- Technischer Bericht NTB 99-12Download
The aim of the Benken borehole was to investigate the properties of the Opalinus Clay and surrounding rock formations and to correlate the seismic data with these results.
Following a licensing procedure which lasted several years, accessing and construction of the drillsite began in April 1998. As a result of the poor ground conditions, additional piling of the foundation for the drilling equipment was necessary, as was installation of a string of auxiliary casing down to a depth of 30 m.
Drilling began on 3rd September 1998 and, including the scientific investigations carried out in the borehole, continued till 12th May 1999. With a final depth of 1007.00 m, the borehole passed through 983.3 m of Neozoic and Mesozoic sediments and 23.7 m of gneisses of the crystalline basement.
With the exception of a salt horizon and crystalline basement in the deepest part of the borehole, geological predictions were largely confirmed by the drilling results.
The borehole was drilled with three-cone-bits for the uppermost 394.8 m; the remaining section was continuously cored using the wire line technique. The drilling rig used was a hydraulic rotary drillig rig, type Wirth B5-R.
For the cored sections, triple core barrels with synthetic liners were used almost exclusively and core recovery was around 99%. The main cored diameter and the final diameter of the borehole were 6 1/4" (158.8 mm).
A total of four permanent casings were installed in the borehole and, with the exception of the anchor casing, were cemented up to ground level. It was not necessary to install a further reserve casing.
The upper section of the borehole down to the base of the Malm limestones and the Upper and Middle Muschelkalk were drilled using a clay-freshwater drilling fluid. The section up to the installation of the anchor casing was marked by unfavourable geohydraulic formation properties.
For the first time in such an exploratory borehole, the Opalinus Clay and its neighbouring formations were drilled using a sodium silicate fluid. Despite the excellent functioning of this fluid in inhibiting the swelling of clay formations that are sensitive to freshwater, stability problems did occur. The reason for this was the hydraulic borehole tests, which were intensive in terms of time and procedure and partly involved several exchanges of drilling fluid. The difficult situation also led, on several occasions, to time-consuming fishing jobs.
Besides the two types of drilling fluid (clay-freshwater and sodium silicate), it was also necessary to use a saturated saline fluid for drilling through the salt horizon as stabilisation of the salt zone by cementation proved unsuccessful.
Once the final depth of 1007.0 m had been reached and the active borehole investigations were completed, the borehole was backfilled with cement up to 827.7 m.
With the perforation of the end casing and preparation of the borehole for installation of the long-tem monitoring system, work was completed 250 working days (continuous 24-hour operation) after spud-in. Of this total, the time used for scientific investigations in the borehole amounted to around 44% and the drilling activities to 56%.
Once the drilling equipment had been removed, a system for long-term monitoring of deep groundwaters was installed in the borehole and the drill-site was partly recultivated.