Evolution with time of the chemistry of steam from three typical Larderello wells is interpreted to indicate that three sources contribute progressively to steam production. Initially and during the first ten to fifteen years of production (about 1940-1955 for two wells; 1962-1970 for the third), most steam came from vaporization of condensate just above the vapor-dominated reservoir. Initially this steam had a high flow because it originated close to the well bottoms and was law in temperature and gas and high in boron and ammonia. During this period, amonia and boron content and flaw rate decreased, and temperature and gas content increased as the condensate near the well was exhausted and the source of steam shifted to the vapor-dominated reservoir. Ten to twenty years later (after 1963) chloride appeared in the steam, and gas content decreased as vaporization of deep brine became a major steam source. Chemical and isotopic compositional variations with geographic location are interpreted as resulting from pre-exploitation lateral steam flow from central boiling (or ihflow) zones toward the edges of the system. During this lateral flow, conductive heat loss to the surface results in partial condensation; gases are concentrated in the residual steam and slightly volatile salts are removed into the condensate. condensation obeying the equation: This process is modeled as a Raleigh where at any point C/Co is the ratio of the concentration of a steam component to its original concentration; m/a, is the fraction of steam condensed; and K is the distribution coefficient of the component between steam and water (Cs/Cw). variations at Larderello (-3'/00 in @O, +5x in CO2 and +3x in "3) suggest that the productive field is limited by excessive liquid water and non-condensable gas when 80 percent of the original steam flow has condensed. a 7'/00 decrease in 6l80 suggest that the limit is at 95 percent condensation. Lateral steam movement and condensation may have created the large lateral extent and cavernous porosity of these systems by leaching the reservoir rock. with steam analyses from existing wells to guide field development. The magnitudes of the At The Geysers, a 17x increase in COP and a 7o/oo decrease in 6180 suggest that the limit is at 95 percent condensation. Lateral steam movement and condensation may have created the large lateral extent and cavernous porosity of these systems by leaching reservoir rock. This model may be used with steam analyses from existing wells to guide field development.

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