Characterization of Acid Water in the Otake-Hatchobaru Geothermal Field

Abstract

Various kinds of acid waters have been recognized by well drilling in the Otake-Hatchobaru geothermal field and their formation mechanism has been studied. Shallow acid water that contains sulfate ion higher than chloride ion in content is formed by dissolved- air oxidation of hydrogen sulfide gas. Contrarily, deep acid water-which has enthalpy higher than approximately 1,000 jlg contains chloride ion of high content. Zones of the latter acid water are located in and around neutral water reservoir tapped by production wells. Clarification of formation mechanism and extent of the latter acid water is indispensable for stable operation of the power plant and future development. Using recent isotopic study techniques, the deep acid water was revealed to have formed by contact of high-temperature CI type water with sulfur-containing rock, hydrolysis of sulfur to hydrogen sulfide and sulfuric acid. Furthermore, combining the chemical and isotopic data with geological and geophysical data, the deep acid waters are judged to be stored in restricted reservoir at relatively deep level.