Estimation of the Preferred Geothermometer for Taiwan's Predominantly Slate Geothermal Reservoirs of the Chingshui Region, Taiwan

Abstract

Estimation of the geothermal energy contained within geothermal reservoirs using geothermometers is a well-established practice. The most commonly used geothermometers at hot-spring environments are SiO2, Na-K and Na-K-Ca. The preferred geothermometer varies with the type of environment in which it is to be used. In Taiwan, geothermal reservoirs of the Chingshui Region of northeastern Taiwan exist in predominantly slate environments. Therefore, this study examines water-rock interactions in a slate environment to determine the best geothermometer for measuring reservoir temperature. Water-rock interaction experiments at between 100 and 300°C were conducted for varying time periods with the longest duration being 60 days. The varying duration were given for equilibria to be established. Cations: Si4+, Na+, K+, Ca2+, Mg2+ and anions: F-, Cl-, PO43-, SO42-, Br-, NO3- were analyzed after water-rock reactions had taken place and steam and liquid products had passed from the reaction chamber into a separate collection chamber in a two-tank autoclave setup. The concentrations of Si4+, Na+, F-, PO43-, and SO42- varied positively with increased temperature. Cations, Ca2+ and Mg2+, however, showed a negative relationships. Others were independent of temperature variation. These results were compared to a variety of silica, Na-K and Na-K-Ca geothermometers. The comparative results supported silica geothermometers as giving the most accurate representation of the slate environment of the Chingshui region, northeastern Taiwan. The Na-K and Na-K-Ca geothermometers did not fit the results of experiments. Further comparisons with downhole temperature testing at geothermal reservoirs in the region showed that steam conditions were very close to the results achieved. The result of these experiments strongly support a silica geothermometers as the appropriate tool for estimating temperature conditions in the slate environment on Taiwan’s northeastern hot-springs region.