Great Geysers (The Geysers)
GREAT GEYSERS (EN: The Geysers; RU: большие гейзеры) is the largest geothermal field of the world (USA, California state), at the distance of 140 kilometres from San Francisco city to the north, within the Valley of the Great Geysers. The area of the field is 55 kilometres. There is the source of the heat at the field, apparently, the magma chamber (or the system of the chambers) of the Pleistocene age, which is embedded at the depth of 5-8 kilometres. The forecasted reserves of steam within the explored part of the field can ensure the work of the electrical power plants with the power capacity of more than 1000 megawatts (according to certain estimates, up to 4800 megawatts).
The exploration of the field has been started during the 1921. By the 1925, at the place of intense fumarolic activity, there have been drilled 8 boreholes (the maximal depth is 195 metres). During the 1960, there has been built the first geothermal combined heat and electrical power plant (GPP) with the power capacity of 12.5 megawatts. By the start of the 70-ies, the quantity of the boreholes, which have been drilled, has reached 110 (of them, 85 have turned out to be productive; the depth of the boreholes is mainly 1500-1800 metres). The vaporous phase exists to the depth of 2000-3000 metres. The temperature of the vapour, which is arriving from borehole, is 200-260 degrees Celsius, less often 290 degrees Celsius. There are contained within the vapour 1-2% of the non-condensable gases (mainly 70% of carbon dioxide, 12% of methane, 12% of hydrogen, 3% of hydrogen sulfide, and others). They drill boreholes using the rotary method, with the washing at the small depths by the clayish drilling fluid, and after the casing of the upper parts, with the usage of the air. To the depth of 90 metres, there are used the casing pipes with the diameter of 508 millimetres, to the depth of 500-600 metres, with the diameter of 339 millimetres, to the depth of 1000-1200 metres, with the diameter of 244 millimetres, and yet deeper, the hole is not fixated, and has the diameter of 222 millimetres.
Taking into account the geological conditions, they use the inclined directional drilling, with the offset for the bottomhole of up to 400 metres at the depth of 1500 metres. The excessive pressure of steam at the mouth of the closed boreholes is 3.3-3.5 megapascals, the working pressure at the mouth of the functioning boreholes does not exceed 1.18 megapascals. The diameter of the steam-conducting pipelines from individual boreholes is 305 millimetres, the diameter of the collecting steam-conducting trunk pipelines is 610-762 millimetres. The steam is delivered to the power-generating blocks through the steam-conducting pipeline (the diameter is 914 millimetres, the length is less than 2 kilometres). The pressure within the steam-conducting pipelines is 0.60-0.99 megapascals. The consumption of steam within the turbine is 820 tonnes per hour, the pressure at the inlet is 0.785 megapascals, the temperature is 179 degrees Celsius. The power capacity of the individual energetical blocks is up to 110 megawatts, the total power capacity of the electrical power plants at the field is more than 900 megawatts (1981). Within the condensate after the turbine, there is contained the significant quantity of boron and ammonia, because of which the vapour may not be discharged into the surface water flows. Because of the purposes for the protection of the environment, they pump the condensate into the steam-containing cracked collector, through the system of the injective boreholes, with the depth of up to 2000 metres, with the total discharge rate of several hundreds cubic metres per hour.
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