A "window" in the underground

The small 1x1 of the drilling technique

KTB bore. © KTB project group
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Research drilling is an indispensable tool in modern geosciences. They provide a "window" into the underground and thus into the history of the earth. Only through them can be obtained direct information about the conditions in the upper crust and the processes running there. But how are these "windows" actually realized technically in the underground?

"A drilling rig must fulfill different tasks, " explains Lothar Wohlgemuth of the GeoForschungsZentrum Potsdam. "The actual drilling (mining: sinking), the transport of the drilled rock to the surface as well as the stabilization or securing of the borehole." The derrick is the central hub of the work and is used in addition to the installation and removal of the Drill strings also the provision of necessary tools.

Drill bit smashes rocks

Scheme of the drilling process. © KTB project group

"The drill string is made up of individual drill rods bolted together, the drill pipe, and connects the surface to the bottom of the hole. It hangs in the derrick on the drilling hook, "explains Wohlgemuth. At its lower end is again the drilling tool: the drill bit. The rock destruction takes place by the rotation of the drilling tool. Either this is done by rotating the entire drill string from the derrick to the bottom hole (Rotary drilling method) or alternatively by a bottom hole motor. This is located directly above the drill bit and is hydraulically driven.

The drilling drilled in this way is secured by the installation of pipes whose number and technical design depends on lithology and geology. This casing is anchored by cement in the mountains and forms in the composite as a "casing-cement jacket borehole wall" a solid element for securing the bore. Once the drill is worn, the entire drill string for replacement of the drill must be removed or reinstalled. For this purpose, the drill string is disassembled with a so-called "pipe handling system" into individual boom trains and parked in the tower.

It depends on the conditioner

Bohrmeiel. GFZ, L. Wohlgemuth

"The rotation of the drill bit destroys the rock, and the necessary pressure is generated by thick-walled drill rods, the drill collars, " explains Wohlgemuth. Now, however, the destroyed rock, the so-called cuttings or cuttings, must be transported from the bottom of the hole to the surface. For this purpose, the so-called circulation system is used: From above, a liquid is pumped through the interior of the rig to the bottom of the hole and exits at the bottom through nozzles on the bit. There, it cleans the bottom of the hole, cools and lubricates the drilling tool and transports the cuttings back up in the annulus between the drill string and the borehole wall, Wohlgemuth explains. Over the course of days, sieves and centrifuges remove the cuttings from the rinsing, which are then returned to the circulation. As a side effect, when using a drilling motor, the washing also drives the drilling tool at the same time. display

Directional drilling by joystick

Today, modern drilling technology is capable of accurate, vertical, inclined and horizontal drilling of several kilometers of length accurately. This is achieved by means of controllable systems, which are located above the drill bit and interact with drilling motors. For comparison: In a large conference room, this would correspond to the centimeter-accurate control of any point with only a bare pipe. The basics of these techniques were already developed at the beginning of the 1990s in the German research program Continental Drilling Program of the Federal Republic of Germany KTB . Today, with this technique, it is possible to perform accurate drilling both for finding deposits or geothermal underground hydraulic systems, as well as answering scientific questions by drilling down on directional drilling, such as Drilling through fault zones such as the San Andreas fault in the US or from volcanic cones such as the Shimabara in Japan).

How does science benefit?

In addition to geophysical deep exploration methods, research drilling is the most important means of obtaining more information about the subsurface, emphasizing Wohlgemuth's importance of the technology. With their help, solid, liquid and gaseous substances such as cuttings, drilling dust, cores or drilling fluids can be promoted, which can then be examined thoroughly over the course of a few days. For example, the use of a drill bit leads to a ring-shaped rock destruction and allows the drilling of cylindrical drill cores. One to two meters of earth history can be drilled per hour in this way, whereby the cores usually have a length of about six meters. A special core tube picks up the samples, which are then transported to the surface for further investigation.

In addition to this sampling, however, the drill holes also allow direct measurements and tests at depth, ie under in-situ conditions "on site". With special measurement technology, the characteristics of the mountains such as electrical resistance, natural gamma radiation, rock density, sound wave velocities, chemical elements, magnetic fields, gravitational field or fracture and fault zones can be clearly determined. For this purpose, borehole probes are retracted into the borehole on a steel cable provided with electrical conductors. The measurement is carried out by the fact that sensors press against the borehole wall and measure the desired data when ascending in the borehole. The data transfer takes place directly via the measuring cable to the measuring station for days, where they are processed and stored by computer. "Without research drilling, science would depend on a pure remote diagnosis and many a secret about the upper crust would have remained closed forever, " said Wohlgemuth.



GeoForschungsZentrum Potsdam (GFZ)

(Lothar Wohlgemuth / GeoForschungsZentrum Potsdam, 02.03.2007 - AHE)