Page 61 - Geothermal Energy Systems Exploration, Development, and Utilization

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2
Exploration Methods
David Bruhn, Adele Manzella, Franc ¸ois Vuataz, James Faulds, Inga Moeck,
and Kemal Erbas

2.1
Introduction

Most of the world’s geothermal fields exploited today are associated with volcanic
and/or recent tectonic activity and have commonly been discovered through surface
expressions, such as hot springs. Typically, springs and fumaroles are sampled, and
geochemical analyses are used to estimate the maximum temperatures of fluids
in the subsurface. The areas with highest geochemically derived temperatures are
further explored in detail and eventually developed.
Some geothermal fields have been found even though there were no obvious
surface manifestations; generally, by exploration drilling for some other resource,
for example, in a sedimentary basin; or by drilling heat flow holes on a regular
grid. Other areas are inferred to have a high geothermal potential on the basis of
known heat flow data. To explore such areas and reservoirs in more detail and
better establish their potential and extent, both geological and geophysical methods
are usually applied.
In general, geothermal exploration strives to detect hot water or steam reservoirs
in a hot and highly permeable environment. The concept of enhanced geothermal
system (EGS) slightly modifies this concept, as fluid pathways can be enhanced
or even generated by suitable stimulation methods. The focus has also broadened
in terms of temperature, as attention is no longer reserved to the absolute hot
spots. Instead, the proximity to the end user is a critical aspect for the profitability
of a project, especially at lower enthalpy reservoirs, where geothermal district
heating is a component of the overall economic calculations. In particular, the
main utilization scheme for sustainable and efficient production from low enthalpy
geothermal reservoirs is a combined heat and power generation with production
and injection wells. Thus, the targets of exploration have become more diverse,
just as the tools to characterize them have evolved in the past decades.
Nonetheless, the general geochemical and geophysical exploration approach is
similar to that described, for example, by Lumb (1981), but is generally combined
with comprehensive assessment of the geologic setting, especially of the tectonic

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