Technischer Bericht NTB 84-38

Sondierbohrung Leuggern Arbeitsprogramm

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 Boettstein in October 1982. At this location, an extensive testing programme has been completed and the well is now in the observation phase. The second well in the sequence, Weiach, was spudded in January 1983 and is about to enter the observation phase. The third well, Riniken, started in June 1983 and was suspended for the time being at a depth of 1800.5 m. The forth well, Schafisheim (spudding date November 26, 1983), and the fifth well, Kaisten (spudding date February 13, 1984), have reached total depth at 2006 m and 1306 m respectively.

The next well on the programme, Leuggern, will be located on coordinates 657'633.9/271'207.9 at an altitude of 358.8 m above sea level (ground floor elevation). This lies to the SW of the confluence of the rivers Rhine and Aare in the Tabular Jura near the southern border of the Black Forest Massif.

According to the geological prognosis the well will penetrate underneath approx. 49 m of Quaternary gravels the gently southward dipping Triassic sediments of the Muschelkalk and Buntsandstein formations. The crystalline basement will be reached at a depth of approx. 250 to 260 m to be penetrated vertically for at least 1000 m.

The present working 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 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 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 Leuggern, the sixth well of the drilling campaign, it is again quite extensive and varied as it has to cater for a wide range of conceivable geological conditions, particularly in the crystalline basement. It is planned to core the whole crystalline sequence as well as the sedimentary overburden. The cores will be oriented with the help of a Sonic Televiewer-log. In addition to the lithological and sedimentological analysis of the potential sedimentary host and barrier rocks, this will provide a complete petrographic record of the potential crystalline host rock in the immediate neighbourhood of the well. The oriented cores will also permit the spatial evaluation of the various fracture systems that are 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 production tests of the Muschelkalk and Buntsandstein aquifers 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 subsurface.

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 low-permeability zones in crystalline rocks. The duration of the observation phase will depend on the hydrogeological conditions encountered during the drilling and testing phases and the experience gained during the observation phase of the wells already drilled by Nagra.

The total drilling and testing activities, from the spudding of the well to the begin of the observations phase, are estimated to take from 8 to 13.5 months.