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

David Banks,Adrian J Boyce,Rob Westaway,Neil M Burnside

doi:10.1016/j.geothermics.2020.102003

David Banks, Adrian J Boyce + Show 2 more

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https://doi.org/10.1016/j.geothermics.2020.102003

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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.

Highlights

The Pohang geothermal site (129◦22′46.08′′E, 36◦06′23.34′′N) is located c. 6 km north of the city of Pohang, on the east coast of the Korean peninsula (Fig. 1)
For the August 2017 PX-1 stimulation, water was pumped from the pond, with no treatment, to a storage tank, from which it was injected under pressure into the well
The inorganic chemistry of the flowback water following the August 2017 well stimulation attempt has already been reported (Burnside et al, 2019; Westaway et al, 2020; Banks et al, 2019); only selected parameters will be discussed in detail in this paper

Summary

Introduction

The Pohang geothermal site (129◦22′46.08′′E, 36◦06′23.34′′N) is located c. 6 km north of the city of Pohang, on the east coast of the Korean peninsula (Fig. 1). Two 4.2 km deep wells have been drilled into a concealed granodiorite below the site, with the intention of creating an enhanced geothermal system (EGS). Most of the published studies of the chemistry of flowback fluid come from the hydrocarbon sector, especially from the hydraulic fracturing of deep organic shales to produce “shale gas”. Most of these studies conclude that the dominant factor determining flowback chemistry is mixing between the injected water and a deep, connate, highly reducing brine, potentially of considerable age (Haluszczak et al, 2013; Zolfaghari et al, 2015; Balashov et al, 2015; Vazquez et al, 2014). The study found an early excess of sulphate in the flowback water, which the au thors ascribed (albeit speculatively) to pyrite oxidation

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