Rolf will be undergoing some general maintenance in the area of his left hip in May 2006, this will improve his scrap value but inhibit his participation on this year’s bike trip with Christian, Sigi and I, along the Mosel Valley. As a short alternative, we set out by car on Wednesday 19th April for Prichsenstadt in Frankonia and Windischeschenbach in the Ober-pfalz. Our first port of call was in the “Gasthof & Weingut Zum Storch” in Prichsenstadt, to retry their asparagus and Silvaner wine, with an outing with the landlord's daughter in her horse carriage planned for the next morning. The fourlegged participants on this trip had been put out in the paddock to get rid of some of their excess energy, collected during a pretty inactive winter, and one of them promptly lost a shoe. As such, we set off to Windischeschenbach slightly earlier than planned.
The first part of the journey took us through the region of the Bavarian Jura known as the “Fraenkische Schweiz”, very picturesque and a favourite area for rock climbers. The transition onto volcanic cover rocks is abrupt and marked by a sharp change in the vegetation and topography. It was in this area that the first of a chain of volcanic cones became visible, these were scheduled to play a significant role in the story of the Continental Deep Drilling Project or KTB carried out between 1986 and 1994 close to the small town of Windischeschenbach
The project was finally approved by the Federal Ministry for Research and Technology in 1985, with a budget of 450 million Marks. Its aim was to reach rocks with a temperature range of 250-300°C, expected at around 10,000m in depth and to support the series of international research programmes, set up to cover the following themes:
- Evaluation of geophysical structures and phenomena
- Investigation of the thermal structure of the continental crust
- In-situ investigation of rock-fluids and their contribution to the formation of ore deposits
- Elucidation of the structure and evolution of the continental crust
- Determination of the Earth’s internal stress Field
In addition, the development of the leading-edge technology required for ultradeep drilling and the experience gained during the project are of great value in the drilling and service industries.
Windischeschenbach was selected from a number of possible drilling sites, as it is situated above what is thought to be the suture between the Saxothuringian and Moldanubian tectonostratigraphic units, associated with the Variscan orogeny during the Carboniferous, some 320 Ma ago. Subsequent erosion has brought once, deeply buried rocks close to the surface, making it an ideal place for the study of deep crustal processes. In addition, geophysical surface experiments have shown that the region around the drilling site is characterized by high electrical conductivity and gravimetric and magnetic anomalies. The KTB drill rig is the biggest and technically most advanced on-shore drill rig, worldwide and it was especially constructed with an automatic vertical drilling system to meet the requirements of an ultradeep borehole in crystalline rocks.
Some of the technical data are:
Total height 83m
Total weight ~ 2,500t
Weight of the drill string at 10,000 m length 400t
Maximum hook load 800t
Motor power 9,500 kW or 12,000HP
Mud pump capacity 1,000-4,000 l/min
Working pressure of the mud pumps 350bar
Mud tank volume 450,000l
In addition, the rig is designed with an automated pipe-handling system which saved time by handling three 13m lengths of pipe per roundtrip and with a maximum drill-string length of 12,000m.
In 1989, the KTB pilot hole was completed, down to a depth of 4,000m. The pilot hole is still used in experiments today, water is pumped out of the pilot hole and the miniearthquakes set off by this process are detected and measured by sensors in the main hole. In 1990, drilling started on the main hole and a depth of 5,500m was reached by the end of 1991. In 1992, the installation of the deepest 13 and 3/8 of an inch casing worldwide, was completed, with a total length of 6,000m and a total weight of 700 tons. By the end of 1992, the depth of 7,000 m was reached. In 1993, the vertical drilling concept was abandoned at a depth of 7,500m, after successfully crossing numerous diagonal fault zones with a total deviation of less than 15m, thereby setting new standards in vertical drilling technology. A depth of 8,000m was reached by the end of the year. In October 1994, at a depth of 9,101m and a rock temperature of 280°C, drilling was stopped.
You may be wondering what all the mud was about in the technical data of the drilling rig, it surprised me. In fact the mud is an essential part of the drilling process, it is pumped downwards through the drill pipe and drives a downhole motor which transmits the energy to the drill bit and causes it to rotate between 80-120 revolutions per min. The mud then flows around the core bit, both cooling and lubricating it. Apart from the cylindrical bore core which enters the core barrel above the bit, rock fragments cut by the drill bit must be removed from the bottom of the hole, this task is also carried out by the mud which is recirculated back to the surface. If it is necessary to stop the pumps, the mud immediately becomes highly viscous, thus preventing any rock fragments from sinking back to the bottom, when the pumps are restarted, the mud regains its liquid form again. That doesn’t cover all of the tasks carried out by the mud. It stabilizes the bore hole wall, it carries fluids and dissolved gases entering the bore hole to on-line analysis equipment at the surface and is even used to transmit functional data regarding the operation of the drill bit to the surface by "mud pulse" technology. Genial!
The KTB field laboratory used highly sophisticated equipment for the first-level investigation of bore cores, rock fragments and mud composition, the results of which were passed on to other research groups with the basic imformation required for further experiments. Four petrophysical parameters were routinely determined from rock fragments, thermal conductivity, natural radioactivity, density and magnetic susceptibility. A core recovered from great depth starts to expand when reaching the surface. Highly sensitive devices were used to measure the rate and extent of expansion of the core, which can be related to the Earth's internal stress field and the results of this experiment, together with rock mechanical tests may contribute at some time to earthquake prediction. Over sections of the drilled hole, no coherent core could be recovered. Nevertheless, using a new combined X-ray fluorescence and X-ray diffraction technique the mineral and chemical composition of the rock fragments could be determined within an hour after sampling, thus allowing a reliable reconstruction of the lithology of the basement rocks penetrated, even in the absence of a coherent core. On-line analysis of the dissolved gasses in the mud was completed every three minutes by mass spectrometer, allowing immediate recognition and localization of fluid inflow zones.
In addition to experiments with the bore core, borehole measurements provided additional information that cannot be obtained from rock samples. At defined intervals, socalled logging tools were run into the borehole, which apart from registering physical properties such as electrical conductivity, density or natural radioactivity, also provided information regarding the structure and lithology of the drilled sequence. Another type of device allowed direct sampling within the borehole by side wall coring or fluid sampling at certain fluid inflow horizons.
A key problem which had to be overcome by all of the steering and measurement systems was their ability to operate at temperatures up to 300°C. Even after the mud cooling, the core bit steering system was still subject to temperatures of 200°C and the electronic measuring devices had to be temperature resistant up to 260°C. Where do the volcanic cones come into the story? During the Eocene, the Alpine orogeny caused the reactivation of volcanic activity in the northern Bavarian region, which manifested itself in a chain of basaltic cones, the nearest of which are within 10 km of Windischeschenbach. There was little or no magma erupted from the cones, but the influx of magma was sufficient to elevate the temperature gradient in the region, such that the target temperature of the basement rocks, 250 - 300°C, was reached at about 9,000m depth and not at 10,000m as originally estimated.
The Research and Information Centre KTB Oberpfalz, now known as the “Geo-Zentrum an der KTB” is open to the public the whole year. They are currently setting up facilities for guided field trips in the surrounding area, possibly including the Czech Republic, so watch this space, maybe a new trip venue is coming along, especially for the igneous and mud freaks.
The pamphlet, “KTB Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland”, published by the Research and Information Centre KTB Oberpfalz.
Link to Geo-Zentrum an der KTB unfortunately in German only.