Composition of Reservoir Fluids in Well IDDP-2

Abstract

As part of the Horizon 2020 project DEEPEGS (Deployment of deep enhanced geothermal systems for sustainable energy business), well IDDP-2 was drilled into the Reykjanes geothermal field to a depth of about 4.5 km by deepening the 3 km deep vertical well RN-15. The well has aquifers at 2.3, 3.4 and 4.5 km depth, the deepest of which is expected to yield supercritical fluids. Following injection testing, and a subsequent period of heating up, a discharge test of the well is planned in autumn 2019. In this contribution, an attempt is made to estimate the chemical composition of the well fluids upon production. This was carried out by estimating the chemical composition at each depth by analogies with fluid from the currently produced Reykjanes geothermal system and by reported composition of fluids from hydrothermal vents on the ocean floor, followed by mixing calculations and calculations of depressurisation boiling from the mixing temperature (350°C) to an estimated wellhead pressure of 70 bar-a. This was carried out using the SOLVEQ-XPT and CHIM-XPT geochemical codes. The reconstructed Reykjanes reservoir fluid at 350°C has seawater salinity and a pH of about 5.5. The sulphide concentration is somewhat higher and copper, iron, zinc and lead concentrations substantially higher, compared to the conventional Reykjanes fluid at 295°C. Mixing of this heated conventional fluid with dilute and concentrated supercritical seawater yields fluids with slightly lower pH, higher silica and substantially higher sulphide concentrations. During boiling to 286°C (70 bar-a), the fluid mixtures precipitate 1–6 mg per kg of produced fluid, mainly anhydrite, haematite, bornite and phyllosilicates. The boiled fluid has pH ranging from 3.8 to 4.5, and high sulphide concentrations. It is somewhat undersaturated with respect to amorphous silica. Upon further boiling of the fluid, to 210°C (19 bar-a) further deposits are formed, mainly amorphous silica (20–70 mg per kg) although the mixtures containing dilute vent fluid are also predicted to precipitate chalcopyrite, chalcocite and galena. After boiling to 210°C, the in situ pH of the fluid ranges from 3.4 to 4.1.