Geothermal Energy
John Sass and Wendell Duffield

Geothermal energy use is growing at a modest yet steady pace worldwide. Geothermal electrical developments are mostly concentrated in areas of Quaternary volcanism, reflecting the presence of shallow magma or hot plutons (or both), heating groundwater within the shallow crust. Direct applications of geothermal heat are dispersed across all continents (except Antarctica), reflecting the fact that even Earth's below-boiling warm springs and areas of "normal" temperatures can be put to use in effective economic projects. The latest available figures indicate that current installed geothermal capacity worldwide is more than 9,000 megawatts (electric), according to Marnell Dickson, associate editor of Geothermics. Geothermal capacity of direct use projects (agriculture, spas, space-heating and cooling, fish-farming, etc.) is estimated at more than 11,000 megawatts (thermal). The corresponding values for the United States are about 2,800 megawatts (electric) and 600 thermal megawatts.

Contemporary developments in the U.S. geothermal industry have been fueled principally by changes in the structure of power markets, by increasing environmental awareness, and by adoption of renewable energy portfolio standards (RPS) — which mandate that a certain percentage of the electricity sold be from renewable sources — in many states. There is also a move at the federal level to provide geothermal operators with production tax credits analogous to those granted to other renewable energy producers. In states with an RPS in place, investment in new generating capacity is taking place.

The domestic focus for geothermal electrical projects has been to improve the productivity and longevity of known hydrothermal systems, and to re-evaluate systems hitherto thought to be uneconomic. These improvements involve a wide spectrum of approaches and technologies, which can all be classified as Enhanced Geothermal Systems (EGS). The EGS concept was adopted by the U.S. Department of Energy (DOE) in the late 1990s at a time when funding for hot-dry-rock (HDR) research and development ended.

Commercially viable geothermal-electric systems are characterized by extraordinarily high permeability which is rarely found in nature. On the other hand, the hot-dry-rock (HDR) concept envisages creating an engineered reservoir from impermeable rock. EGS refers to hot rocks whose permeability is not high enough to sustain production of geothermal fluids at a commercial level without additional engineering efforts to increase productivity. It also refers to injection of water into systems whose natural fluids are being depleted by production so ast to extend their useful life.

Many hydrothermal systems have been discovered and evaluated, only to be left undeveloped, usually because of insufficient permeability, and therefore, limited productivity. Some of these systems, though not economic under present market conditions, could become competitive with increased permeability and (or) with increased efficiency for conversion of heat to electricity. EGS occupies a large part of the permeability spectrum between high-permeability, commercially successful hydrothermal systems, and the very low permeability systems that formed the basis of early HDR efforts. Currently uneconomic zones around the margins of developed hydrothermal reservoirs also are targets for EGS enhancements.

Stimulation of permeability in an EGS project can be undertaken in a variety of geologic environments. The technology includes fault and fracture analysis, hydraulic fracturing to increase permeability, directional drilling to intersect fractures that are oriented favorably, and injection of groundwater or wastewater at strategic positions to replenish depleted, naturally occurring fluids and to reverse reservoir-pressure declines. Taken together, these enhancement and remedial measures can extend the productivity and longevity of an existing hydrothermal reservoir, allow hitherto uneconomic reservoirs to be brought on line, or increase the size and output of existing reservoirs by allowing development of previously unproductive portions of the field.

The "HDR" projects currently being pursued in Europe which countryies (a joint French-German effort at Soultz sous Fôrets) and those recently terminated in Japan (Hijiori, Ogachi) are probably better described as EGS projects because they aim to increase the productivity of existing hydrothermal reservoirs rather than create new ones from impermeable rock. A project currently underway in the Cooper Basin of South Australia is termed an HDR project, but if sufficient permeability exists in the target reservoir zone, it too may become an EGS project. The programs to inject water at The Geysers in California and Dixie Valley in Nevada are also examples of successful EGS projects. Currently, the U. S Department of Energy is co-sponsoring several EGS projects in the western United States to prove up additional geothermal resources. These include a project to increase permeability on the margins of the reservoir and strategically target injection at Coso, California, a program to investigate the potential for hydraulically stimulating a large volume of rock adjacent to Desert Peak, Nevada, and a chemical stimulation project in a well near Four Mile Hill in the Medicine Lake prospect in Northern California.

The Geysers dry steam system in northern California is the largest geothermal electrical development in the world. It has been producing electricity for 40 years, and it is a major EGS success story. Steam production peaked in 1988, and declined for the next decade due to loss of pressure in production wells. Most of the heat remains, so that restoring lost productivity is a question of replacing the depleted water supply of the reservoir.

In 1997, a 50-kilometer-long pipeline from Lake County began delivering about 30 million liters of wastewater a day, for injection into the southern part of The Geysers geothermal field. This slowed the pressure decline and ultimately resulted in the recovery of 75 megawatts of generating output. Construction of a 65-kilometer long pipeline from the City of Santa Rosa is on schedule to deliver another 40 million liters a day, by late 2003, to the central part of the field. Together, these two sources of "recharge" water will replace nearly all of the geothermal fluid being lost to electricity production. The injection program is expected to reduce the decline in electrical output from The Geysers resulting in about 150-200 megawatts of additional output for at least two more decades, and possibly much longer.

The Iceland Deep Drilling Project (IDDP) is, in part, an EGS project. Iceland is producing about 200 megawatts of electric power from geothermal systems along the part of the mid-Atlantic Rift system that traverses that country. This output is likely to double within the next decade or so, mostly by enhancement of currently exploited geothermal reservoirs, which are dilute water-brine systems at temperatures between 200 and 350°C. The primary goal of IDDP is to find out if hydrothermal resources at supercritical conditions (400 to 600 degrees Celsius) can be tapped within the deep upflow zones feeding the conventional geothermal reservoirs.

Most EGS projects are jointly funded by governmental bodies (such as the U.S.Department of Energy) and industrial partners, but some are being supported solely by industry. For example, after a hiatus of several decades, Shell International has re-entered the geothermal energy sector through the creation of a "Hot Fractured Rock" research group, which is a partner in the Soultz EGS project and is active in El Salvador.

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Sass was educated in Canada and Australia and enjoyed a 32-year career in geothermal studies with the U.S. Geological Survey. He is currently a scientist emeritus with USGS, and an adjunct professor of earth sciences at University of Western Ontario and Northern Arizona University.

In 1967, with a new geology PhD in hand from Stanford University, Duffield began a 30-year career with the USGS, studying volcanoes, volcano hazards and geothermal energy. He continues USGS research as scientist emeritus, and is an adjunct professor of geology at Northern Arizona University

The authors thank many colleagues in industry, government, and academia for their comments and input. Unfortunately, space does not permit listing their names and affiliations here.

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