ALBERT

Description: Rock at temperature 〉400 °C can be reached at 〉10-20 km depth globally. Although natural rock permeability at such depths is prohibitively low for energy extraction, laboratory studies have shown that high-pressure fluid injection into superhot rock under high confining pressures (〉500 bar) can generate fine-scale fracture networks permeable to fluid flow. In this study, we focus on modeling the thermal and hydraulic behavior of an enhanced geothermal system at such depths to evaluate its potential for thermal energy extraction. Our models show that such systems can achieve high power output with a low spatial footprint if bulk permeability in the stimulated volume can be maintained near 10-15 – 10-14 m2. While this permeability range is several orders of magnitude higher than the natural maximum permeability of ductile crust at this depth during natural fluid-driven processes (e.g. fault zone metamorphism), much remains to be understood about the response of nominally ductile rock to fluid injection and thermal stress cracking. As additional rock mechanical data describing the permeability response of nominally ductile rock undergoing pressurization and thermal shocking become available, the experimental constraints can be incorporated into the model. Although our existing model points at an exciting potential, the development of such a model would enable more accurate predictions of the long-term sustainability and commercial viability of deep geothermal systems. The integration of interdisciplinary research and collaboration between academia, industry, and government agencies will be critical to the success of future deep EGS projects.

Description: Continental crust at temperatures 〉 400 °C and depths 〉 10–20 km normally deforms in a ductile manner, but can become brittle and permeable in response to changes in temperature or stress state induced by fluid injection. In this study, we quantify the theoretical power generation potential of an enhanced geothermal system (EGS) at 15–17 km depth using a numerical model considering the dynamic response of the rock to injection-induced pressurization and cooling. Our simulations suggest that an EGS circulating 80 kg s−1 of water through initially 425 ℃ hot rock can produce thermal energy at a rate of ~ 120 MWth (~ 20 MWe) for up to two decades. As the fluid temperature decreases (less than 400 ℃), the corresponding thermal energy output decreases to around 40 MWth after a century of fluid circulation. However, exploiting these resources requires that temporal embrittlement of nominally ductile rock achieves bulk permeability values of ~ 10–15–10–14 m2 in a volume of rock with dimensions ~ 0.1 km3, as lower permeabilities result in unreasonably high injection pressures and higher permeabilities accelerate thermal drawdown. After cooling of the reservoir, the model assumes that the rock behaves in a brittle manner, which may lead to decreased fluid pressures due to a lowering of thresholds for failure in a critically stressed crust. However, such an evolution may also increase the risk for short-circuiting of fluid pathways, as in regular EGS systems. Although our theoretical investigation sheds light on the roles of geologic and operational parameters, realizing the potential of the ductile crust as an energy source requires cost-effective deep drilling technology as well as further research describing rock behavior at elevated temperatures and pressures.