Superhot Geothermal Resources

Figure 1. Conceptual sketch of a magma-driven geothermal system (left) with the conventional 250-300°C resource (blue) and the inferred location of superhot resources (red). The pressure-enthalpy diagram on the right indicates that producing from superhot resources will encounter very high enthalpies and may allow direct production without intersecting the two-phase liquid+vapor region. 

A key control on the occurrence of superhot resources is the brittle to ductile transition temperature, TBDT, of the host rock (Scott et al., 2015) as it induces a strong decrease in permeability due to plastic behavior of the rock. High TBDT allow the fluids to access high temperatures and are expected for quartz-poor host rocks such as basalts in Iceland, and/or for conditions of active tectonic extension.

In systems with saline fluids, possible exploitable superhot resources are restricted to cases with deep magma bodies (≥ 4.5 km) as the phase relations of saltwater then allow development of high-enthalpy, low-salinity mobile fluid of vapor-like density. Above shallower magma bodies, phase relations enforce the development of a dense, low-mobility, low-enthalpy brine layer that suppresses the formation of economically attractive resources (Scott et al. 2017).

Current research problems include understanding permeability beyond a simplistic porous medium approximation, development of a thermodynamic framework for modelling the chemistry of solutes in the superhot fluid, and applied numerical modelling for optimal utilization (e.g., direct production. vs. recharge vs. doublets).