A Vapor-Dominated Reservoir Exceeding 600' F at the Geysers Sonoma County, California


M.A. Walters

Geo Location:

The Geysers, California


Stanford Geothermal Workshop




Field Development



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A high-temperature vapor-dominated reservoir underlies a portion of the Northwest Geysers area. Sonoma County, California. The hightemperature reservoir (HTR) is defined by flowing fluid temperatures exceeding 50O0Ff rock temperatures apparently exceeding 600?F and steam enthalpies of about 1320 BTU/lb. Steam from existing wells drilled in the Northwest Geysers is produced from both a "typical" Geysers reservoir and the HTR. In all cases, the HTR is in the lower portion of the wells and is overlain by a "typical" Geysers reservoir. Depth to the hightemperature reservoir is relatively uniform at about -5900 f t subsea. There are no identified lithologic or mineralogic condit ions that separate the HTR from the "typical" reservoir, although the two reserv o i r s are vertically distinct and can be located in most wells to within about 200 f t by the use of downhole temperature-depth measurements. Gas concentrations in steam from the HTR are higher (6 to 9 w t %> than from the "typical" Geysers reservoir (0.85 to 2.6 w t %). Steam from the HTR is enriched in chloride and the heavy isotopes of water relative to the "typical" reservoir. Available static and dynamic measurements show pressures are subhydrostatic in both reservoirs with no anomalous differences between the two: the HTR pressure being near 520 psia at sea level datum. The small observed differences in pressure between the reservoirs appear to vary along a steam density gradient. It is postulated that the Northwest Geysers area evolved more slowly toward vapor-dominated conditions than other parts of The Geysers field because of its poor connection with the surface. In this paper. a model is presented in which the boundary, between the HTR and "typical" reservoir is a thermodynamic feature only, resulting fromi recent deep venting of a liquid-dominated system in which conduction is still an Important component of heat transfer.

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