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

• The 4.2 km deep Pohang PX-1 well was subject to hydraulic stimulation. Sulphate concentration and δ 34 S increase during post-stimulation flowback. Excess SO 4 2− demonstrates mineral dissolution Isotopes modelling indicate anhydrite as probable sulphate source. A stimulation carried out on a granodiorite reservoir in an enhanced geothermal system August 2017 Pohang, Korea. Water injected into the contained c. 330−360 mg/L sulphate, with negative S. resulting flowback water became more saline time, chloride concentrations dissolved all increasing. Compared conservative advective-dispersive mixing models, surplus elevated fluid is saturated respect at downhole temperatures pressures. Dissolution by surface of secondary along fracture surfaces, most likely reflecting fluid, have resulted excess S-enriched fluid. An alternative hypothesis involving oxidation pyrite also plausible but stoichiometrically inadequate account for observed excess, unlikely from sulphur isotopic perspective. This analysis thus contributes evidence water-rock reactions granodiorite.