Page 188 - Geothermal Energy Systems Exploration, Development, and Utilization

164 3 Drilling into Geothermal Reservoirs
Time [d]
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
0
500
1000
1500
2000
Depth [m] 2500

3000
3500
4000
4500
5000
Figure 3.29 Depth versus time (red – planned, blue – real,
colors indicate lithostratigrafic members). (Please find a
color version of this figure on the color plates.)

cooling towers, etc.). It is obvious that the cost of the downhole part has a
high impact on economics of a geothermal project; this is particularly valid for
low-enthalpy projects. So, it is mandatory to have a close look on cost-influencing
parameters in conjunction with the wells.

3.10.1
Influence of Well Design on Costs

Besides a proper drilling operation a good planning is of high importance, and
inside the planning process the well design may have the highest impact on overall
well cost.

3.10.1.1 Casing Scheme
Early investigations had shown that there is a certain relationship between drilled
diameter and overall well cost. At this time it was found out that there is a direct
relationship between total volume of destroyed rock and the cost, which was investi-
gated for oil and gas wells in sedimentary rock. Whether this is still true or also valid
for nonsedimentary rock and geothermal wells is not so important but the general
influence of borehole size on cost. So, it should always be checked which casing
scheme is to be favored for a specific drilling project as discussed in Section 3.5.
Generally the casing diameters of a deep geothermal well have to be larger than
for oil and gas wells. The reason is that typical flow rates have to be higher for
an economic hot water production than for normal production rates of oil wells.
Additionally particularly the upper hole section needs a large diameter when an
ESP has to be installed for production.