Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 551 Modeling internal deformation of salt structures targeted for radioactive waste disposal ZURAB CHEMIA ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 UPPSALA ISBN 978-91-554-7281-8 2008 urn:nbn:se:uu:diva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hough we are justices and doctors, and churchmen, Master Page, we have some salt of our youth in us..." W. Shakespeare Dedicated to: salt of the earth... List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I Chemia, Z., Koyi, H., and Schmeling, H. (2008) Numerical modelling of rise and fall of a dense layer in salt diapirs. Geophysical Journal International, 172(2):798–816 II Chemia, Z. and Koyi, H. (2008) The control of salt supply on entrain- ment of an anhydrite layer within a salt diapir. Journal of Structural Geology, 30(9):1192–1200 III Chemia, Z., Schmeling, H., and Koyi, H. (2008) The effect of the salt viscosity on future evolution of the Gorleben salt diapir. Tectonophysics, submitted Reprints were made with permission from the publishers. Contents 1 Introduction .......................................... 11 1.1 Objectives........................................ 11 1.2 Rock Salt......................................... 12 1.3 Processes of diapir growth............................ 12 1.4 Dense inclusions................................... 15 1.5 The evolution history of the Gorleben diapir.............. 15 1.6 Long-term safety................................... 16 2 Numerical modeling.................................... 17 2.1 Fundamental principles.............................. 17 2.1.1 Conservation of mass............................ 17 2.1.2 Conservation of momentum....................... 18 2.1.3 Conservation of composition...................... 19 2.2 Symmetric boundary condition........................ 20 3 Rheology ............................................ 21 3.1 Rocksalt......................................... 21 3.2 Anhydrite........................................ 22 3.3 Overburden....................................... 22 4 Sedimentation technique ................................ 23 5 Summary of the Papers ................................. 25 5.1 PaperI: Rise and fall of a dense layer.................... 25 5.1.1 Modeling concept.............................. 25 5.1.2 Summary..................................... 26 5.1.3 Conclusions................................... 28 5.2 PaperII: The parameters that influence salt supply.......... 30 5.2.1 Modeling concept.............................. 30 5.2.2 Overview..................................... 31 5.2.3 Conclusions................................... 31 5.3 Paper III: Evolution of the Gorleben salt diapir............ 34 5.3.1 Modeling concept.............................. 34 5.3.2 Summary..................................... 34 5.3.3 Conclusions................................... 37 6 Concluding Remarks ................................... 39 7 Summary in Swedish ................................... 41 Acknowledgments ........................................ 43 References.............................................. 45 List of Figures 1.1 Global distribution of basins containing salt structures . 13 1.2 Modes of diapir piercement . 14 1.3 Line drawing of the Gorleben salt diapir . 16 4.1 Sediment aggradation . 24 5.1 The initial geometric condition . 26 5.2 Regions of the diapir piercement . 27 5.3 Snapshots of models . 29 5.4 Illustrated areas of the diapir and entrained anhydrite . 31 5.5 Shapes of passive diapirs . 32 5.6 Salt supply and entrainment of the anhydrite layer . 33 5.7 The initial geometry of the model for Gorleben diapir. 35 5.8 Evolution of the models for different salt rheologies . 36 5.9 Internal displacement field of the diapir (PF-Model) . 37 1. Introduction 1.1 Objectives For the last 40 years, scientists and engineers have been searching for geologi- cally suitable repositions for radioactive waste. Salt layers and structures have been targets as repositories for hazardous waste [e.g. Gorleben and Morsleben salt diapirs in Germany; WIPP site in USA and Anloo, Gasselte (Drenthe) and Winschoten (Groningen) in Netherlands]. Currently only one repository in salt is in use, the Waste Isolation Pilot Plant (WIPP) where long-lived radioactive waste is buried in deep salt beds (40 kilometres east Carlsbad, New Mexico). However, many other salt structures are targets for waste storage (e.g. Gor- leben diapir in Germany). Tectonic stability of a salt structure is a significant factor in evaluating its suitability as a repository (e.g. hydrocarbon storage, waste disposal, etc.). To evaluate safety of a repository many different scenarios have been consid- ered such as: suberosion and diapirism, brine migration, thermo-mechanical fractures, flooding of a disposal mine, large brine inclusions, diapirism to the biosphere, solution mining etc (e.g. onshore disposal committee, 1989). Many of these scenarios have been applied to Gorleben salt diapir, which is currently used as an intermediate storage facility and was targeted as a future final repos- itory for high-grade radioactive waste. However, the influence of dense inclu- sions, anhydrite layer, which is present within the Gorleben salt diapir, was not considered as a possible disturbance of the repository. Based on analogue and numerical models Koyi[2001] suggested that Gorleben diapir might be active internally due to presence of a dense anhydrite layer (blocks) within it. According to Koyi’s [2001] models, the denser blocks entrained by the diapir, sink within the externally inactive Gorleben diapir. Indications for movement of the anhydrite blocks comes from acoustic emission measurement that have recorded displacement on the boundary between rock salt and the anhydrite blocks [Spies and Eisenblätter, 2001]. Understanding the structural evolution of an initially tabular salt layer with intercalated anhydrite layer of high density and high viscosity and investiga- tion of the parameters that influence its development are required for evalua- tion of salt structures as a "safe repository". Salt tectonics is therefore critical for radioactive waste storage in rock salt in many parts of the globe. First, a short introduction to the modern understanding of the salt tectonics is presented below, followed by geology of the Gorleben diapir, which has been used as a general guideline in this work. Second, the applied modeling 11 procedures, together with the theoretical background for the modeling are ex- plained. Finally a brief summary of each paper including conclusions is given. 1.2 Rock Salt Halite, a colorless or white mineral sometimes tinted by impurities that is commonly known as "rock salt" is found in beds as evaporites. The term "rock salt" is used to include all rock bodies composed primarily of halite [Jackson, 1997a,b]. Salt naturally occurs in immense deposits, and occasionally in sur- face deposits in arid areas as the mineral halite. Salt deposits are widespread. There are major salt basins in the different parts of the world (Fig. 1.1). A number of these salt deposits are mined for halite. Salt has mechanical properties different from those of most clastic and car- bonate rocks. Under unusually high strain rates, salt fractures like most other rocks. However, under typical geologic strain rates, salt flows like a fluid in the subsurface and at surface [Weijermars et al., 1993]. Salt is also relatively incompressible so it is less dense than most carbonates and all moderately to fully compacted siliciclastic rocks. Salt rheology and incompressibility make it inherently unstable under a wide range of geologic conditions. Diapirs are a major type of salt structures resulting from tectonic deformation. Bedded salt deposits are nearly horizontal, although some contain fault zones and other anomalies. Salt beds range in thickness from a few tens of meters to several thousands of meters. The differential compaction of the sediments that cover salt may produce instabilities and flows within the salt layer that may result in salt domes or diapirs. Furthermore, salt is also an effective conductor of heat, elevating the ther- mal maturity of rocks above salt structures and cooling