Development of Study Methods for Clarifying Formation Mechanism and Distribution of Acid Geothermal Fluid -Case Studies of Geothermal Areas in Kyushu, Japan

Abstract

In some geothermal fields in Kyushu, Japan, strongly acid water are being produced from geothermal wells, causing terrible corrosion of casing pipes of geothermal wells and transportation pipelines of geothermal fluid. In order to develop the study methods for evaluating acid geothermal fluid, case studies for geothermal areas in Kyushu were carried out. It was ascertained that geochemical techniques using isotope data are useful to reveal the origin and characteristics of acid fluids enriched in SO4. In particular, the integrated interpretation using several indices based on isotope and chemical data of sulfur species, i.e. ‰34S(S total), ‰34S fractionation factor and SO4 and H2S concentration in total fluid, are effective. A recent data set of sulfur isotope analyses including ‰34S(H2S) values of the acid fluids occurring in the Hatchobaru field was used for this study with to the existing data of ‰34S(SO4) value of the discharge waters. Sulfur isotope ratios of the acid fluid are relatively high, suggesting that the SO4-rich fluid is not caused by the simple oxidation of H2S. On the basis of their isotopic and chemical characteristics, the acid fluids are thought to be the result of mixing of neutral-pH fluid with low temperature SO4-rich fluids from shallower levels. Taking into consideration the occurrence of native sulfur in some core samples from shallow levels, a possible explanation to the origin of SO4-rich fluids is the hydrolysis of native sulfur at a temperature lower than 224?C. However, the partial oxidation of H2S accompanied with isotopic reequilibration at temperatures lower than 240?C is alternatively able to explain the origin of SO4-rich fluid. In either case, the distribution of acid fluid reservoirs in the Hatchobaru field is believed to be restricted to the relatively shallow level. Isotopic compositions and chemical characteristics of sulfur species in acid fluids in the Takigami field indicate that its formation mechanism is similar to that of the Hatchobaru field. Although acid fluids occurring in the Shiramizugoe are likely to be also produced by mixing of SO4-rich fluid, more detailed geochemical studies are necessary to reveal the formation mechanism of the acid fluids.