To start the drilling with the goal to tap the thermal water, a drilling site with a drilling rig is built. The area required for a drilling site is about 3000-6000 m2, or about 100 m x 50 m. The drilling site is sealed and has its own drainage and waste disposal system to avoid environmental pollution. Often both, the production well and the injection well, are drilled as deviated wells from one drilling site. The structure of a drilling rig can be seen in the figure. No pressure needs to be applied during drilling, as the dead weight of the drill pipe is sufficient to propel the drill. In fact, the drilling rig must actually hold the drill pipe so that the drill bit does not break due to the surcharge weight. During drilling, a drilling fluid is used to cool the bit and support the borehole wall with its water pressure. The depth of drilling depends on the depth of the thermal water-bearing layer. In the Bavarian Molasse Basin, geothermal boreholes reach a depth of between 825 meters in Straubing and 5079 meters in Holzkirchen. In fact, the actual drilling distances are usually even longer, because the wells are usually deviated horizontally.
For deep geothermal drilling, the rotary drilling method is normally used. The drill bit rotates in this process at 100 – 1000 rpm, driven by motors that transmit the motion either at the derrick or directly behind the bit in the borehole. This drilling method yields drill cuttings (rock fragments), which are transported from underground to the surface by the mud. The drill cuttings are usually sampled and provide conclusions about the rock composition in the subsurface. If the sequence of rocks and their properties are to be studied in even greater detail, cores must be drilled using the core drilling method. The cores are cylindrical samples that are continuously extracted from the subsurface and are usually stored as 1-meter-long pieces for further study. The core drilling method is much slower and more cost-intensive than the rotary drilling method and is very rarely used in geothermal drilling.
Parts of a drilling rig (Bildquelle: Merkur.de – Enex Power Germany)
The borehole is not drilled completely in one go. Different sections with successively smaller diameters are drilled and casing is installed section by section. The following steps are carried out for each section:
- Drilling of a section
- Pulling out the drill pipe: after thorough cleaning or flushing of the borehole, the drill pipe is pulled out.
- Borehole measurements: Various measurements, mostly geophysical, are made in the open borehole; for example, photos of the borehole walls can be taken or measurements of density and porosity can be made.
- Casing: steel casing is inserted into the open borehole. The casing stabilizes the borehole and seals the borehole from the rock. The casing is suspended from the previous casing using a liner hanger system to save material.
- Cementation: to fix the casing and anchor into the rock, the area between the steel casing and the borehole wall is filled with a special cement. The cement casing prevents fluids from entering or leaving the borehole.
- Borehole measurements: Geophysical and pressure measurements are again taken in the borehole to check whether the casing and cement casing are intact and thus the borehole is completely sealed.
- The drilling of the next section starts, which has a smaller diameter.
- In the last section, when the well has reached the planned position in the reservoir, a perforated liner is used for the casing, which is not cemented in place. The perforated casing allows water to flow into the wellbore for longer distances and eventually be pumped to the surface. Another commonly used method is to complete the borehole without casing in the reservoir as a so-called “open hole”.