| Title | Estimation of Long-Term CO2 and H2S Release During Operation of Geothermal Power-Plants |
|---|---|
| Authors | Niels Giroud and Stefán Arnórsson |
| Year | 2005 |
| Conference | World Geothermal Congress |
| Keywords | carbon dioxide, hydrogen sulfide, environmental aspects, geothermal power-plant |
| Abstract | The concentrations of CO2 and H2S in undisturbed liquid-dominated high-temperature geothermal reservoir waters are generally controlled by temperature dependent equilibria with various mineral buffers. These equilibria cause the concentrations of these gases to increase with temperature. The presence of equilibrium steam in the reservoir (two phase reservoir) will cause the gaseous concentrations in the fluid to be higher than the aqueous equilibrium concentrations at any particular temperature. In the range of about 230-300?C, the CO2 buffer is considered to be clinozoisite + prehnite + quartz + calcite. In high-temperature waters of low salt content, which are strongly reducing, the H2S buffer is considered to be pyrite + pyrrhotite + epidote + prehnite. In waters of higher salinity, the respective H2S mineral buffer may consist of pyrite + magnetite + hematite. The concentrations of CO2 and H2S in steam of wet-steam wells producing from liquid-dominated reservoirs are higher than those of the parent fluid, frequently in the range 50-300 and 2-20 mmoles/kg of steam, respectively. However, values as high as 1000 mmoles/kg for CO2 and 50 mmoles/kg for H2S are not uncommon. The concentrations of these gases in steam from wet-steam wells depend on 1) their concentration in the parent geothermal water, 2) the steam fraction, which has formed by depressurization boiling, 3) the reservoir steam fraction, if present, 4) the steam separation pressure, and 5) the boiling processes, which lead to the steam formation. Long-term utilisation of geothermal reservoirs may lead to decline in the concentrations of CO2 and H2S in the steam. The decline can be caused by recharge of cooler water into producing aquifers and/or progressive boiling of water retained in the aquifer rock by capillary forces. Further, enhanced boiling, which is a consequence of reservoir pressure draw down, and steam separation during lateral flow into production wells may cause the well discharge to become depleted in gas. The separated steam may form a steam cap over the liquid reservoir and/or enhance fumarolic activity. Although gas emissions from geothermal power plants may be enhanced much during the early years of production relative to natural discharge, in the long run, the integrated gas emission may not exceed that of the natural gas flux. A steady state may be reached between the flux of gases from the magma heat source into the geothermal system and from the geothermal system into producing wells and fumaroles. The source of noble gases, apart from He, in geothermal fluids is air saturated meteoric water. The relative abundance of noble gases in geothermal steam may aid assessment of which processes are responsible for changes in the concentrations of the environmentally important CO2 and H2S. |