Assuming that the brittle-ductile transition (BDT) drastically reduces rock permeability, potentially exploitable geothermal resources (permeability: >10−16 m2) consisting of supercritical water or superheated steam (≥~400 °C) could occur only in rocks with unusually high transition temperatures such as basalt. However, fracturing is possible even in ductile rocks, and some permeability-depth relations proposed for the continental crust show no drastic permeability reduction due to the BDT. Here we introduce our published results of permeability measurements on high-temperature fractured granite, suggesting that the BDT is not the first-order control on rock permeability, and that potentially exploitable resources may occur in granitic rocks with much lower BDT temperatures (~360 °C) that comprise the bulk of the continental crust. We additionally introduce our published results of hydraulic fracturing experiments on granite from subcritical/brittle to supercritical/ductile conditions, confirming the possibility of well stimulation to create excellent fracture patterns that should allow the effective extraction of thermal energy in the superhot geothermal environments of ≥~400 °C.
The IDDP-1 well in Iceland found rhyolitic magma at a depth of ~2 km, and well tests showed flow rates of up to 50 kg/s at temperatures of up to 440 °C (Ingason et al., 2014). At such high enthalpies of ~3.2 MJ/kg, sustained flow rates of ~30 kg/s could produce ~20 MWe. This finding generated interest in the possibility of extracting energy from superhot geothermal environments of ≥~400 °C. On the other hand, it has been proposed that permeability decreases drastically due to the brittle-ductile transition, BDT (Hayba and Ingebritsen, 1997). Assuming that the permeability is drastically reduced due to the BDT, potentially exploitable geothermal resources (permeability: >10−16 m2) consisting of supercritical water or superheated steam (≥~400 °C) can form only in rocks with unusually high transition temperatures (≥450 °C), such as basalt (Scott et al., 2015). However, even rocks that are ductile in compression can be brittle in extension (Heard, 1960), and evidence of intensive melt-driven hydraulic fracturing in the middle crust beneath the BDT zone has been found in outcrops in Antarctica (Engvik et al., 2005). Moreover, some permeability-depth relations proposed for the continental crust show no drastic permeability reduction due to the BDT (Manning and Ingebritsen, 1999). Therefore, potentially exploitable geothermal resources in superhot geothermal environments may be more widespread than has been assumed.