Technischer Bericht NTB 88-27

Nagra is conducting a comprehensive geologic exploration program in northern Switzerland with the goal to provide the basic scientific information which is necessary to assess the suitability of subsurface formations for the safe storage of high-level radioactive wastes. The exploration program is composed of: a deep drilling and geophysical survey program to examine the regional subsurface geological and structural setting; a hydrogeological investigation program to delineate groundwater flow system(s) within the deep subsurface; and a neotectonic study program to examine recent tectonic activity and to identify zones where active earth movements maybe occurring.

On June 24, 1980, a request for twelve exploration boreholes was submitted by Nagra to the Federal Traffic and Energy Department. The objective of these deep boreholes was to provide information concerning the geological characteristics of the crystalline basement and overlying sedimentary cover, for a 1200 square kilometre region within northern Switzerland. In addition, they also provide regional hydrodynamic and geochemical data for the development of mathematical models of the hydrogeological regime for the region between the northern slope of the central and eastern Swiss Alps to the Black Forest.

Following federal approval for eleven of the twelve exploration boreholes on February 17, 1982, by the Bundesrat, the deep drilling program was initiated with the Böttstein exploratory borehole in October 1982. Currently, six boreholes have been drilled and intensively tested. They include: Böttstein (drill depth = 1501 m), Weiach (2482 m), Riniken (1801 m), Schafisheim (2006 m), Kaisten (1306 m), and Leuggern (1689 m). After completion of drilling and associated testing, all boreholes were equipped with multi-packer systems for the long-term monitoring of hydrologic pressure levels in selected aquifers and permeable zones. The long-term monitoring phase is currently on-going at all of the borehole sites, and will be completed in a few years. The scientific results have, to a large extent, been reported or will be published in the near future in the form of individual test or final investigation reports.

With Siblingen (the seventh in the series of northern Swiss exploration boreholes), the objective is to obtain the relevant information for assessing the long-term storage characteristics of the crystal line basement and overlying Mesozoic sedimentary rocks within the region northwest of Schaffhausen.

The borehole site (coordinates 68'090/28'693/574) is located in a slightly elevated area, approximately 1.5 km northwest of the Siblingen village center. Directly underlying the Siblingen borehole site are argillaceous marls and limestones of the Lias formation. No significant surficial Quaternary deposits are present. Approximately 350 m of Jurassic and Triassic sediments, mainly marls and argillaceous carbonates, are expected to underlie the Lias, which generally dip a few degrees towards the southeast. Below the Buntsandstein, to the projected completion depth of 1500 m, is estimated to be crystalline rock of unknown lithology. Currently available information does not exclude the possibility that a small thickness of Permian sediments may overly the crystalline basement. Significant tectonic faults are not known to exist within the vicinity of the planned borehole.

The work program for the Siblingen borehole contained in this report presents a detailed technical description of its design aspects, and describes the scientific objectives, the scope of investigations, test methods to be used, and the sequence of foreseen field and laboratory tests. The detailed work program has a dual purpose. Firstly it provides to participants involved, the general guidance and basis for all work activities performed within the borehole project. Secondly, it serves to inform government agencies, supervisory organizations, and the interested public of the planned work.

Aspects of the scientific investigation program, presented in Chapters 6 and 7, can be described in two ways: objective-oriented and from an operational perspective. Table 1 lists the Siblingen investigation objectives and identifies the established evaluation methods to be used in achieving these objectives. From an operational perspective, the scientific investigation can be divided into three main program parts: (1) geological studies of retrieved core, (2) wireline geophysical surveys within the borehole, and (3) hydraulic tests and groundwater sampling of selected test intervals.

It is currently planned to core the entire borehole length (i.e., to 1500 m). Based on analysis of the obtained core, (which will be oriented based on sonic-televiewer surveys of the borehole walls), a detailed petrographic, structural and stratigraphic description of intersected sedimentary sections will be obtained. Of particular importance will be the accurate detection and characterization of fractures and water-bearing features within the crystalline rock. Initial macroscopic studies of the retrieved core, conducted at the borehole site, will be expanded through results obtained from more detailed and comprehensive geological/mineralogical laboratory microscopic analyses.

The extensive borehole geophysical program (with its variety of different types of geophysical surveys) is expected to provide valuable information for many facets of the overall Siblingen investigation program. First, a direct contribution to fulfilling project objectives is expected; and secondly, results from various geophysical surveys should complement and support non-geophysical studies planned at Siblingen (e.g., for hydraulic tests: identification and location of borehole fracture zones; selection of test intervals; etc.). The breadth of information obtained from the borehole geophysical program will be extensive and will provide information on a wide range of rock and/or fluid characteristics and properties. These include: petrophysical characteristics of the intersected formations (e.g., density, elastic properties, electrical conductivity, natural gamma emission, etc.); identification of structural features (i.e., location of planar discontinuities) through borehole imagery; and the quantitative determination of fluid influx to the borehole, through electrical conductivity and temperature profile measurements within the fluid column (fluid logging).

A particularly important aspect within the scope of the Siblingen investigation program is the determination of the hydraulic characteristics of the crystalline basement rock and the overlaying sedimentary aquifer system(s). Single and double packer tests are planned for the determination of hydraulic parameters, in particular transmissivity (or associated hydraulic conductivity) and formation pressure. For borehole sections intersecting Mesozoic aquifers and fluid inflow zones within the crystalline, the hydrologic testing program will also include the taking of groundwater samples with the downhole testing-string. The water samples will be analyzed for a large suite of chemical and isotopic constituents. The sample analyses will provide important information concerning groundwater age and resident flow time within associated rock formations. The borehole drilling fluid will be "marked" with selected hydrochemical tracers which will enable the quantitative determination of sample representativeness (through on-site chemical analysis). Besides the monitoring of tracer concentration, gas content and drilling fluid mass-balance (i.e. groundwater produced versus drilling fluid lost) will also be continually monitored during the borehole drilling phase.

Contained also in this report is a technical description of the borehole construction phase of the work program (in Chapter 9), which describes the technical details and specifications of the planned drilling rig (Typ Wirth B8 F24). The technical data and specifications refer to the actual drilling phase from the surface, through the predicted geological formations, to completion depth; details are also given of the equipment and materials to be used, e.g., core bits, tubing strings, casing, blowout-preventers, drilling fluids, and cement characteristics. All technical specifications and materials listed, including the borehole diameter/depth profile, casing scheme, and drilling fluid program have been selected based on the diverse requirements of the planned measurement and testing programs.

The total time required for borehole drilling and conducting the comprehensive scientific investigation program (i.e., drilling and testing phase) is estimated to be about nine months. Depending on the existing hydrologic conditions and results from hydraulic testing, it may be decided to equip the borehole (i.e., after the testing phase) with a multi-packer system to monitor the long-term pressure fluctuations within selected deep groundwater zones. The possible installation of a long-term monitoring system and technical discussion of aspects of the associated "monitoring phase", however, are not part of this work program.