MINERAL ALTERATIONS CAUSED BY OXIDISING ACCESSORY GASES IN THE GEOLOGICAL STORAGE OF CO2

The German project COORAL (“CO₂ Purity for Capture and Storage”) investigates the effects of accessory gases during the processes of carbon sequestration, i.e., power generation, capture, transport, injection and CO₂ geological storage. At BGR we concentrate on geochemical experiments at in-situ pressure-temperature conditions to elucidate the occuring geochemical processes after injection of CO₂ into saline aquifers. To understand the occuring mineral alterations it is essential to investigate fluid-rock interactions that include the minerals of potential storage formations, CO₂ and accessory gases that occur in the captured CO₂ gas stream. The latter will contain minor amounts of gases such as O₂, N₂, NO_x, SO_x, CO, and H₂S. However, quantitative data on mineral alterations due to these accessory gases are scarce at relevant conditions.

The experiments at BGR are carried out using static batch reactors equipped with chemically inert flexible gold-titanium-cells. The investigated mineral phases are carefully crushed, sorted and cleaned natural mono-minerals whereas the natural formation water is simplified to a Na-Cl solution (150 g NaCl /l). A first set of experiments on carbonates in pure water or salt solution allowed testing the laboratory set-up and adjusting the modelling environment using the numerical code PHREEQC. While experimental data without CO₂ addition are well represented by thermodynamic simulations, discrepancies occur between measured and simulated data in the presence of CO₂. Duplicate dolomite-brine-CO₂ experiments exhibited a very good reproducibility showing release rates for both, Mg and Ca, between 2*10^-10 mol s^-1 cm^-2 at the very beginning and 4*10^-13 mol s^-1 cm^-2 just before approaching steady state.

The main target of the ongoing experiments is a) to continue the work on the carbonates using a binary gas mixture of CO₂–SO₂ thus looking into effects caused by an increased acidity and the presence of SO₄ and b) to investigate the effects of accessory O₂ on redox sensitive minerals, especially clay minerals.