
Open this publication in new window or tab >>A new scripting library for modeling flow and transport in fractured rock with channel networks
2018 (English)In: Computers & Geosciences, ISSN 0098-3004, E-ISSN 1873-7803, Vol. 111, p. 181-189Article in journal (Refereed) Published
Abstract [en]

Deep crystalline bedrock formations are targeted to host spent nuclear fuel owing to their overall low permeability. They are however highly heterogeneous and only a few preferential paths pertaining to a small set of dominant rock fractures usually carry most of the flow or mass fluxes, a behavior known as channeling that needs to be accounted for in the performance assessment of repositories. Channel network models have been developed and used to investigate the effect of channeling. They are usually simpler than discrete fracture networks based on rock fracture mappings and rely on idealized full or sparsely populated lattices of channels. This study reexamines the fundamental parameter structure required to describe a channel network in terms of groundwater flow and solute transport, leading to an extended description suitable for unstructured arbitrary networks of channels. An implementation of this formalism in a Python scripting library is presented and released along with this article. A new algebraic multigrid preconditioner delivers a significant speedup in the flow solution step compared to previous channel network codes. 3D visualization is readily available for verification and interpretation of the results by exporting the results to an open and free dedicated software. The new code is applied to three example cases to verify its results on full uncorrelated lattices of channels, sparsely populated percolation lattices and to exemplify the use of unstructured networks to accommodate knowledge on local rock fractures.

Keywords
Channel network, Groundwater flow, Solute transport, Graph theory
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-343655 (URN)10.1016/j.cageo.2017.11.013 (DOI)000423005900018 ()
Available from: 2018-05-09 Created: 2018-05-09 Last updated: 2018-05-09Bibliographically approved
Figueiredo, B., Tsang, C.-F., Rutqvist, J. & Niemi, A. (2018). Corrigendum to “The effects of nearby fractures on hydraulically induced fracture propagation and permeability changes”[Eng. Geol. 228 (2017) 197–213]. Engineering Geology, 239, 344-344
Open this publication in new window or tab >>Corrigendum to “The effects of nearby fractures on hydraulically induced fracture propagation and permeability changes”[Eng. Geol. 228 (2017) 197–213]
2018 (English)In: Engineering Geology, ISSN 0013-7952, E-ISSN 1872-6917, Vol. 239, p. 344-344Article in journal (Other academic) Published
National Category
Geology
Identifiers
urn:nbn:se:uu:diva-359373 (URN)10.1016/j.enggeo.2018.03.006 (DOI)000432769300030 ()
Funder
The Geological Survey of Sweden (SGU), 1724EU, Horizon 2020, 636811
Note

WoS title: The effects of nearby fractures on hydraulically induced fracture propagation and permeability changes (vol 228, pg 197, 2017)

Correction to: Engineering Geology, vol. 228, pages 197-213. DOI: 10.1016/j.enggeo.2017.08.011

