Thermal Fluid Changes after Operating a Geothermal System: A Case Study of the Chingshui Geothermal Field, Taiwan

The operation of Taiwan's first geothermal power plant from 1981 to 1993 affected the chemistry of hot fluids in the Chingshui geothermal field. Long-term chemical and isotope data from the production wells have been analyzed since 1973, and the rising steam ratio and enthalpy indicate that shortages of recharging fluids lead to a serious boiling and scaling of carbonates within the first few years of production. Furthermore, the progressively depleted hydrogen and oxygen isotope ratios and the high chloride concentrations in the southern part of the reservoir after discharge indicate that the hot fluids may have been derived from high altitudes and circulated through a deep layer during the discharge period. Decreasing chloride concentrations, lowered silica temperatures, and enriched hydrogen isotope values in the northern part of the reservoir indicate that there is a shallower meteoric fluid circulation from another fractured system that has seeped into and discharged the reservoir when the pressure of the main conduit dropped. After a long non-discharging period, the isotope values are enriched, the concentration of chloride is halved, and silica temperatures are lower than initially state. These observations indicate that the reservoir after the pressure drop was recharged by a shallower meteoric fluid, but the residence time is too short for heating to the initial condition.