The Steam field
NCPA's Steam Field is the fuel source for the geothermal plants. The steam is both naturally occurring and man-made as NCPA continues to successfully mine heat from the hot rock by injecting water in order to augment steam production. The Steam Field, which is located in Sonoma and Lake Counties, has a productive area of 1,200 acres and includes 69 steam wells, (up to two miles in depth) seven wells available for water injection, and 25 miles of steam transmission, water injection, and condensate collection pipelines. The Steam Field lease is located on federal lands that are managed by the Bureau of Land Management. The geothermal reservoir rock consists of fractured greywacke and greenstones heated to a temperature of 460 to 480 F. The fractures are filled with superheated steam.
NCPA operates the Steam Field under a "Protocol" which specifies a variety of operating parameters, allowing the Agency to save or "bank" steam for future use while better maintaining an average annual output of 150 MWs, which has been maintained since 1988. The Geothermal Operating Protocol has again been revised this year to allow the minimum generation level to be lowered and the maximum generating levels increased so as to provide additional flexibility. Altogether, these recent adjustments further enhance the optimum scheduling of the Geothermal Project. The Protocol also allows for dependable and increased generation for Reliability Must Run (RMR) generation from Plant #2.
The Steam Field continues to perform well under a Four-Zone production control system specified in the Protocol. This strategy recognizes and takes advantage of the differing reservoir pressure that vary geographically and range between about 100 and 240 psig. High, low and intermediate pressure wellhead settings and multiple steam gathering system line pressures allow the turbine inlet pressures to differ between the four units, by as much as 40 psi, resulting in overall higher steam flow rates, increased generation efficiencies, and a reduced rate of pressure decline in the reservoir. For example, the average reservoir pressure loss for 1999 was 2 psi compared to an average annual pressure loss of 12 psi per year since 1990. This is the lowest annual loss of reservoir pressure since the start of generation in 1983. The decrease is due to the start of Multi-Zone operation, combined with Steam Field enhancements that include augmented and expanded injection.
The most significant effect on increasing NCPA's recoverable steam reserves came from the construction and operation of the Southeast Geysers Effluent Pipeline project. This $34.1 million project with Calpine, Unocal and the Lake County Sanitation District as partners has delivered over 6 billion gallons of lake water and treated sanitation plant effluent to the Geysers since startup in September 1997, for increased injection and a resulting increase in injection-derived steam. NCPA's one-third share of the water doubled its availability of injectate, and the total amount of steam capable of being produced over the net twenty years is expected to be increased from 238 to 401 billion lbs., resulting in a 70 percent increase in the amount of electricity capable of coming from NCPA's geothermal operations.
To fully utilize some of the lower pressure steam found in certain sections of the NCPA reservoir, NCPA modified its Plant 1, Unit 2 turbine to a low pressure turbine in 1996, along with the steam field piping and computer control systems. This allowed the lower pressure steam to be dedicated to the low-pressure turbine whereby turbine-generator outputs were maintained even at the lower pressure.
NCPA found that while reservoir pressure was declining, heat within the reservoir was not. Water injected into the reservoir via strategically selected production wells (typically low steam producing wells and/or wells with a communicating fracture system within the reservoir to other production wells) was readily converted to steam, helping to lessen the effects of the pressure decline. To this end, NCPA constructed two water retention dams near each of the power plants that collect rainwater runoff for injection into the reservoir. This rainwater supplemented the condensate blow down from each plant that was already being injected. Reservoir testing found that the reservoir was capable of converting even more injection water to steam than there was water available. To maximize the benefits of augmented injection, NCPA partnered with other operators at the Geysers and the Lake County Sanitation District to pump treated effluent from the City of Clearlake, California some 28 miles up to the Geysers field for injection. This Effluent Pipeline Project has helped to further extend the life of the NCPA geothermal project by offsetting some of the pressure decline effects of the reservoir. Five to eight wells are utilized as injection wells to handle the augmented injection program.
A byproduct of the conversion of geothermal energy to electricity at the Geysers is elemental sulfur. The sulfur is produced during the chemical abatement of hydrogen sulfide gas, a naturally occurring non condensing gas with a noxious order that is produced along with the geothermal steam. In the early years of the NCPA project, this sulfur was treated as a hazardous waste due to trace amounts of mercury, also a natural occurring constituent of the geothermal steam produced at the Geysers. Through technological innovation, a method to remove mercury was developed that now renders the sulfur non hazardous. The sulfur is now utilized as a feedstock in the California agricultural fertilizer industry, benefiting the environment by eliminating the need to landfill the material. NCPA maintains an on-site laboratory certified by the California Department of Health Services as an Environmental Testing Laboratory. The laboratory tests every shipment of sulfur leaving the site to ensure that it meets all regulations connected to its classification as a non hazardous byproduct.
The NCPA geothermal project has established itself as a leader in the industry, and through the dedicated efforts of the NCPA staff, will continue to be a leader for many more years into the future.
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