Available from: 2018-09-04 Created: 2018-09-04 Last updated: 2018-09-04Bibliographically approved
Birkholzer, J. T., Bond, A. E., Hudson, J. A., Jing, L., Tsang, C.-F., Shao, H. & Kolditz, O. (2018). DECOVALEX-2015: an international collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems. Environmental Earth Sciences, 77(14), Article ID 539.
Open this publication in new window or tab >>DECOVALEX-2015: an international collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems
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2018 (English)In: Environmental Earth Sciences, ISSN 1866-6280, E-ISSN 1866-6299, Vol. 77, no 14, article id 539Article in journal, Editorial material (Other academic) Published
Place, publisher, year, edition, pages
Springer, 2018
National Category
Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-366672 (URN)10.1007/s12665-018-7697-7 (DOI)000439245400001 ()
Available from: 2018-11-27 Created: 2018-11-27 Last updated: 2018-11-27Bibliographically approved
Tsang, C.-F., Figueiredo, B. & Niemi, A. (2018). Importance of stress effects on inputs to fracture network models used for subsurface flow and transport studies. International Journal of Rock Mechanics And Mining Sciences, 101, 13-17
Open this publication in new window or tab >>Importance of stress effects on inputs to fracture network models used for subsurface flow and transport studies
2018 (English)In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 101, p. 13-17Article in journal (Refereed) Published
National Category
Geosciences, Multidisciplinary
Identifiers
urn:nbn:se:uu:diva-334138 (URN)10.1016/j.ijrmms.2017.11.012 (DOI)000418717200002 ()
Funder
Swedish Radiation Safety Authority, SSM2016-763The Geological Survey of Sweden (SGU), 1724
Available from: 2017-11-21 Created: 2017-11-21 Last updated: 2018-01-18Bibliographically approved
Doughty, C., Tsang, C.-F., Rosberg, J.-E., Juhlin, C., Dobson, P. F. & Birkholzer, J. T. (2017). Flowing fluid electrical conductivity logging of a deep borehole during and following drilling: estimation of transmissivity, water salinity and hydraulic head of conductive zones. Hydrogeology Journal, 25(2), 501-517
Open this publication in new window or tab >>Flowing fluid electrical conductivity logging of a deep borehole during and following drilling: estimation of transmissivity, water salinity and hydraulic head of conductive zones
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2017 (English)In: Hydrogeology Journal, ISSN 1431-2174, E-ISSN 1435-0157, Vol. 25, no 2, p. 501-517Article in journal (Refereed) Published
Abstract [en]

Flowing fluid electrical conductivity (FFEC) logging is a hydrogeologic testing method that is usually conducted in an existing borehole. However, for the 2,500-m deep COSC-1 borehole, drilled at re, central Sweden, it was done within the drilling period during a scheduled 1-day break, thus having a negligible impact on the drilling schedule, yet providing important information on depths of hydraulically conductive zones and their transmissivities and salinities. This paper presents a reanalysis of this set of data together with a new FFEC logging data set obtained soon after drilling was completed, also over a period of 1 day, but with a different pumping rate and water-level drawdown. Their joint analysis not only results in better estimates of transmissivity and salinity in the conducting fractures intercepted by the borehole, but also yields the hydraulic head values of these fractures, an important piece of information for the understanding of hydraulic structure of the subsurface. Two additional FFEC logging tests were done about 1 year later, and are used to confirm and refine this analysis. Results show that from 250 to 2,000 m depths, there are seven distinct hydraulically conductive zones with different hydraulic heads and low transmissivity values. For the final test, conducted with a much smaller water-level drawdown, inflow ceased from some of the conductive zones, confirming that their hydraulic heads are below the hydraulic head measured in the wellbore under non-pumped conditions. The challenges accompanying 1-day FFEC logging are summarized, along with lessons learned in addressing them.

Place, publisher, year, edition, pages
SPRINGER, 2017
Keywords
Hydraulic testing, Fractured rock, Hydraulic head, Well logging, Drilling
National Category
Oceanography, Hydrology and Water Resources
Identifiers
urn:nbn:se:uu:diva-320848 (URN)10.1007/s10040-016-1497-5 (DOI)000395001300015 ()
Funder
Swedish Research Council, 2013-94
Available from: 2017-04-26 Created: 2017-04-26 Last updated: 2018-01-13Bibliographically approved
Lei, Q., Latham, J.-P., Xiang, J. & Tsang, C.-F. (2017). Role of natural fractures in damage evolution around tunnel excavation in fractured rocks. Engineering Geology, 231, 100-113
Open this publication in new window or tab >>Role of natural fractures in damage evolution around tunnel excavation in fractured rocks
2017 (English)In: Engineering Geology, ISSN 0013-7952, E-ISSN 1872-6917, Vol. 231, p. 100-113Article in journal (Refereed) Published
Abstract [en]

