Supersaline and metal-rich brine obtained from the Quaternary Kakkonda Granite by NEDO WD-1a in the Kakkonda geothermal field, Japan

Abstract

A supersaline, metal-rich brine (ca. 40 weight% total chloride salt) was extracted from 3708 m depth of well WD-1a in the Kakkonda Granite by reverse circulation after a standing time about 196 hours. The estimated borehole temperature exceeds 500 °C near the bottom. Tritium content and the relationship between δD and δ18O show that the injected borehole fluid (river water) evidently mixed with an isotopically heavy and ancient fluid. The phase analysis showed that a gas phase separated from a brine and that a brine concentrated in the borehole as the borehole temperature recovered after cooling by drilling fluids. We think the original fluid was trapped in the Kakkonda Granite and mixed with the borehole fluid through fine fractures induced by thermal stress.