Page 62 - Geothermal Energy Systems Exploration, Development, and Utilization

38 2 Exploration Methods
and structural framework. Thus, fruitful exploration strategies typically involve the
following:
• assessment of the geologic and geodynamic setting;
• geochemistry including fluid and rock isotope chemistry;
• structural analysis of faults, fractures, and folds;
• determination of the regional stress field;
• potential methods, mainly gravity and magnetic surveys;
• electrical and electromagnetic methods (EMs);
• seismic methods, both active and passive.
Previous exploration efforts have focused primarily on obvious geothermal
targets with good surficial expressions and relatively high temperatures. However,
many high enthalpy systems are yet to be developed and others simply do not
have obvious surficial expressions. Consequently, exploration efforts today are
commonly focused on hidden (or blind) geothermal systems and unconventional
low enthalpy resources, which commonly require enhancement in permeability
(i.e., EGS).
Exploration of potential EGS reservoirs encompasses a broad spectrum of
geological settings and therefore requires a wide variety of approaches, often a
combination of several methods. For example, potential EGS targets now include
deep sedimentary basins, which have previously been the almost exclusive domain
of hydrocarbon (HC) exploitation. Because of the breadth of potential settings for
geothermal activities, the geological characterization of an area becomes even more
important, especially if no surface expressions indicate geothermal activity at depth.
There has also been considerable improvement in both technology and methods,
often borrowed from the HC industry, and driven by increasing activity in the
geothermal sector. For example, magnetotellurics (MTs), still in its infancy as
an exploration method in the early 1980s, has become a primary tool for the
detection and characterization of deep geothermal reservoirs. Similarly, seismic
methods, often considered too expensive and not very useful in the traditional,
mostly volcanic environments of conventional geothermal activity, have increased
in popularity in the geothermal sector. Examples include well-established regions
such as Larderello in Italy and the deep reservoirs in sedimentary basins such as the
Molasse Basin in Southern Germany or the Rhine Graben, where development of
EGS systems can become particularly important. Efforts to better characterize the
geologic and structural settings of geothermal activity have also recently increased,
including analyses of surficial geothermal features (Coolbaugh et al., 2007, 2006)
and favorable structural settings (Curewitz and Karson, 1997; Faulds et al., 2004,
2006; Micklethwaite and Cox, 2004; Fairley and Hinds, 2004).
Enhancing a geothermal system generally involves drilling along deviated well
paths and with large diameters, drilling with formation damage mitigating tech-
nologies, stimulating the reservoir by hydraulic fracturing, and/or targeting fault
zones that will produce with high flow rates, which are usually higher than those in
HC production (Huenges and Moeck, 2007). Thus, one of the key geological issues,
especially critical for EGS development, is knowledge of the stress field and an