Three-dimensional geological and geomechanical modelling of a repository for waste disposal in a domal salt structure

Transcription

1 GOSPODARKA SUROWCAMI MINERALNYMI Tom Zeszyt 3/2 MICHAEL HOFMANN*, OTTO BORNEMANN*, SANDRA FAHLAND*, STEFAN HEUSERMANN* Three-dimensional geological and geomechanical modelling of a repository for waste disposal in a domal salt structure Wprowadzenie Over the last three decades, the Federal Institute for Geosciences and Natural Resources (BGR), Germany, has been carrying out extensive geological and geomechanical research and practical project work on domal salt structures to prove their suitability for the disposal of radioactive wastes as well as for the construction of salt caverns used for solution mining and the storage of oil or gas. Especially for waste disposals, the natural geological barrier is an important part of the multiple-barrier system of the repository. Thus, the load-bearing capacity and geomechanical integrity of the rock, its geological and tectonic stability, and its geochemical and hydrogeological development are important aspects of the required safety analysis. This analysis must include several basic steps and items, e.g. geological investigations to provide the basic geological data for two- and three-dimensional modelling of the salt structure, mining observations and experience, geotechnical in-situ measurements to provide the necessary parameters of the host rock and the overburden, geomechanical laboratory investigations to determine the relevant properties of the rock and to develop adequate material models, two- and three-dimensional geomechanical model calculations, and, if necessary, additional thermomechanical or hydromechanical model calculations to analyse the stability of the repository and the integrity of the geological barrier, and a safety assessment taking all geological, experimental and theoretical investigation results into account (Bornemann, Heusermann 2005). * Federal Institute for Geosciences and Natural Resources, Hannover, Germany; michael.hofmann@bgr.de

2 64 As an example, the results of geological and geomechanical investigations of the Morsleben repository are presented including three-dimensional modelling of the geological structure of the central part of the mine as well as three-dimensional geomechanical model calculations to evaluate the stability of old mining rooms as well as the integrity of the salt barrier. The Morsleben repository was established in the old Bartensleben mine, a former salt and potash mine consisting of several mining parts, e.g. the southern, western, and eastern part located at the periphery of the mine and used for waste disposal (Heusermann et al. 2007). From a geomechanical point of view, the most critical and important part of the repository is the central part which is not used for disposal, but shows the most considerable degree of excavation and numerous large old mining rooms (Fahland et al. 2007). Based on the two dimensional model of the Morsleben repository (Behlau et al. 1997, 1998) a three dimensional model was designed in the BGR in the years 2006 and The intention of the activity was the improvement of the visualisation and the advancement of the interpretation from the existing geological basic data. Also new drillings were introduced to built the 3D-model. The construction of the 3D-model occurs in the view of the prospective closing of the repository. Within the frame of the backfilling of the repository a lot of new galleries and drillings are necessary. The 3D-models helps as an efficient planning tool for the exact positioning of these activities. Especially the interpretation of new data, which were created during the monitoring, will be improved in the future e.g. microacoustic and groud penetrating radar (GPR) data. Different basic data (Fig. 1) are used for the purpose of the modelling. All of these data are based on real coordinates and be correlated with each other and interpreted by the geologist in the 3D-space. In the 3D-model a discrete 3D-body exists for every geological unit. The assignment of the stratigraphy (Borneman 1991) for the different salt rocks were combined to nine main units in the 3D-model. The geological units can be displayed in any combinations. The digital 3D-mining layout (Zerna 2004), which also exists in a digital version, can be visualized together with the geological model. It is also possible to generate intersections of the geology and the drift. With the construction program opengeo TM a complex geological 3D-model was generated. Every single point within the 3D-modell is well defined. The interpretation of the basic data in the three-dimensional space led to a consistent and high grade image of the underground. Further basic data can be easily included to the 3D-model. The development of the geomechanical model based on the two dimensional geological model. Workings on the interface to exchange the 3D-geometry are in process. To establish a finite-element model for purposes of stability and integrity evaluation, the model of the geological structure had to be idealized on the basis of a characteristic geological cross section perpendicular to the axis of the structure and the mining rooms. The idealized geological layers were classified with respect to the steady-state creep behaviour.

