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Geothermal Energy
John Sass and Wendell Duffield

The year 2003 has been a time of "breathing-space" consolidation within the geothermal energy industry, as developers try to create realistic plans for future projects. Currently, worldwide geothermal generating capacity is about 8,000 megawatts (electric). Worldwide direct use (including the rapidly increasing deployment of geothermal heat pumps) is about 15,000 megawatts (thermal). The values for the United States are about 2,000 megawatts (electric) and 4,000 thermal megawatts (thermal).

One of several moving targets for planners are Renewable Energy Portfolio Standards (RPS) being legislated in some states. Already in place in a few states, these standards mandate that a certain percentage of electricity sold be generated from renewable resources . At the federal level, production tax credits for geothermal energy, analogous to those already granted to other renewable energy sources, are part of the energy bill being considered by Congress. Together, federal tax credits and state mandated RPS could prompt significant new geothermal development, but developers and utilities must await resolution of these significant economic issues before moving forward. These prospects have, however, rekindled interest and investment in geothermal projects. The renewed interest has not yet resulted in increased generating capacity, but some promising new projects are under development.

In last year's Highlights story, we discussed importance of developing techniques to lengthen the productive life of a hydrothermal system. The primary reason that an exploited hydrothermal reservoir becomes less productive with time is a slow, yet continuous, loss of water from the system, typically seen as a white cloud of water vapor escaping from the condensing towers of a power plant. However, even as steam pressure and thus productivity drop in wells, most of the original thermal energy of the hydrothermal system still resides in reservoir rocks. Thus, identifying sources of "make-up" water can be critical to extending the life of a geothermal resource.

In a pioneering and groundbreaking project at The Geysers of northern California (the largest geothermal electrical development in the world), engineers from Calpine Corporation are turning around a decrease in productivity. As we reported last year, a 50-kilometer-long pipeline from nearby Lake County towns began delivering daily about 30 million liters of water to The Geysers in 1997. This flow is made up of highly treated water reclaimed at sewage treatment plants and water taken from Clear Lake, for injection into the southern part of The Geysers geothermal field. This injection quickly slowed pressure decline and has so far resulted in the recovery of an additional 68 megawatts electric of generating output.

A somewhat larger 65-kilometer-long pipeline went into service in December 2003. This pipeline carries daily 40 million liters of highly treated reclaimed water from Santa Rosa and neighboring communities to the central and northern part of The Geysers steam field. More time is needed to evaluate the effect of injecting this reclaimed water, but based on experience from the Lake County pipeline, 85 megawatts of electrical power area is expected to be recovered within this part of The Geysers. Together, the injection of the reclaimed water, surface water and condensed steam should reduce the decline in output at The Geysers and help preserve this unique and valuable resource for decades to come.

Two projects to enhance permeability in rocks that have marginal to non-commercial permeability are also in progress — one at Soultz sous Fôrets in the northwestern Rhinegraben and the other in the Cooper Basin of South Australia. Enhancement of permeability 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.

The European experiment began in 1987, with the first successful forced circulation between two wells in 1997. The current goal of the experiment is to construct a "scientific pilot plant," involving two injection wells and a production well. The schedule calls for the completion of the pilot plant with a net power output of 4.5 megawatts (electric) by the end of 2005.

The Australian project, launched in 2003, had its first well completed at a depth of 4,421 meters, with an estimated bottom-hole temperature exceeding 250 degrees Celsius. Hydraulic stimulation tests were begun in November 2003, and thousands of acoustic events related to the hydraulic fracture have been recorded and analyzed. These seismic studies suggest the creation of a large permeable reservoir, but flow tests are required to confirm this suggestion and to assess commercial potential.

Projects are also moving forward in Iceland, a country situated on perhaps the largest potential geothermal resource on Earth, the mid-Atlantic Ridge. Producing steam from heat exchangers emplaced in magma has been a goal of geothermal engineers for decades. Problems related to corrosion, heat-exchanger design and safety issues have kept this goal elusive. Iceland's energy community is now leading an international effort to assess the feasibility of reaching an intermediate goal — that of exploiting the high-temperature (about 400 to 500 degrees Celsius) environment between conventional (250 to 300 degrees Celsius) hydrothermal reservoirs and underlying magma bodies. This project (the Iceland Deep Drilling Project) plans to drill into a high temperature zone to investigate the deeper resource potential of the Reykjanes high-temperature geothermal system in southwestern-Iceland.

This geothermal system contains seawater and thereby has a global interest, as 70 percent of Earth's crust is oceanic and most of the worldwide high-temperature hydrothermal systems are saline. Temperatures at 5 kilometers depth are estimated to be 500-700 degrees Celsius. The field developer will provide a 2.5 kilometers deep well. Plans call for deepening this well in 2005 by coring to 3.7 kilometers and after evaluation of flow tests from this interval, coring down to 5 kilometers in 2006 to permit extensive flow testing of the deep reservoir.

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Sass enjoyed a 32-year career in geothermal studies with the U.S. Geological Survey (USGS). He is currently a scientist emeritus with USGS, and an adjunct professor of earth sciences at University of Western Ontario and Northern Arizona University. Duffield, who had a 30-year career with the U.S. Geological Survey, now continues geothermal and volcano research as an adjunct professor in the geology department of Northern Arizona University in Flagstaff.

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