Geothermal energy can be duly called ‘Hydrogeothermal Energy’ considering the critical role of sustainable water circulation in permeable reservoir. An enhanced geothermal system (EGS) is a system that uses hydraulic stimulation of a hot, comparably-impermeable rock mass at great depths to create an artificial geothermal reservoir for electricity or direct heat. The key technological components in an EGS are drilling, seismic monitoring, hydraulic stimulations and reservoir characterization. Outstanding issues of hydraulic stimulation, induced seismicity and geomechanical characterization are presented. Challenges in hydraulic stimulations includes but not limited to less-than-expected permanent increase of permeability. Improvements on monitoring, mechanisms and management of induced seismicity are pre-requisite for safe operation of EGS. Intrinsic data-limited nature add uncertainties in the geomechanical characterization such as in situ stress estimation, and estimation of hydraulic regime. In order for geothermal energy or hydrogeothermal energy to be a universal energy, grand challenges of EGS has to be tackled.
Geothermal energy supplies 70 GW and 12.7 GW of installed capacity to the global energy requirement for direct use and electricity, respectively (Lund and Boyd, 2015; Bertani, 2016), and it is the only base load power among other major renewable energy sources. Currently, most geothermal power is produced through hydrothermal systems around the basin of the Pacific Ocean (Bertani, 2016). In order for geothermal energy to be a universal option, its applicability to a non-volcanic area must be demonstrated. An enhanced geothermal system (EGS) is a system that uses hydraulic stimulation of a hot, comparably-impermeable rock mass at depths typically deeper than 3 km to create an artificial geothermal reservoir (Tester et al., 2006). The geothermal potential of EGS in US alone is considered to be 100 GW (Tester et al., 2006) making it as a promising alternative energy.
The key technological components in an EGS are drilling, seismic monitoring, reservoir characterization and hydraulic stimulations.
Water is the main medium to transport the heat and reservoir permeability is critically important component of geothermal energy. EGS target to achieve sufficient permeability to maintain economical supply of geothermal power or heat. Therefore, ‘geothermal energy’ can be duly called ‘hydrogeothermal energy’ in order to do justice on the underlying principle and emphasize the critical role of securing sufficient permeability.