Calcimetry as a useful tool for a better knowledge of flow pathways in the Soultz-sous-Forêts Enhanced Geothermal System

The Soultz-sous-Forêts’ Enhanced Geothermal System: A Granitic Basement Used as a Heat Exchanger to Produce Electricity

“…Petrographic data were obtained on cuttings by mano-calcimetry (for the calcite content) and X-Ray diffraction (for the illite content). In the Soultz granite, like in many other granites (Ledésert et al, 2009), the base level of calcite amount is around 1.8 wt % Ledésert et al, 2009). As a consequence, calcite contents over 2% are considered as calcite anomalies by these authors.…”

“…The Soultz-sous-Forêts' Enhanced Geothermal System: A Granitic Basement Used as a Heat Exchanger to Produce Electricity 481 thanks to thorough characterization of the fracture network (Hébert et al, , 2011Ledésert et al, 2009). When the rock heat exchanger is finally operated, careful reservoir engineering and monitoring has to be performed to ensure the viability of the EGS (Satman, 2011).…”

“…At Soultz, thorough petrographic studies of the fracture network (Dubois et al, 2000;Genter et al, 1995;Hébert et al, 2010Hébert et al, , 2011Ledésert et al, 1999Ledésert et al, , 2009Ledésert et al, , 2010 have shown that more than 90% of the fractures are sealed by minerals that precipitated because of natural fluid flow. K-Ar dating of illite (K-bearing clay mineral) found in a fractured and altered zone located at 2200 m in the Soultz granite (Bartier et al, 2008) indicate at least two episodic illitization at 63 Ma or slightly more for the coarsest particles and at 18 Ma or slightly less for the smallest.…”

“…Recently we have focused on calcite since this mineral is thought to impair the permeability between the 3 deep wells and especially between GPK3 and GPK4 (Hébert et al, , 2011Ledésert et al, 2009Ledésert et al, , 2010. To this aim, we performed a compared study of calcite-content, other petrographic data (alteration degree of the rock and illite content) and fluid flow from well-tests.…”

“…Careful management of the water resource is unlikely to induce subsidence (lowering of the ground's level as in shallow mining activity; MIT, 2006). Seismic activity linked to engineering of EGS reservoirs during hydraulic stimulation (injection of water under pressure to create or open pre-existing joints) has conducted managers of these sites to prefer chemical stimulations (use of chemicals to dissolve minerals responsible for the sealing of joints) in or close to urban areas (Hébert et al, 2011;Ledésert et al, 2009) in order to avoid earthquakes. Another force of the EGS technology is the possible use of CO 2 instead of water because of its favourable thermodynamic properties over water in EGS applications (Brown, 2000;Magliocco et al, 2011) thus leading to a possible sequestration of carbon dioxide produced by the use of fossil fuel.…”