Technischer Bericht NTB 82-10

Sondierbohrung WeiachArbeitsprogramm

Nagra, the National Cooperative for the Storage of Radioactive Waste, has started a comprehensive geological research programme in northern Switzerland. This will provide the scientific know­ ledge 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 24.06.1980 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. 1'200 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 17.02.1982 and drilling has started at Böttstein in October of this year. The second well in the sequence, Weiach, will be located on coordinates 676 750/268 620. This lies near the northern boundary of the Molasse basin 900 m South of the river Rhine and approx. 8 km Northwest of the town of Bülach.

According to the geological prognosis, the well will penetrate underneath a 30 m thick layer of fluviatile gravels a slightly South dipping 820 m thick sedimentary series of tertiary marls and sandstones, jurassic limestones, marls and clays as well as triassic marls, evaporites and sandstones. A preliminary evaluation of new seismic reflection data suggests the presence of remnants of a permo-carboniferous trough under the triassic sequence. It could be filled with up to 750 m of predominantly argillaceous deposits. In this case the well would drill only about 400 m into the crystalline basement of granites and gneisses. Should the permo-carboniferous sediments be absent, however, the vertical penetration of the crystalline rocks would be approx. 1'000 m. In both cases the well is scheduled to reach a total depth of 2'000 m.

The present work programme consists of a technical part, the drilling programme, and a scientific part, the sampling, measuring and testing programme.    lt also lists the various directions and provisions 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 band, 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 Weiach, the second 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. Moreover, several of the plan­ ned investigations have similar aims. This will allow a comparison of the interpretative qualities and predictive capabilities of the various methods used so that the work programme for the follow-up wells can be optimized.

As it is the intentions to take oriented cores throughout the whole rock sequence, a detailed lithological-sedimentological analysis will be possible, particularly of those sedimentary units which can form migration barriers for radionuclides. These oriented cores will also permit the spatical evaluation of the various fracture systems that are most probably providing the preferred paths for groundwater flow in the granites and  gneisses.

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 an open-hole production test of the 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.

To round off the scientific investigations, a series of rock­ mechanical 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 the crystalline rocks.

The total drilling and testing activities, from the preparation of the drillsite to the begin of the observation phase, are estimated to take from 11.5 to 20 months.