Sulphur isotopes in deep groundwater reservoirs : evidence from post-stimulation flowback at the Pohang geothermal facility, Korea

Abstract A hydraulic stimulation was carried out on a granodiorite reservoir in an enhanced geothermal system in August 2017 in Pohang, Korea. Water injected into the 4.2 km deep PX-1 well contained c. 330−360 mg/L sulphate, with a negative δ34S. The resulting flowback water became more saline with time, with sulphate and chloride concentrations and dissolved sulphate δ34S all increasing. Compared with conservative advective-dispersive and mixing models, the flowback contained surplus sulphate with an elevated δ34S. The PX-1 reservoir fluid is saturated with respect to anhydrite at downhole temperatures and pressures. Dissolution by injected surface water of secondary anhydrite along fracture surfaces, most likely with elevated δ34S reflecting the reservoir fluid, is likely to have resulted in an excess of 34S-enriched sulphate in the flowback fluid. An alternative hypothesis involving oxidation of pyrite is also plausible but is stoichiometrically inadequate to account for the observed sulphate excess, and unlikely from a sulphur isotopic perspective. This analysis thus contributes to the evidence for water-rock reactions during stimulation of the Pohang granodiorite.