3 65 Unit cr z2+z3 z2sf z2ds-z3lk z3ha z3bs-bk/bd z3am z3ss-z3tm z4 Description Cap rock Staßfurt- und Leine-formation, structureless Kaliflöz Staßfurt Decksteinsalz bis Leine-Karbonat Hauptanhydrit Basissalz bis Bank-/Bändersalz Anhydritmittelsalz Schwaden- bis Tonmittelsalz Aller-formation, structureless Fig. 2. Combined stratigraphic units in the 3D-model Rys. 2. Zestawione jednostki stratygraficzne dla modelu 3D The main units of the Zechstein strata (salt layers z2hs, z2sf, z3ls, z3os, z3bk/bd, z3am/ss, and anhydrite layers z3ha) and, if necessary, composites of the main units (z3os/bk/bd) were considered. The Hauptsalz z2hs was separated into two several parts (z2hsw and z2hso), due to different creep behaviour. The structure of the overburden was idealized taken into account the main layers caprock cr, DGL layer (including layers of Deckanhydrit, Grauer Salzton, and Leinekarbonat) within the caprock, Keuper k, Jurassic- -Cretaceous j-kr and Quaternary q (Fig. 4). The deformation behaviour of the ductile rock salt layers was described by a constitutive equation including both elastic and steady-state creep deformations. In addition, the dilatant behaviour of rock salt was considered using a new dilatancy concept according to HUNSCHE & SCHULZE (2003). Calculations were made using the finite-element codes ANSALT developed by BGR and the new commercial JIFE code for THMC processes developed by SRD company, Berlin. Pre- and post processing of the data was done with the INCA/PATRAN tool. The three-dimensional finite-element model included half of the length of the rooms and of the pillar at the head of the rooms for reasons of symmetry. Figure 5 shows a plot of the entire 3-D model, 750 m in height, 850 m wide and 75 m in length, comprising about 120,000 nodal points and 120,000 isoparametric 8-node elements. It was assumed that the rooms were instantaneously excavated in the year Thus, up to now a time elapse of about 67 years had to be regarded to analyse the recent stress and deformation state. As an example, the calculated dilatant rock zones are plotted in Figure 6. Excavation of the rooms and creep of the salt rock cause the development of dilatancy in major parts of the roofs and pillars around the rooms as well as in larger rock zones between rooms and the anhydrite layers. Comparing the results of 3-D modelling to former 2-D calculations, a certain reduction of dilatancy in the salt rock is obtained. This is caused by the more favourable three-dimensional structural behaviour and the related lower amount of de-

4 66 Fig. 6. Three-dimensional finite-element model of the central part Rys. 6. Trójwymiarowy skoñczony element modelu centralnej czêœci wysadu viatoric stresses. The geomechanical 3-D calculations show that the integrity of the salt barrier is given for most parts of the salt rock, especially at the top of the salt structure. Since significant zones of dilatancy occur between the rooms and the anhydrite layers, potential migration of brine from the caprock into the mining rooms via the anhydrite layers and the dilatant salt rock cannot be excluded. Combined geological and geotechnical investigations including geological modelling, geotechnical in-situ measurements, laboratory tests, and geomechanical modelling have been successfully carried out by BGR for several waste disposal projects, e.g. Gorleben salt dome and Morsleben repository, as well as for a couple of salt cavern projects in several European countries. REFERENCES Behlau J.,Mingerzahn G.,1998 ERAMorsleben Erarbeitung eines geologischen Lagerstättenmodells. 2. Anhang zum Abschlußbericht Struktureller Bau der Westflanke der Hauptmulde im Bereich des Abbaues 1a.- BGR, unveröffentl. Ber., : 17 S., 2 Tab., 13 Anl.; Hannover. B e h l a u J., M i n g e r z a h n G., B o r n e m a n n O., 1997 ERA Morsleben Erarbeitung eines geologischen Lagerstättenmodells Morsleben. Abschlußbericht.- BGR, unveröffentl. Ber., : 73 S., 1 Tab., 61 Anl.; Hannover.

