Page 186 - Geothermal Energy Systems Exploration, Development, and Utilization

162 3 Drilling into Geothermal Reservoirs
0

270 90

180
3845.0

Cal
240.000 240.00
230.000 232.00
220.000 224.00
210.000 3850.0 216.00
200.000 208.00
200.00
−999.000 0 90 180 270 360
Figure 3.27 Example of multifinger caliper results in the
damaged casing section. (Please find a color version of this
figure on the color plates.)
sediments have been cased and cemented as planned. The obviously impermeable
volcanics have been saved against cement intrusion instead of using an external
casing packer by a highly viscous pill of mud weighted by chalk to 1600 kg m −3
which could be flushed out after drilling float collar and float shoe. Despite the loss
of one casing dimension the target of the well could be reached finally (Figure 3.28).
With many other technical and drilling problems (e.g., four weeks top drive repair,
fishing of boulders just beneath the shoe of the surface casing, successful side-
tracking only after four attempts and modification of the anchor system, poor ROP
and long round trip times, unexpected encountering of H 2 S-gastracesinthe lowest
member of the Zechstein formation) drilling time and costs increased considerably
in comparison with the planned ones based on the adjacent smaller offset well.
Nevertheless, the well reached the targets though real drilling time was beyond
the time plan (Figure 3.29). After the multiple fracs carried out successfully in 2007
(Zimmermann et al., 2008) it is now ready for thermal brine production within the
intended communication experiment of the in situ geothermal laboratory.

3.10
Economics (Drilling Concepts)

Geothermal drilling projects normally are done to use the geothermal energy for,
for example, heating and electric power generation. Very often (in most of the
cases) the cost for the downhole part (drilling the wells) of a deep geothermal
project is higher than the cost for the surface part (heat exchanger, power plant,