The geology and water chemistry of the Hellisheidi, SW-Iceland carbon storage site

Abstract

The subsurface rocks at the Hellisheidi carbon injection site are primarily olivine tholeiite basalts consisting of lava flows and hyaloclastite formations. The hyaloclastites are low permeability glassy rocks formed under ice and melt water during glaciations that serve as the cap rock at the injection site; the boundaries between hyaloclastites and lava flows and those between individual lava flows boundaries are preferential fluid flow pathways. Some alteration is observed in the hyaloclastite cap rock situated at 100–300m depth consisting primarily of smectite, calcite, Ca-rich zeolites, and poorly crystalline iron-hydroxides. Alteration increases with depth. These alteration phases lower the porosity and permeability of these rocks. Carbon dioxide injection will be targeted at a lava flow sequence at 400–800m depth with the main aquifer located at 530m depth. Loss on ignition suggests that over 80% of the primary rocks in the target zone are currently unaltered. The target zone rocks are rich in the divalent cations capable of forming carbonates; on average 6 moles of divalent cations are present per 1kg of rock.The water in the target zone ranges in temperature from 15 to 35°C; the in situ pH ranges from 8.4 to 9.8. The partial pressure of CO2 and O2 suggest that the water in this system is isolated from the atmosphere. The concentration of Ca and Mg in these waters are limited by secondary mineral precipitation. All the waters are supersaturated with Ca-zeolite, analcime, Ca–Mg–Fe smectite, calcite, and aragonite, and some are supersaturated with respect to dolomite and Fe–Mg carbonates.Pure commercial CO2 and a 75%–24.2%–0.8% mixture of CO2–H2S–H2 gases will be dissolved into water prior to its injection into this system. The injected water will have a temperature of ∼25°C and be equilibrated with ∼25bar pressure of the CO2 gas, and ∼14bar pressure of the CO2–H2S–H2 mixture. The injected gas will have total CO2 concentrations of ∼0.8–0.42mole/kg H2O and a pH of 3.7–4.0, depending on the H2S concentration of the injected gas. All host rock minerals and glass will be strongly undersaturated with respect to the gas charged injection waters. The water will therefore create porosity in the near vicinity of the injection by dissolving primary and secondary minerals. Further from the injection, secondary minerals will precipitate, potentially clogging the system. Reaction path modeling shows that 1–2 moles of basaltic glass are needed to lower the injected CO2 concentration down to natural pre-injection concentrations, but less than 1 mole is needed for the sequestered H2S. Major carbonates formed were Ca–Mg–Fe–carbonate and dolomite at pH <5, whereas ankerite and calcite formed later at higher pH. Associated minerals at lower pH were chalcedony, kaolinite and iron hydroxide, followed by smectite and zeolites at higher pH. Modeling result suggest that the first sulfur bearing phase to precipitate is elemental sulfur, followed by greigite and mackinowite upon further basalt dissolution.