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Engineering CO2 Storage with Co-Contaminants

Abstract

Geochemical CO2-water-rock reactions may positively or negatively affect CO2 geological storage sites by; 1) modifying porosity and permeability of rock and hence movement of CO2, 2) causing acidification and redox changes that result in evolution of water quality (e.g. pH, metal ion concentrations), 3) dissolution of silicates providing divalent cations for ionic and subsequent mineral trapping of CO2 as carbonates, and 4) precipitation of carbonates and other minerals or the movement of clays fines that can plug pores and self-seal. Impurity gases such as SO2, O2, H2S and NOx, are present in industrial CO2 capture streams including from coal combustion and oxyfuel firing, natural gas processing and combustion, or cement, steel, iron or coke production. Although their co-injection may present a cost benefit, the majority of studies assume pure CO2 injection. A suite of integrated experiments, geochemical and physical analyses, and geochemical modelling informed by natural analogue studies has been performed. Target core from a proposed sequestration site in the Surat Basin, Queensland, indicate a reservoir rock with a clean mineralogy and good porosity, and a sequence of baffled and impermeable units above. The geochemical response of core to reaction with dissolved supercritical CO2-SO2 and CO2-SO2-O2 gas mixtures at reservoir conditions has indicated that impurity gases result in greater acidification and hence enhanced dissolution or ion leaching of silicates providing higher concentrations of divalent cations for CO2 ionic and mineral trapping. The presence of SO2 and O2 also resulted in sulphate and oxide precipitation on core from the reservoir seal boundary. The potential long term benefits of manipulating gas streams to optimise safe storage outcomes, and the effects of NOx are so far mainly untested. Recent CO2 storage natural analogue studies have identified mineralisation at the reservoir- seal interface as well as in the gas-leg interface region where supercritical CO2 containing dissolved water (and potentially impurity gases) may be structurally trapped under the seal. This low water availability region provides another opportunity to positively engineer the storage system.