| Title | High-Temperature Water Adsorption on Geothermal Reservoir Rocks |
|---|---|
| Authors | Miroslaw Gruszkiewicz, Juske Horita, John M. Simonson and Robert E. Mesmer |
| Year | 2000 |
| Conference | World Geothermal Congress |
| Keywords | geothermal, reservoir, adsorption, capillary condensation, The Geysers, Awibengkok |
| Abstract | The mechanism of water storage in geothermal reservoirs was investigated by direct measurements of water adsorption on reservoir rocks under simulated reservoir conditions. Since the capacity for water adsorption was found to vary widely across the reservoir, multiple measurements are necessary to map water retention capacities and build reliable models. Isotherms of water adsorption on well cores from The Geysers (California, USA) and Awibengkok (West Java, Indonesia) geothermal fields were obtained at 150, 200, and 250 ?C in the whole pressure range from vacuum to over 98% of the water saturation pressure at each temperature. Both adsorption (increasing pressure) and desorption (decreasing pressure) runs were made in order to investigate the adsorption hysteresis. The amount of water adsorbed was determined by direct in situ weighing of solid samples using the Oak Ridge National Laboratory isopiestic apparatus. Additionally, adsorption and desorption isotherms of nitrogen at its boiling point (ñ196 ?C) were obtained for the same samples. The low temperature adsorption data were used to determine BET specific surface areas, pore volume distributions with respect to pore sizes, and porosities. Mercury intrusion porosimetry was also used to obtain additional information about pore size distributions extending to very large pores (macropores) and fractures. The results indicated that the dominant water storage mechanism at high temperatures is better described as multilayer adsorption rather than capillary condensation. The adsorption branches of the isotherms were essentially temperature independent in the range investigated, but the amounts of water retained on the desorption branches decreased with increasing temperature. At 250 ?C capillary condensate exists only at pressures very close to the saturation pressure for bulk water. The remaining hysteresis loop is significantly reduced in area in comparison with its size at temperatures up to 150 ?C. Water adsorption capacities are generally higher for the AWI 1-2 core than for any previously investigated well at The Geysers. One of the AWI 1-2 samples adsorbed more water at p/p0 = 0.5 (12 mg/g) than any other geothermal well core sample, exceeding even water adsorption on pure powdered chlorite. The capacity of the rocks for water adsorption depends on both mineral compositions and pore system features. The BET specific surface areas for nitrogen adsorption were found to be a poor predictor of the capacity of geothermal rocks for water adsorption, since water adsorption on polar adsorbents involves not only weak dispersion forces, but also electrostatic and chemical interactions with the solid. |