5 67 B o r n e m a n n O., 1991 Zur Geologie des Salzstocks Gorleben nach den Bohrergebnissen.- BfS-Schriften, 4/91: 67 S., 13 Abb., 5 Tab., 24 Anl.; Salzgitter. B o r n e m a n n O., H e u s e r m a n n S., 2005 Geological and geotechnical investigation methods to characterize domal salt structures. IV. Int. Congress Brown Coal Mining, June 6 8, 2005, Belchatow, Poland. Fahland S., Heusermann S., Eickemeier R., Nipp H.-K., Preuss J., 2007 Three-dimensional geomechanical modelling of old mining rooms in the central part of the Bartensleben salt mine. The Mechanical Behavior of Salt Understanding of THMC Processes in Salt (Eds. Wallner, Lux, Minkley & Hardy, Jr.), , Taylor & Francis Group, London. H e u s e r m a n n S., N i p p H.-K., E i c k e m e i e r R., F a h l a n d S., P r e u s s J., 2007 Geomechanical integrity of waste disposal areas in the Morsleben repository. REPOSAFE 2007 Int. Conf. on Radioactive Waste Disposal in Geological Formations, Nov. 6 9, 2007, Braunschweig, Germany. H u n s c h e U., S c h u l z e O., 2003 The dilatancy concept a basis for the modelling of coupled TMH processes in rock salt. European Commission CLUSTER Conference on the Impact of EDZ on the Performance of Radioactive Waste Geological Repositories, Nov. 3 5, 2003, Luxembourg. GEOLOGICZNE I GEOMECHANICZNE MODELOWANIE 3D DLA SK ADOWISKA ODPADÓW ZLOKALIZOWANEGO W WYSADZIE SOLNYM S³owa kluczowe Modelowanie 3D, wysady solne, sk³adowanie odpadów radioaktywnych, Niemcy Streszczenie Przez ponad trzy dekady Federalny Instytut Nauk Geologicznych i Naturalnych Zasobów (BGR Niemcy) prowadzi³ intensywne geologiczne i geomechaniczne badania i praktyczne prace projektowe w strukturach wysadowych w celu okreœlenia ich przydatnoœci dla sk³adowania odpadów radioaktywnych jak i dla budowy w nich zbiorników kawernowych do magazynowania paliw oraz gazu. W szczególnoœci dla odpadów radioaktywnych, naturalna geologiczna granica jest wa n¹ czêœci¹ systemu multi-barier sk³adowiska. St¹d te, pojemnoœæ noœna i w³aœciwoœci geomechaniczne ska³, ich geologiczna i tektoniczna stabilnoœæ, a tak e geochemiczne i hydrogeologiczne w³aœciwoœci s¹ wa nymi aspektami wymaganej analizy bezpieczeñstwa. Tego typu analiza musi zawieraæ kilka podstawowych kroków, takich jak: geologiczne rozpoznanie w celu okreœlenia podstawowych danych geologicznych do modelowania 2D oraz 3D struktur solnych, obserwacje kopalniane, geotechniczne pomiary in situ w celu prognozowania niezbêdnych parametrów wysadu oraz ska³ nadk³adu, geomechaniczne badania laboratoryjne w celu okreœlenia istotnych w³aœciwoœci ska³ i opracowywania odpowiedniego modelu materia³owego, 2- i 3-wymiarowe geomechaniczne obliczenia i jeœli to konieczne dodatkowe termomechaniczne i hydromechaniczne obliczenia modelowe do analizy stabilnoœci sk³adowiska i integralnoœci geologicznej bariery. THREE-DIMENSIONAL GEOLOGICAL AND GEOMECHANICAL MODELLING OF A REPOSITORY FOR WASTE DISPOSAL IN A DOMAL SALT STRUCTURE Key words 3D modeling, salt diapirs, disposal of radioactive wastes, Germany Abstract Over the last three decades, the Federal Institute for Geosciences and Natural Resources (BGR), Germany, has been carrying out extensive geological and geomechanical research and practical project work on domal salt

6 68 structures to prove their suitability for the disposal of radioactive wastes as well as for the construction of salt caverns used for solution mining and the storage of oil or gas. Especially for waste disposals, the natural geological barrier is an important part of the multiple-barrier system of the repository. Thus, the load-bearing capacity and geomechanical integrity of the rock, its geological and tectonic stability, and its geochemical and hydrogeological development are important aspects of the required safety analysis. This analysis must include several basic steps and items, e.g. geological investigations to provide the basic geological data for two- and three-dimensional modelling of the salt structure, mining observations and experience, geotechnical in-situ measurements to provide the necessary parameters of the host rock and the overburden, geomechanical laboratory investigations to determine the relevant properties of the rock and to develop adequate material models, two- and three-dimensional geomechanical model calculations, and, if necessary, additional thermomechanical or hydromechanical model calculations to analyse the stability of the repository and the integrity of the geological barrier, and a safety assessment taking all geological, experimental and theoretical investigation results into account (Bornemann, Heusermann 2005).

7 Fig. 1. Basic data sources for the 3D-model Rys. 1. Podstawowe dane Ÿród³owe dla modelu 3D Fig. 3. Geology at the salt level (top of salt dome) Rys. 3. Geologia na poziomie zwierciad³a solnego (strop wysadu solnego) Fig. 4. Special view to the 3D-model of the central part of the Morsleben repository Rys. 4. Widok specjalny modelu 3D centralnej czêœci sk³adowiska w Morsleben

8 Fig. 5. Idealized geological structure of the central part Rys. 5. Idealizowana geologiczna struktura centralnej czêœci wysadu Fig. 7. Spatial distribution of dilatant zones in the rock salt around the mining rooms Rys. 7. Przestrzenne rozmieszczenie stref dylatacji w soli kamiennej wokó³ komór