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Technischer Bericht NTB 83-25

Sondierbohrung Schafisheim Arbeitsprogramm

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Nagra, the National Cooperative for the Storage of Radioactive Waste is carrying out a comprehensive geological research programme in northern Switzerland. This will provide the scientific knowledge which is required for assessment of the feasibility of safe disposal of highly radioactive waste in the geological formations of the area.

The various investigations comprise a programme of 12 deep boreholes, a regional geophysical reconnaissance of the petrographical and structural conditions, a hydrogeological programme for the clarification of groundwater movements in the deep subsurface and neotectonic observations to detect and measure active crustal movements.

On June 24, 1980 Nagra submitted to the Swiss federal authorities a request for permission to drill 12 deep boreholes for scientific purposes. This drilling campaign aims at the geological evaluation of the crystalline basement and its sedimentary cover in an area of approx. 1200 km2. In addition, the wells should supply much needed hydrodynamical and geochemical data for the construction of a mathematical model simulating the hydrogeological conditions in the earth's crust between the northern slope of the central and eastern Swiss Alps and the Black Forest.

The federal permits for 11 of the 12 boreholes were granted on February 17, 1982 and drilling began at Bottstein in October 1982. The second well in the sequence, Weiach, was started in January 1983 and the third well, Riniken, spudded in June 1983. At present, drilling has been completed at Bottstein and there the testing phase is about to begin. The wells Weiach and

The fourth well, Schafisheim, will be located on coordinates 653 620/246 760 at an altitude of 421 m (ground elevation) above sea level. This lies in the very gently folded and generally SE dipping Swiss Molasse Basin close to its northwestern border, approx. 13 km ENE of the eastern plunge of the Born - Engelberg anticline. In this part of the tertiary basin the ancient glacier of the river Reuss has carved several troughs into the Lower Freshwater Molasse. These have been filled subsequently by thick quaternary gravel beds and fine-grained lacustrine deposits.

According to the geological prognosis the well will penetrate underneath an approx. 180 m thick layer of quaternary beds a slightly SE dipping 1300 m thick sedimentary series of tertiary sands, silts and marls, jurassic limestones, marls and clays as well as triassic evaporites, clays, limestones and sandstones. It is possible that the well will encounter a thin permian sequence before entering the crystalline basement. The vertical penetration of the crystalline rocks is planned to be approx. 500 m.

The total depth of the well will be dependent on the geological and hydrogeological condition encountered as it is imperative to procure sufficient data from the crystalline basement to underpin the regional hydrogeological model.

The present work programme consists of a technical part, the drilling programme, and a scientific part, the sampling, measuring and testing programme. It also lists the various directions and provisons imposed by the federal and cantonal authorities for the conduct of the drilling operations.

The drilling programme contains detailed instructions for the drilling contractor concerning, on the one hand, the technicalities of drilling through different rock types to the planned total depth and, on the other hand, the equipment and materials to be used, such as blow-out preventers, casing, core barrels, drill bits, chemicals and cements. These instructions are determined, within the technical capacity of the stationary and fully electrified rig to be utilized, by the requirements of the sampling, measuring and testing programme.

In Table 1 the scientific part of the work programme is summarized. For Schafisheim, the fourth well of the drilling campaign, it is also quite extensive and varied as it has to cater for a wide range of conceivable geological conditions, particularly in the still little known crystalline basement.

As it is the intention to take oriented cores in selected larger intervals of the sedimentary overburden and throughout the crystalline sequence, a detailed lithological, sedimentological and petrographical analysis of all potential barrier and host rocks will be possible. The oriented cores will also permit the spatial evaluation of the various fracture systems that are most probably providing the preferred paths for groundwater flow in the crystalline basement.

An extensive geophysical borehole logging programme will serve to verify the results of the core analysis and complement the core data. Numerous borehole logs are to be registered with different types of tools. These allow one to determine various parameters essential for the full description of the rock sequences penetrated. A first category of logs enables the petrographical identification of the different rock types and indicates porous zones that are either water- or hydrocarbon-bearing. A second category provides data e.g. on the degree of pore and fracture fill, rock density and rock temperature, natural gamma radiation and rock­mechanical properties. Other logs measure strike and dip of the sedimentary layers and the position of rock fractures. A fourth category provides information on the diameter and the deviation of the borehole, the quality of casing cementations and the position of casing joints. In addition, well shooting surveys will supply exact values of seismic velocities for the various rock units; data that are needed for the depth correction of the reflection profiles from Nagra's regional seismic network.

With numerous hydrological tests ranging from a production tests of the triassic Muschelkalk aquifer to labelled slug tests in low-permeability crystalline sections, the hydraulic conditions of deep groundwater flow will be investigated. The recovered water samples will undergo full physical and geochemical analysis. Furthermore, their isotope content is to be measured in order to estimate the age of the various formation waters and their time of residence in the various aquifers.

To round off the scientific investigations, a series of rockmechanical and geotechnical laboratory tests will provide characteristic values to be applied eventually in the design and construction of shafts and caverns for an underground repository.

After completion of the various tests and before the final plugging of the well, there will be an observation phase of at least 12 months. During this time the pressure variations in the aquifers encountered will be monitored. If required, long term production tests will provide additional water samples from lowpermeability zones .in crystalline rocks.
 
The total drilling and testing activities, from the spudding of the well to the begin of the observation phase, are estimated to take from 11 1/2 to 14 months.

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