Geochemical modelling of experimental O2–SO2–CO2 reactions of reservoir, cap-rock, and overlying cores

Abstract

Carbon dioxide streams stored geologically from industrial sources e.g. coal oxy-fuel firing sources, steel or cement processing, may contain gas impurities including O2, NOx and SOX which may have higher reactivity to rock than pure CO2. Supercritical CO2 with O2 and SO2 impurities (0.16 and 2% respectively) was reacted with core from the Precipice Sandstone, Evergreen Formation, and Hutton Sandstone of the Surat Basin, Australia. The Precipice Sandstone is a low salinity target reservoir for CO2 storage, where the Evergreen Formation would be the overlying cap-rock, and the Hutton Sandstone is an overlying aquifer. High and low water regimes were investigated with core sections either submerged in bulk gas saturated water, or suspended in the water saturated supercritical gas head at reservoir in situ conditions of 60 °C and 12 MPa. During O2–SO2–CO2 water rock reactions in bulk solution, a low solution pH (~1–2) was measured owing to the relatively high gas impurity concentrations used. Dissolution of reactive minerals from the core including Fe-chlorite, ankerite, siderite, and plagioclase was directly observed. The Evergreen Formation cap-rock and Hutton Sandstone cores reacted here contained ~ 5–10 times higher total concentrations of elements such as Fe, Al, Ti, Ba than the Precipice Sandstone core. Several cations were mobilised to solution including Mn, Co, Zn, Sr, Si, and Mg. Increasing dissolved concentrations of Mn, and Co were strongly correlated with Fe (R2 = 0.99) during reaction of the Precipice Sandstone core. Dissolved Co and Sr were strongly correlated with Fe during reaction of the Evergreen Formation core. Dissolved Mn was somewhat correlated with Fe (R2 = 0.99) from reaction of the Hutton Sandstone core, from both carbonate and chlorite dissolution. During gas-water reaction with the Hutton Sandstone core, dissolved element concentrations subsequently decreased, by incorporation into newly formed Fe oxide precipitates observed with Cr and Ni signatures. Geochemical models were built and validated with good agreement attained with experiments, predicting Fe-chlorite and minor carbonate minerals were the main minerals dissolving. Reactive surface areas initially estimated by a modified geometric method needed substantial increases for example in the case of chlorite (up to 2000x), and decreases for carbonates (2–10x) to match experimental data. A 30 year simulation of reservoir rock reactivity showed that the lowered pH was buffered by mineral dissolution. Compared to previously published pure CO2-water-rock reactions, the concentrations of ions such as Si generally were initially higher through reaction of more silicate minerals at the lower generated pH, and the oxidation of sulphides. Compared to SO2–CO2-water reactions, the concentrations of cations were similar, but with O2 present dissolved Fe subsequently decreased as Fe-oxide, sulphate and clay minerals precipitated. Cap-rock showed reactivity to wet supercritical O2–SO2–CO2 fluid in the supercritical gas headspace. Corrosion of minerals including ankerite and Fe-chlorite were observed, along with precipitation of oxide, sulphate and silicate minerals on rock surfaces. These results have implications for impacts in CO2 storage sites where gas impurities may be co-injected with CO2 and accumulate in the near wellbore region.