This paper studies the role of pre-existing fractures in the damage evolution around tunnel excavation in fractured rocks. The length distribution of natural fractures can be described by a power law model, whose exponent a defines the relative proportion of large and small fractures in the system. The larger a is, the higher proportion of small fractures is. A series of two-dimensional discrete fracture networks (DENs) associated with different length exponent a and fracture intensity P-21 is generated to represent various scenarios of distributed preexisting fractures in the rock. The geomechanical behaviour of the fractured rock embedded with DFN geometry in response to isotropic/anisotropic in-situ stress conditions and excavation-induced perturbations is simulated using the hybrid finite-discrete element method (FEMDEM), which can capture the deformation of intact rocks, the interaction of matrix blocks, the displacement of natural fractures, and the propagation of new cracks. An excavation damaged zone (EDZ) develops around the man-made opening as a result of reactivation of preexisting fractures and propagation of wing cracks. The simulation results show that when a is small, the system which is dominated by large fractures can remain stable after excavation given that P-21 is not very high; however, intensive structurally-governed kinematic instability can occur if P-21 is sufficiently high and the fracture spacing is much smaller than the tunnel size. With the increase of a, the system becomes more dominated by small fractures, and the EDZ is mainly created by the coalescence of small fractures near the tunnel boundary. The results of this study have important implications for designing stable underground openings for radioactive waste repositories as well as other engineering facilities that are intended to generate minimal damage in the host rock mass.

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Fluid Mechanics and Acoustics Natural Sciences
Identifiers
urn:nbn:se:uu:diva-335606 (URN)10.1016/j.enggeo.2017.10.013 (DOI)000418975600011 ()
Available from: 2017-12-07 Created: 2017-12-07 Last updated: 2018-02-01Bibliographically approved
Figueiredo, B., Tsang, C.-F., Rutqvist, J. & Niemi, A. (2017). Study of hydraulic fracturing processes in shale formations with complex geological settings. Journal of Petroleum Science and Engineering, 152, 361-374
Open this publication in new window or tab >>Study of hydraulic fracturing processes in shale formations with complex geological settings
2017 (English)In: Journal of Petroleum Science and Engineering, ISSN 0920-4105, E-ISSN 1873-4715, Vol. 152, p. 361-374Article in journal (Refereed) Published
Abstract [en]

Hydraulic fracturing has been applied to extract gas from shale-gas reservoirs. Complicated geological settings,such as spatial variability of the rock mass properties, local heterogeneities, complex in situ stress field, and preexistingbedding planes and faults, could make hydraulic fracturing a challenging task. In order to effectivelyand economically recover gas from such reservoirs, it is crucial to explore how hydraulic fracturing performs insuch complex geological settings. For this purpose, numerical modelling plays an important role because suchconditions cannot be reproduced by laboratory experiments. This paper focuses on the analysis of the influenceof confining formations and pre-existing bedding planes and faults on the hydraulically-induced propagation ofa vertical fracture, which will be called injection fracture, in a shale-gas reservoir. An elastic-brittle model basedon material property degradation was implemented in a 2D finite-difference scheme and used for rock elementssubjected to tension and shear failure. A base case is considered, in which the ratio SR between the magnitudesof the horizontal and vertical stresses, the permeability kc of the confining formations, the elastic modulus Epand initial permeability kp of the bedding plane and the initial fault permeability kF are fixed at reasonablevalues. In addition, the influence of multiple bedding planes, is investigated. Changes in pore pressure andpermeability due to high pressure injection lasting 2 h were analysed. Results show that in our case during theinjection period the fracture reaches the confining formations and if the permeability of those layers issignificantly larger than that of the shale, the pore pressure at the extended fracture tip decreases and fracturepropagation becomes slower. After shut-in, the pore pressure decreases more and the fracture does notpropagate any more. For bedding planes oriented perpendicular to the maximum principal stress direction andwith the same elastic properties as the shale formation, results were found not to be influenced by theirpresence. In such a scenario, the impact of multiple bedding planes on fracture propagation is negligible. On theother hand, a bedding plane softer than the surrounding shale formation leads to a fracture propagationasymmetrical vertically with respect to the centre of the injection fracture with a more limited upward fracturepropagation. A pre-existing fault leads to a decrease in fracture propagation because of fault reactivation withshear failure. This results in a smaller increase in injection fracture permeability and a slight higher injectionpressure than that observed without the fault. Overall, results of a sensitivity analysis show that fracturepropagation is influenced by the stress ratio SR, the permeability kc of the confining formations and the initialpermeability kp of the bedding plane more than the other major parameters.

