Development and Verification of a Method to Forecast Hot Springs Interference Due to Geothermal Power Exploitation

Abstract

When proposing a geothermal power development, it is extremely important in terms of environmental conservation to forecast the possible influence of geothermal power exploitation on the surrounding hot springs. For mutual understanding, cooperation, and agreement between developers and hot spring owners as well as environmentalists, the environmental measures to conserve hot springs based on scientific study are particularly required in Japan, where hot springs represent important sightseeing resources. As such, The New Energy Foundation (NEF) has endeavored to develop a method to predict the influence of geothermal exploitation on the surrounding hot springs, by applying and modifying a method taken from geological, geochemical and reservoir engineering approaches. The procedure consists of two steps. First, based on a large scale conceptual model, the possibility of interference on hot springs is qualitatively assessed from the view points of the structural setting of the model and of the fluid flow connection between the geothermal reservoir and hot springs. The former is evaluated from geological analysis and the latter is derived from the similarity of origin and mixing of fluids, which are determined by geochemical analysis. Then, a quantitative estimation of the influence on hot springs is calculated by numerical simulation taking the exploitation scenarios into account.

In order to assure the reliability of the forecasting method, verification studies were conducted using two model fields: the Wairakei geothermal field in New Zealand and the Palinpinon geothermal field in the Philippines. Both fields had experienced decline of water tables and changes in the concentration of chemical components of thermal water at some surrounding hot springs after the exploitation commenced. In these verification studies, we made conceptual and numerical reservoir models, including hot springs, using limited data obtained only during the investigation stage. Then, we attempt to predict the above changes in surrounding hot springs after the geothermal exploitation. We succeeded in both qualitatively and quantitatively predicting the tendency of actual changes in the hot springs, which means that the forecasting method is effective. Based on the verification studies, we distinguished the differences between affected and unaffected types of hot spring aquifers. It is expected that the forecasting method will contribute to environmental conservation in geothermal exploitation.