Page 149 - Geothermal Energy Renewable Energy and The Environment

8 Drilling

Accessing geothermal heat usually requires drilling. Drilling is used to sample the rock and soil
in the subsurface in order to determine such things as thermal conductivity, porosity and perme-
ability, temperature gradients, and other parameters that influence resource evaluation. Drilling
is also required to access geothermal fluids and assure a consistent supply of heat at a controlled
rate. Finally, drilling is required when reinjection of fluids is necessary. Drilling is often one of the
most expensive single undertakings encountered when developing a geothermal application. For
this reason, careful consideration is required when selecting a drill site and the type of drilling tech-
nology to be used. The material in this chapter is intended to be an introduction to basic principles
that apply when drilling holes for a variety of geothermal applications. The material covered will
provide background sufficient to allow further, detailed pursuit of information needed for specific
applications. But, it is important to understand that drilling remains a skill-intensive enterprise.
Although technological advances have been impressive in reducing uncertainty about what is pres-
ent in the subsurface, detailed knowledge of what will be encountered tens, hundreds, or thousands
of meters underground is very rarely available. For this reason, it is important that experienced drill-
ing teams with knowledge of the local and regional drilling lore be utilized, if at all possible. Such
teams should be able to anticipate potential challenges, have access to the materials that would be
needed to flexibly respond to them, and act responsibly.

backGroUnd

A rule of thumb that is often mentioned is that drilling accounts for about 50% of the cost of a
geothermal project. Although the accuracy of such a statement may not be high, it is a certainty
that drilling is expensive. Figure 8.1 is a graph that shows the drilling costs for various types of
geothermal drilling projects. For comparison, the costs of completing oil and gas holes by the petro-
leum industry are also shown. Note that the curve for completing boreholes for ground source heat
pumps, which are discussed in Chapter 10, is extrapolated to depths much greater than most such
boreholes are drilled. It is shown for comparison.
The broad range in costs is notable, as is the fact that hydrothermal and enhanced geothermal
system (EGS) geothermal wells are, in general, more costly to drill to a given depth than wells for
oil and gas. Boreholes for ground source heat pumps are comparable in cost to drilling water wells,
currently about $15 per foot. These differences are a reflection of the technological and physical
challenges encountered for these different enterprises. We will consider these challenges on the
basis of the type of project being developed. Since these different suites of projects more or less
correspond to the low-cost and high-cost curves in Figure 8.1, we will consider them from this per-
spective and describe drilling techniques for the low-cost projects first.

drIllInG For GroUnd soUrce heaT pUmp
and dIrecT Use applIcaTIons
These types of applications require access to low to moderate temperatures. Ground source heat
pump installations need only the stable temperature zone found at depths of a few meters to a few
hundred meters. Such systems are best installed where subsurface temperatures fall in the range
of about 10–25°C (50–75°F). Other applications, such as aquaculture, spas, and greenhouses need
access to similar temperatures. Since the conditions are relatively benign and the drilling depths

135