HR: 1340h
AN: T23B-0559    [Abstracts]
TI: Condensation Processes in Geothermal Systems
AU: * Norman, D I
EM: dnorman@nmt.edu
AF: New Mexico Tech, Dept. of Earth & Environmental Science, Socorro, 33 87801
AU: Moore, J N
EM: jmoore@egi.utah.edu
AF: EGI, University of Utah, Salt Lake City, UT 84108
AB: We model condensation processes in geothermal systems to understand how this process changes fluid chemistry. We assume two processes operate in geothermal systems: 1) condensation of a vapor phase derived by boiling an aqueous geothermal fluid into a cool near surface water and 2) condensation of a magmatic vapor by a deep circulating meteoric thermal fluid. It is assumed that the condensation process has two stages. Initially the condensing fluid is under saturated in gaseous species. Condensation of the vapor phase continues until the pressure on the fluid equals the sum of the partial pressures of water and the dissolved gaseous species. At that time bubbles flux through the condensing fluid. In time the fluid and fluxing gas phase come to equilibrium. Calculation shows that during the second stage of the condensation process the liquid phase becomes enriched in more soluble gaseous species like CO₂ and H₂S, and depleted in less soluble species like CH₄ and N₂. Stage 2 condensation processes can therefore be monitored by ratios of more and less condensable species like CO₂/N₂. Condensation of vapor released by boiling geothermal fluids results in liquids with high concentrations of H₂S and CO₂ like is seen in geothermal system steam-heated waters. Condensation of a magmatic vapor into circulating meteoric water has been proposed, but not well demonstrated. We compare to our models the Cerro Prieto, Mexico gas analysis data set collected over twelve years time by USGS personnel. It was assumed for modeling that the Cerro Prieto geothermal fluids are circulating meteoritic fluids with N₂/Ar ratios about 40 to which is added a magmatic vapor with N₂/Ar ratio = 400. The Cerro Prieto analyses show a strong correlation between N₂/Ar and CO₂/N₂ as predicted by calculation. Two dimensional image plots of well N₂/Ar + CO₂/N₂ show a bull's-eye pattern on the geothermal field. Image plots of analyses collected over a year or less time show N₂/Ar and CO₂/N₂ hot spots. Plotting data for individual wells show a hysteresis like loops on time vs. CO₂/N₂ diagrams. Our analysis demonstrates that condensation of magmatic vapor into convecting meteoric waters is a viable process. Condensation explains variations in Cerro Prieto geothermal system gas chemistry and is compatible with helium isotope data. Locally condensation appears to wax and wane over a time periods of about 10 years.
DE: 8130 Heat generation and transport
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
SC: Tectonophysics [T]
MN: Fall Meeting 2005