Keywords
Shale-gas, Hydraulic fracturing stimulation, Fracture propagation, Elastic-brittle model, Bedding plane, Fault reactivation
National Category
Geosciences, Multidisciplinary
Identifiers
urn:nbn:se:uu:diva-321462 (URN)10.1016/j.petrol.2017.03.011 (DOI)000401042000030 ()
Funder
EU, Horizon 2020, 636811The Geological Survey of Sweden (SGU), 1724
Note

Correction about funding information in Journal of Petroleum Science and Engineering, Volume 166, July 2018, Page 797

https://doi.org/10.1016/j.petrol.2018.03.016

Available from: 2017-05-05 Created: 2017-05-05 Last updated: 2018-12-10Bibliographically approved
Figueiredo, B., Tsang, C.-F., Rutqvist, J. & Niemi, A. (2017). The effects of nearby fractures on hydraulically induced fracture propagation and permeability changes. Engineering Geology, 228, 197-213
Open this publication in new window or tab >>The effects of nearby fractures on hydraulically induced fracture propagation and permeability changes
2017 (English)In: Engineering Geology, ISSN 0013-7952, E-ISSN 1872-6917, Vol. 228, p. 197-213Article in journal (Refereed) Published
Abstract [en]

Fracture propagation caused by hydraulic fracturing operations can be significantly influenced by adjacent fractures. This paper presents a detailed coupled hydro-mechanical analysis to study the effects of nearby natural fractures on hydraulically induced fracture propagation and changes in fracture permeability. Two rock domains were considered in comparison: FD1, with one fracture, and FD2, with two adjacent parallel or non-parallel fractures. It was assumed that water injection occurred in a borehole that intersected the single fracture in FD1 and one of the two fractures in FD2. Simulations were made for a time period of 3 h with an injection period of 2 h followed by 1 h of shut-in. An elastic-brittle model based on the degradation of material properties was implemented in a 2D finite-difference scheme and used for elements of the intact rock subjected to tension and shear failure. The intact rock was considered to have a low but non-negligible permeability. A verification study against analytical solutions showed that the fracture propagation and stress concentrations due to differential boundary stresses could be accurately represented by our model. Next, a base case was considered, in which the stress ratio (SR) between the magnitudes of the maximum and minimum boundary stresses, the permeability kR of the intact rock and the initial permeability kTF of the tension failure regions were fixed. In FD2, the distance dF between the two natural fractures defined by the closest distance was also fixed. The results showed that in both rock domains, the fracture started to propagate when the pore pressure was approximately 85% of the magnitude of the minimum boundary stress. The propagation of a single fracture was significantly greater than the propagation of a double fracture system because, in the latter case, the pore pressure decreased when the two fractures connected. As a result, changes in permeability in FD2 were smaller than in FD1. At the end of injection, the maximum ratios between the final and initial permeability of the natural fractures were found to be approximately 3 and 2 for rock domains FD1 and FD2, respectively. For non-parallel fractures, the controlling factor for fracture propagation was the separation between the tips of the pressurised fracture and the neighbouring non-pressurised fracture. A sensitivity study was conducted to study the influence of the key parameters dF, SR, kR and kTF on the simulation results. Fracture propagation showed more sensitivity to dF and SR than to the other parameters.

Keywords
Hydraulic fracturing stimulation, Coupled hydro-mechanical effects, Fracture propagation and connectivity, Permeability changes, Elastic-brittle model
National Category
Geology
Identifiers
urn:nbn:se:uu:diva-340135 (URN)10.1016/j.enggeo.2017.08.011 (DOI)000413282000017 ()
Funder
EU, European Research Council, 640979EU, Horizon 2020, 640979
Note

Correction in: Engineering Geology, vol. 239, pages 344-344. DOI: 10.1016/j.enggeo.2018.03.006

Available from: 2018-01-29 Created: 2018-01-29 Last updated: 2018-09-04Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-2355-4861

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