Page 197 - Geothermal Energy Systems Exploration, Development, and Utilization

173

4
Enhancing Geothermal Reservoirs
Thomas Schulte, G¨ unter Zimmermann, Francois Vuataz, Sandrine Portier, Torsten
Tischner, Ralf Junker, Reiner Jatho, and Ernst Huenges

4.1
Introduction

In many cases, drilling operations will not open up a geothermal reservoir under
such conditions that an extraction of geothermal energy is economically viable
without any further measures. Geothermal wells often have to be stimulated, in
order to increase well productivity. Different stimulation concepts have been applied
to enhance the productivity of geothermal wells. Formally, stimulation techniques
can be subdivided with respect to their radius of influence. Techniques to improve
the near-wellbore region up to a distance of few tens of meters are chemical
treatments, and thermal fracturing. The only approved stimulation method with
the potential to improve the far field, up to several hundreds of meters away from
the borehole is hydraulic fracturing.
The foundations, on which today’s enhanced geothermal systems (EGS) projects
are built were laid out in the early 1970s, when a hot dry rock (HDR) development
concept was worked out at Los Alamos National Lab, entailing drilling a well into
hot crystalline rock, using water under high pressure to create a large vertical
fracture by interaction with the in situ stress field, and finally to drill a second well
to access that fracture at some distance above the first wellbore. The Rosemanowes
experiment, performed by the Camborne School of mines, was set up in 1976,
aiming at testing at lower temperatures and at lower depth, and some of the ideas
that had emerged from earlier investigations, were experimented at Fenton Hill.
One of the main findings from these two pioneering endeavors was that deep
granites often are fractured and that potentially existing natural fracture systems
will determine the orientation and the propagation of the stimulation. Both projects
have been comprehensively presented by Armstead and Tester (1987), who also
give a broad overview of the state of stimulation technologies at the end of the
1980s.

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