Page 165 - Geothermal Energy Renewable Energy and The Environment

Drilling 151

The experience obtained through the Kakkonda drilling program has provided important lessons
for drilling such systems. Overall, the drilling technology, which included monitoring of the hole
orientation while drilling, top head drilling, and motor-driven directional controls can be success-
fully employed for drilling in systems such as this high temperature field. However, it is also clear
from this experience that significant work remains to develop drilling fluids that can function at
these elevated temperatures, as well as more robust drilling equipment suitable for prolonged opera-
tion in these hostile environments.

synopsIs

Drilling wells for geothermal applications requires strict attention to assuring that environmental
protections are in place. Especially important is assuring isolation of wells from the environment
they penetrate, restoration of drilling sites to an acceptable condition, and preventing releases of
unwanted or hazardous compounds. Wells for ground source heat pump and direct use applications
are generally shallower than those drilled for power generation purposes. The former are usually
drilled with an auger system (if the soil conditions allow) or using rotary drill rigs. Often the same
equipment used for water well drilling can be employed for these applications. Wells for power gen-
eration are more demanding engineering efforts. Much larger drilling rigs, with capabilities to place
thousands of meters of casing in hostile conditions are required. Equipment performance and opera-
tional issues are challenging because of the extreme temperature and pressure conditions encoun-
tered. Nevertheless, technology transfer from the oil and gas industry has allowed the introduction
and successful use of such things as directional drilling, enhanced drilling fluids, and trajectory
monitoring. These successes need to be expanded upon by the geothermal industry by developing
more robust downhole equipment, especially packers and instrumentation, as well as drilling fluids
that can sustain high temperatures.

Problems

8.1 What are the purposes of drilling “mud”? What affects the ability of drilling “muds”
to perform these functions?
8.2 If you were to hire a driller for installation of a shallow well, what question would you
ask to assure you were dealing with a reputable and experienced driller?
8.3 What is the primary component in drilling muds and why is it used?
8.4 What are the environmental issues that need to be addressed when drilling a well for
power generation? For nonpower applications?
8.5 What is a packer and what is it used for?
8.6 Why is the cost of a deep geothermal well somewhat more expensive than an oil and
gas well drilled to the same depth?
8.7 What determines how long it takes to drill a well to any given depth? Explain why the
penetration rate decreases as drilling reaches deeper levels.
8.8 How might directional drilling have affected whether a geothermal site would be con-
sidered a reserve or not?
8.9 Using the information in Figure 8.10, make a plot of temperature versus depth. What is
the steepest geothermal gradient? The shallowest? At what depth would these two gra-
dients suggest that a temperature of 650 C would be reached? Which of these estimates
°
is reliable and why?

reFerences

Axelsson, G., and S. Thórallsson. 2009. Stimulation of Geothermal Wells in Basaltic Rock in Iceland. IPGT
Nesjavellir Workshop, May 11–12.
Bertani, R. 2007. “World geothermal power generation in 2007.” Proceedings of the European Geothermal
Congress 2007, Unterhaching, Germany, 30 May – 1 June.