Supercritical and Superhot Geothermal Resources - Some Fundamental Insights
Thomas DRIESNER, and The COTHERM Team
[ETH Zurich, Switzerland]
Both wells drilled so far by the Iceland Deep Drilling Project (IDDP) encountered supercritical fluid conditions at depth. A testing campaign at the first well, IDDP-1 at Krafla, indicated that if such resources could be exploited routinely, power output per well may reach almost an order of magnitude more than from conventional wells drilled into high-enthalpy fields (Ref). In this contribution I review key findings of the Swiss-funded COTHERM project and related insights: • Advanced numerical modelling indicates that supercritical resources may be an integral part of many natural high-enthalpy geothermal fields. • Host rock lithology and/or strain rate determines if the supercritical resource is permeable. Basaltic rocks and elevated strain rates favor a higher brittle to ductile transition temperature, allowing geothermal fluids to access hotter rock. • In saline systems, intrusion depth is a major control on whether an economically attractive resource can be encountered. This is due to the phase diagram of saltwater: deeper intrusions favor development of a superhot, dry steam zone below about 4.5 km while shallower intrusions favor the development of a highly saline, low-mobility brine layer. We predict that IDDP-2 at Reykjanes encountered a potentially attractive resource at more than 500°C and in the dry steam + salt phase region that so far could not yet be characterized by tests due to problems with the well. • It is important to realize that due to its low density and dielectric constant, supercritical water is a weak solvent and that possibly corrosive agents (e.g., HCl) will likely have little corrosion or scaling potential at depth. This changes drastically if the supercritical fluid gets mixed with subcritical water in either wells or the periphery of natural high-enthalpy plumes.
|        Topic: Advanced Technology (Magma, Geopressure, etc.)||Paper Number: 37006|