Technischer Bericht NTB 85-09

Sondierbohrung Böttstein Hydrogeologic Testing of Crystalline Rocks

In addition to comprehensive studies in geology, geophysics, hydrochemistry and rock mechanics, a three-phased program for (1) drilling (2) testing and (3) monitoring of the twelve boreholes was proposed. The Boettstein borehole is located in the central part of the target areas. It was the first borehole to be drilled. Drilling in the crystalline granitic basement rocks started at a depth of 315 m below ground surface in November, 1982 and was completed in June, 1983 at 1501 m. The testing phase commenced in June and was completed in December, 1983. The monitoring phase is on-going at this time.

The study reported herein describes the hydrogeologic testing of the crystalline rocks and results of the work done by Gartner Lee AG (GLAG) in the Boettstein borehole on behalf of Nagra. This report describes testing equipment and performance. Also included are sections on the testing and analysis methods that were used to determine the hydrogeologic results.

Testing was conducted using single and double packer tools with associated downhole and surface electronic equipment. Downhole information from pressure transducers and thermistors were converted from frequency signals to pressure and temperature readings that were printed, plotted and stored on magnetic tape at the surface facility. All the testing equipment worked well.

The hydrogeologic testing program consisted of: (1) single packer tests conducted during the drilling phase, (2) hydrogeologic reconnaissance logging carried out during both the drilling and testing phases and (3) double packer tests conducted during the testing phase.

A total of eighteen single packer tests were completed in the Boettstein borehole to provide information on formation pressure and hydraulic head. Seventy-six hydrogeologic reconnaissance logging intervals were tested including repetition of some zones due to anomalous results. The hydrogeologic reconnaissance logging was completed using double packers with straddle intervals of 12.5 m or 25.0 m. This testing was done to provide rapid semi-quantitative assessment of hydraulic parameters of fracture zones and their variation over the borehole length. Double packer sampling and formation testing was carried out utilizing a work-over rig after the Boettstein borehole drilling was completed at 1501 m depth. This testing phase was carried out to isolate discrete fracture sets and to obtain representative groundwater samples for chemical analysis as well as to provide further information for the determination of hydraulic parameters on seventeen selected zones.

Hydraulic head is the driving force for groundwater movement. The main controlling factor for its measurement is the time available to reach equilibrium or static conditions. Consequently less permeable zones require a longer test time than more permeable zones. In addition, temperature and borehole history conditions influence formation static pressure measurement and thus hydraulic head determination. Reliable formation pressures were obtained from seven tests in the Boettstein borehole. The data from the testing program suggest that the fractures are hydraulically influenced by the drilling activity. Thus reliable static pressures are determined only after a long period of testing time (tens of days) or very shortly after drilling a portion of the rock before drilling effects are imposed on the fracture system.

The pressure profile developed for the borehole indicates the rate of pressure change varies with depth between zones of known pressure. No conclusions are drawn from these data, but results of the long-term monitoring at Boettstein should be assessed from this perspective.

Similarly, no conclusions are drawn from the calculated values of hydraulic head pending further information from geochemical characterization and pressure monitoring data.

Hydraulic conductivities were determined from the single packer, hydrogeologic reconnaissance logging and the double packer detailed sampling/testing phases of the hydrogeologic testing program. Hydraulic conductivities can be reliably estimated or interpolated with less than 100 % recovery to formation static pressure conditions. Furthermore, temperature changes can significantly affect the derived hydraulic conductivity value from type-curve solutions and the effect is proportional to the rate of temperature change. Temperature effects are more significant at low hydraulic conductivity values. The hydraulic conductivity values in the Boettstein borehole have been reported as an equivalent porous media value, Kpm.

The hydraulic conductivity values for the crystalline rock portion of the Boettstein borehole ranged from about 1e-13 to 1e-05 m/s and the profile shows discrete zones of high permeability which decrease with depth. In general, where overlap occurs from single packer, hydrogeologic reconnaissance logging and double packer detailed sampling/testing the hydraulic conductivity results show close agreement. Thus repeatability from one testing method to another was found to be good. Accuracy of the hydraulic conductivity values was assessed to be less than +/- one half order of magnitude for values greater than 1e-11 m/s and less than +/- one order of magnitude for values less than 1e-11 m/s.

Assessment of transient hydraulic data has shown that fractured medium testing can yield results that may be interpreted by type-curve matching as equivalent porous medium. However, in other instances pressure responses may only be attributable to fracture flow. Generally, the hydraulic test data from the Boettstein borehole match equivalent porous medium analyses. This suggests that the conditions tested are at a larger scale than individual single fractures.

Pressure transient responses in some test intervals were influenced by temperature variations or from pressure history effects caused by the borehole being open to annulus pressure conditions. In these cases type-curve solutions will not likely provide accurate estimates of hydraulic parameters. Therefore, under these conditions analytical and numerical models that take these effects into account were used for determination of hydraulic parameters.

Groundwater temperature data were used to calculate thermal gradients for the granitic rocks in the Boettstein borehole. The data suggest that the thermal gradient increases with depth. From 350 to 1000 m the gradient is about 2.7°C per 100 m and from 1000 to 1500 m about 3.5°C per 100 m. These results have been confirmed by the geophysical temperature log.

In summary, the hydrogeologic testing activities at the Boettstein borehole were successful in providing information for Nagra's regional assessment of the crystalline basement rocks. In addition, water samples could be obtained from discrete intervals for geochemical characterization. Continuing groundwater monitoring activities at this borehole will add to the data base provided by this report.