The Potential for On- and Off-shore High-enthalpy Geothermal Systems in the USA

Abstract

In the USA, after more than 50 years of development, the electrical production from geothermal resources is overshadowed by that from wind and solar energy. Furthermore, most new geothermal developments involve only moderate temperatures in the range 250 to 300 oC. In contrast, the development of higher-enthalpy, supercritical geothermal systems has obvious advantages: specifically higher temperatures yield higher power outputs per well so that fewer wells are needed, leading to smaller environmental footprints for a given size of power plant. Recent numerical simulations indicate that supercritical water plays a key role both in transporting heat from magmatic intrusions and controlling the thermal structure of high temperature geothermal systems. Although suitable geothermal systems are restricted to young volcanic terrains, the potential for very favorable economic returns suggests that the USA should begin developing supercritical geothermal systems, both on and off-shore. An approach to mitigating the cost issue is being used by the Iceland Deep Drilling Project (IDDP), a consortium of industry, government and academia that is sharing the costs and broadening the scope of investigations of the economic feasibility of producing electricity from supercritical geothermal reservoirs. At Krafla in NE Iceland the IDDP developed the world's hottest geothermal well, capable of generating more than 36 MWe from superheated steam at a well-head temperature of ~450°C and pressures up to 14 MPa. In 2015-16 the IDDP will drill a second deep well designed to reach supercritical conditions at Reykjanes in SW Iceland, where the Mid-Atlantic Ridge comes on land. Plans for deep drilling to explore for much higher enthalpy geothermal resources are also underway in the Taupo Volcanic Zone of New Zealand (Project HADES), and in northeast Japan where the "Japan Beyond the Brittle Project” (Project JBBP) is an ambitious program attempting to create an EGS reservoir in ~500 oC rocks. However, in the USA, although there is significant undeveloped potential for developing high-enthalpy geothermal systems on land in the western USA, Hawaii, and Alaska, there is no comparable national program to develop such resources. Furthermore, very large (multi-gigawatt size) hydrothermal systems lie offshore on the Gorda and Juan de Fuca ocean ridges. Such offshore resources will be expensive to develop but hold the promise of very large returns. There is need therefore to form a consortium to systematically explore, assess, and eventually develop high-enthalpy geothermal resources and to stimulate the necessary technical and engineering investigations.