Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 474

duced during the fracturing process and the second well is directionally drilled
in order to penetrate the fracture system created in the first borehole.

Los Alamos Los Alamos Camborne Camborne Soultz Soultz

Fig. 10.14 Hot-Dry-Rock (HDR) concepts

− The concept proposed by the Camborne School of Mines (Cornwall, England)
("Camborne concept", Fig. 10.14) is based on the existence of a network of
natural fractures (joints). By massive water injection these fractures are opened
up and widened so that sufficient flow is achieved between the wells. Since
natural fractures at great depth are generally vertical to sub-vertical, inclined
boreholes are advantageous.
− The concept applied in the European Hot-Dry-Rock Project Soultz (Fig. 10.14)
is based on the existence of permeable fault zones. These large scale vertical or
sub vertical discontinuities have a lateral extent of at least several square kilo-
metres and a thickness of several meters to decametres. They are generally
composed of highly fractured and altered rock material with a much higher
permeability and porosity than the surrounding bedrock. Within Graben struc-
tures like the Upper Rhine Valley, a dense network of such discontinuities ex-
ists so that large hydraulic fractures created in a well will always connect to at
least one or several of these faults or fracture zones. The purpose of hydraulic
fracturing is therefore not to directly connect the boreholes via hydraulic frac-
tures like in the Los Alamos concept, but to link the boreholes separately to
these major natural faults. The permeability of the faults or fracture zones may
be increased during the fracturing tests due to induced shearing. The Soultz
concept has been successfully applied at Soultz in the Upper Rhine Valley. At
a depth of 3,000 m a Hot-Dry-Rock (HDR) system of commercial size has been
established and has successfully been tested in 1997. A bigger Hot-Dry-Rock
(HDR) system with a central injection borehole and two inclined production
wells has been established in 2004 at 5,000 m depth and a rock temperature of
200 °C. The system will be tested in the upcoming years.