Gas Geothermometry for Drillhole Fluids From Vapor Dominated and Hot Water Geothermal Fields

Abstract

The compositions of steam from the vapor-dominated geothermal systems of Larderello, Italy and The Geysers, California have been shown by previous investigators to vary with position in the field ( Celati et al., 1973; Fanichietal. , 1974; DíAmore et al., 1977; Truesdell et al., 1977; Truesdell and Nehring, 1978, Mazor, 1978). vapor-dominated geotherrnal areas are strong increases or decreases from the center to the edges of constituents carried in the steam. The pattern of these parameters in vapor-dominated systems seem to be controlled mainly by a process of lateral steam movement and condensation (DíAmore and Truesdell, 1979; Calore et a l . , 1980). The most conspicuous chemical patterns observed in the Larderello and The Geysers vapor-dominated geotherrnal are as are strong increases or decreases from the center to the edges of constituents carried in the steam. The pattern of these parameters in vapor-dominated systems seem to be controlled mainly by a process of lateral steam movement and condensation (DíAmore and Truesdell , 1979; Calore et al. , 1980).

The condensation process, at constant temperature and total pressure increases the partial pressure of Cogat the same rate as the gas/stes in ratio, strongly affecting the composition of the total gas ( including steam). The condensation effect should increase contents of C02, A2S, H2 and CH4 interesidual steam to about the same degree because their solubilities are similar. However, the general trend observed is almost constant ratios of 1325, H2, CH4 to HzO a s the C32/H20 ratio increases in Larderello about 5-5 times from the center to the edges at constant temperature and HzS/COz ratios decrease with increasing C02/H20 ratios. explain this behavior is reaction of the sethree gases with other gases and with reservoir minerals so that the partial pressures of these gases are buffered by temperature-dependent reactions with water and rock minerals . This means that the ?i2;C02, CH4/CO2, Apparently the only rnechanism that can explain this behavior is reaction of these three gases with other gases and with reservoir minerals so that the partial pressures of these gases are buffered by temperature-dependent reactions with water and rock minerals.