Thermal-Hydrological-Geochemical modeling of the distribution of acidic fluids at the Onikobe geothermal field, Japan

Abstract

Geothermal development has not been a priority in acidic fluid areas to date. In recent years, Japan's deregulation has allowed wells to be drilled near young volcanoes in national parks where geothermal development investigations have not previously been conducted, and acidic fluids are now being developed in some areas. More areas with acidic fluids are expected to be discovered in the future. Our goal is to predict the chemical characteristics and distribution of acidic fluids prior to well drilling to maximize the use of available acidic fluids (pH 3∼5) so that the exploitable areas of geothermal resources can be expanded. Predicting acidic fluid production is important to plan and implement corrosion mitigation measures to minimize detrimental corrosive effects on wells and surface facilities. In this paper, the Onikobe geothermal field is used as a model field, and a conceptual model of this geothermal system with acidic fluids is developed. Natural-state coupled thermal-hydrological-geochemical (THC) simulations were performed to estimate the acidic fluid distribution zone and to evaluate the validity of the conceptual model. Multiple iterations between TH (thermal-hydrological, both natural-state and exploitation phases) and THC simulations were performed using iTOUGH2 and TOUGHREACT until reasonable pressures, temperatures, enthalpies, and chemical species concentrations could be obtained. The pH of the acidic fluid zone at the Onikobe field remains stable at about pH 3.5, with the acidic fluid understood to extend consistently with the spatial distribution of pyrophyllite.