Page 198 - Geothermal Energy Systems Exploration, Development, and Utilization

174 4 Enhancing Geothermal Reservoirs
4.1.1
Hydraulic Stimulation

Hydraulic fracture stimulations are performed as waterfracs, gel-proppant fracs, or
a combination of both called hybrid fracs (Sharma et al., 2004) The procedures are
well known in the hydrocarbon industry (Shaoul et al., 2007a, b) as well as in the
HDR technology (Hettkamp et al., 2004; Baumg¨ artner et al., 2004; Schindler et al.,
2008). However, the application for geothermal reservoirs requires a technique that
is able to produce considerable higher amounts of fluids than the ones required
for production of hydrocarbon reservoirs.

4.1.2
Thermal Stimulation

Thermal stimulation treatments are performed in order to increase the productivity
or injectivity of a well by either, enhancing the near well permeability, which may
have been reduced by drilling operations itself (drill cuttings or mud clogging feed
zones), or by opening hydraulic connections to naturally permeable zones, which
were not intersected by the well path. This can happen by either reopening of
existing, possibly sealed fractures, or by creation of new fractures through thermal
or additional hydraulic stresses.

4.1.3
Chemical Stimulation

Acid treatments were developed by the oil industry for improving the productivity
of oil and gas wells (Smith and Hendrickson, 2005; Economides and Nolte,
1989; Schechter, 2006) Acid treatments were first applied to wells produced from
limestone formations, about 100 years ago, and they became a technology widely
used in the 1930s. The technique was partially adapted to the geothermal wells,
most often to remove the mineral scaling deposited in the wells after several years
of exploitation (Strawn, 1980; Epperson, 1983; Barrios et al., 2002; Serpen and
T¨ ureyen, 2000), and also to enhance the fractures network in the reservoir.

4.2
Initial Situation at the Specific Location

4.2.1
Typical Geological Settings

EGS can be and are being engineered in a wide variety of geological settings: volcanic
settings like Iceland (Axelsson, Th´ orhallson, and Bj¨ ornsson, 2006), metamorphic
environments like, for example, Larderello (Italy) (Bertini et al., 2005) magmatic
systems like deep granites in Soultz, France, (see Hettkamp et al., 2004); or