Superdeep Tests and Experiments at 9 . 1 km and 4 km GFZ

The Continental Deep Drilling Program of Germany (in German: Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland, abbreviated as KTB) was a scientific drilling project near the town of Windischeschenbach, Bavaria. The KTB Depth Laboratory comprises two 9.1 km and 4 km deep, water-filled boreholes in crystalline basement rocks just 200 meters apart from each other. Available equipment such as cables, winches, geophysical borehole tools as well as workshops and office infrastructure allows for in-situ tests and experiments at different pressure and temperature conditions. The two stable wells are large-diameter steel-cased and have been geophysically monitored in detail since 1996.


Introduction
In present day, earth system science geological and geophysical investigations on surface with support of modeling allow to chart out the underground of Earth.In certain cases however, a proof of concept can only be achieved by sampling and in situ observations at depth which requires drilling into the Earth´s crust.A drillhole opens not only the third dimension but through long-term observations also the critical fourth dimension.Time dependent properties in the deep underground comprise e.g.temperature, fluid composition and flow, stress, strain, among others.Deep and stable boreholes allow also technical test of tools for geophysical, deep marine, geotechnical or high temperature and high-pressure applications.As boreholes are fluid-filled, pressure and temperature stability of instruments can be checked as well as communication along umbilicals or cables.
The deepest accessible test bed into the Earth´s crust is today the deep underground lab KTB of the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences.In operation since 20 years, this lab comprises two adjacent deep boreholes of 4,000 and 9,100 m depth.

Field Conditions
During the KTB drilling project from 1987 to 1995 two deep boreholes were drilled in South-East Germany near the town of Windischeschenbach.The so-called pilot hole, KTB-VB, reached 4 km depth while the ultra-deep main hole, KTB-HB, reached 9.1 km depth.The boreholes truncated a stack of Paleozoic high-grade metamorphic rocks of leucocratic paragneisses and intercalated metabasic rocks (Fig. 1).This crystalline series forms the border between two major tectonic terranes which where amalgamated during Devonian to Carboniferous times in Central Europe.Brittle postorogenic stacking deformed the rock pile during the Tertiary.The metamorphic rock pile is characterized by generally steeply dipping foliation of >60° and a few east-dipping fault zones (Emmermann et al., 1997).However, both wells are almost completely cased with steel-pipes down to the total depth; the VB is accessible to nearly bottom hole while the HB is currently not accessible below 6,700 m.The two deep boreholes are very close to each other at a distance at surface of only 200 m.The KTB-HB shows very low deviation of the vertical (<2°) down to 7,400 m, below the deviation reaches higher values (>5°) as is in the entire VB.The platforms surrounding the two boreholes are completely concrete paved and therefore accessible also with heavy vehicles.High temperatures and pressures of T=190 °C at 6700 m depth, p >65 MPa in the HB below 6 km offer ideal test beds for high-tech developments and testing of new scientific and technical methods.The infrastructure comprises sheaves in the HB drill rig frame and on a crane at the VB allowing guided access to the wellheads via cables or steel ropes (Fig. 2).A fully equipped mechanical workshop and office space are available too.

Data Resources
During the operational phase of the KTB drilling all technical and scientific digital data collected in the drill rigs and the field laboratory have been acquired in data bank systems.In the mid to late 1990 years they were opened to web access through the KTB Information System.Furthermore, all field lab reports published during the drilling were published later as open access reports and are available through web resources.Data available comprise:  Data from Internal KTB Working Groups with geology, tectonics, geochemistry, petrophysics and rock mechanics on all core, cuttings and fluid samples  Drilling engineering data with all daily reports from pilot and main well  Large scale experiment data such hydrofrac/ seismic or fluid hydraulic experiments  Borehole measurement data including all logs registered  Measurements in the KTB with seismic and geoelectric tools  Geology, geochemistry and petrophysics data from external KTB Working Groups The data are available for download from the site: http://www-icdp.icdp-online.org/sites/ktb/.This site also provides combined background information on the KTB program.
The strategy to migrate these data into a modern repository has been evaluated in a paper by Klump et al. (2015).

Typical Applications and Services Offered
The KTB deep laboratory of the GFZ is a primary site for deep borehole geophysical measurements, cross-hole experiments or surface to borehole trials for geoscientific or technical methods and equipment under in-situ borehole conditions.Other utilization comprise for example: The KTB boreholes and the infrastructure are available for external scientific (and commercial) utilization.Due to complex, heavy-duty operations and because of safety regulations all operations will be conducted under the supervision of GFZ personnel.Tools and instruments of the GFZ as described above can be made available according to needs.For scientific purposes only the net costs have to be borne by the external user.The KTB-Deep Lab of the German Research Center for Geosciences Potsdam (GFZ) as a worldwide unique facility for a wide range of experiments under in-situ borehole conditions is available for science and research through requests to ktb-tl@gfzpotsdam.de.

Figure 1 :•
Figure 1: upper left: Aerial photo of KTB site, lower left: sketch of design of the two boreholes and geological profile to the right.

Figure 2
Figure 2: a) sheaves cable guidance into the pilot well, b) photo of HB well head, c) drill rig of the HB

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Testing of deep water instruments or HP-HT (6,700 m, 190 °C, 67 MPa) tools • True in-situ tests of sensors, housings, cables etc. at large depth with high temperature and high pressure (6,700 m, 190 °C, 67 MPa) • Test of cables, umbilicals or wireless communication technologies • Repeat measurements over several days, weeks and even months • Test in just one well or in both wells simultaneously • Cable seasoning up to a length of 6,700 m • Check, verification and calibration of downhole and deep water instruments in boreholes that are thoroughly documented by many measurements