Models for steam chemistry at Larderello and The Geysers

Abstract

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. Chemical and isotopic compositional variations with geographic location are interpreted as resulting from pre-exploitation lateral steam flow from central boiling (or inflow) 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. This process is modeled as a Raleigh condensation obeying the equation: C/C/sub o/ = (m/m/sub o/)/sup (1/K-1)/ where at any point C/Co/sub o/ is the ratio of the concentration of a steam component to its original concentration; m/m/sub o/ is the fraction of steam condensed; and K is the distribution coefficient of the component between steam and water (C/sub s//C/sub w/). The magnitudes of the variations at Larderello (-3/sup 0///sup 00/ in delta/sup 18/O, +5x in CO/sub 2/ and +3x in NH/sub 3/) suggest that the productive field is limited by excessive liquid water and non-condensable gas when 80% of the original steam flow has condensed. At The Geysers, a 17x increase in CO/sub 2/ andmore » a 7/sup 0///sup 00/ decrease in delta/sup 18/O suggest that the limit is at 95% condensation. Lateral steam movement and condensation may have created the large lateral extent and cavernous porosity of these systems by leaching the reservoir rock. This model may be used with steam analyses from existing wells to guide field development.« less