Pure CO2 and impure CO2-SO2-NO-O2 reactions with carbon storage site underlying seals: Coaly mudstones and carbonate cemented sandstones

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The transition to net zero emissions requires the capture of carbon dioxide from industrial point sources, and direct air capture (DAC) from the atmosphere for geological storage. Dissolved CO₂ has reactivity to rock core, and while the majority of previous studies have concentrated on reservoir rock or cap-rock reactivity, the underlying seal formation may also react with CO₂. Drill core from the underlying seal of a target CO₂ storage site was reacted at in situ conditions with pure CO₂, and compared with an impure CO₂ stream with SO₂, NO and O₂ that could be expected from hard to abate industries. Argillaceous sandstones, mudstones, coaly mudstones, and carbonate cemented sandstones of the Moolayember Formation, Bowen Basin, had significant natural alteration of feldspar to kaolinite creating porosity, with clays, siderite and textured ankerite filling and rimming porosities of 3.5 to 15.8 %. Synchrotron XFM quantified Mn mainly hosted in siderite veins and cements, Sr and Rb in feldspar, and Pb, Th and Sr in monazite. Pb was also in siderite; with As mainly in pyrite and associated with ankerite. On pure CO₂ or impure CO₂ reaction, ankerite and siderite dissolution, Fe-chlorite leaching, and apatite or sulphide alteration occurred. With the impure CO₂ stream Fe-oxides precipitated on rock surfaces especially in argillaceous sandstone. Ferroan carbonates, calcite, and Fe oxides containing Cr were also precipitated. Ankerite and siderite dissolution released increasing concentrations of dissolved Ca, Mg and Mn from carbonate cemented core that were higher with mixed gas injection. Argillaceous sandstone however released higher concentrations Si, Rb, Co and Zn. Dissolved Fe initially increased then decreased in impure gas experiments via Fe oxide precipitation, and Pb, Ni, Cr, REE also increased and subsequently decreased. Geochemical modelling predicted that Fe was mobilised mainly from reaction of siderite and Fe chlorite. Mainly carbonates (siderite, ankerite) and chlorite dissolution released trace metals, with several metals also initially mobilised by desorption and exchange. Precipitated Fe oxides provided adsorption sites to adsorb a portion of metals from solution. These reactions are also relevant to CO₂ streams from DAC that could be expected to contain O₂ and to potential reactions in overlying aquifers.