Page 137 - Geothermal Energy Systems Exploration, Development, and Utilization

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3
Drilling into Geothermal Reservoirs
Axel Sperber, Inga Moeck, and Wulf Brandt

3.1
Introduction
Drilling is an essential and expensive part of geothermal exploration, development,
and utilization. The aim of geothermal drilling is not only to access the reservoir at
its most safe but also most low priced way. Cost reduction of geothermal drilling is
therefore a major issue that should be considered for the economic development
of geothermal energy.
The consequences of reducing cost are often impressive, because drilling and
well completion can account for more than half of the capital cost for a geothermal
power project. Geothermal drilling can be even more expensive (in cost/depth)
than onshore oil and gas drilling for two principal reasons:

1) Technical challenge: Geothermal reservoirs may host highly corrosive fluids
of high temperature in great depth, which can mean that special tools and
techniques are required for the harsh downhole conditions.
2) Large diameters: Because the produced fluid (hot water) is of intrinsically low
value, large flow rates and thus, large holes and casing, are required.
Geothermal drilling cost reduction can take many forms, for example, reduction
of downtime (i.e., the NPT = nonproductive time), use of optimized drilling tech-
nique and tools, higher per-well production through multilaterals and less trouble
by understanding geological–technical risk before the start of drilling.
The most important part of a successful geothermal drilling project is however, to
understand the complexity of drilling a well from planning to completion. Drilling
means the interaction of geology and engineering, of rock material and techniques.
Thus, both fields each inherently complex, need to be integrated theoretically and
practically.
The main idea of this chapter is to provide an overview and a systematic approach
to improve the planning and the design of geothermal wells. Since geothermal
environments can differ considerably we also focus on the special demands in
well design and drilling operation concerned with the specific geologic conditions
of geothermal fields. The reader will be introduced to drilling techniques, drilling

Geothermal Energy Systems. Edited by Ernst Huenges
Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-40831-3