Metal mobilisation and fines migration in pure CO₂ and impure CO₂-SO₂-NO reactions of carbon storage site core

Abstract

Carbon dioxide geological storage is proposed as part of the solution to reach net zero emissions. The potential to mobilise heavy metals to low salinity groundwater through CO2 water rock geochemical reactions is a potential environmental risk factor, if CO2 migrates. Previous studies have focused on pure CO2 reactivity, however CO2 streams from hard to abate industries can contain gas impurities. Reservoir sandstone and mudstone drill cores from a proposed low salinity CO2 storage demonstration site were reacted at in situ conditions with pure CO2 or an impure NO-SO2-CO2 stream. Sandstones hosted Rb in illite analysed via synchrotron XFM. Arsenic (As) was hosted in pyrite; and Pb, Cr, Mn in siderite rimming intergranular pores. Mudstone contained Zn, Co, Ni, Cu, As, Pb in sphalerite, and Rb in illite and K-feldspar. In impure NO-SO2-CO2 experiments the lowered pH and oxidising conditions initially released higher concentrations of metals including Pb, Zn, Co into solution compared to pure CO2 reactions. Higher concentrations of Zn (Mn and Co) were released from sphalerite in the mudstone. Fe-chlorite, K-feldspar, and carbonate dissolution released Rb, Si, Fe, Ca, and Mg. Elevated dissolved Pb was mainly from siderite and sulphide mineral reaction in sandstones. Mobilised As was released prior to CO2 addition from desorption and ion exchange. Clay and fines migration into pores occurred in both pure and impure CO2 reactions that has the potential to impact fluid migration. A portion of metals including Fe, Ni, Cr were subsequently incorporated in precipitated Fe hydr(oxy)oxides where the co-injected NO induced oxidising conditions. Rock mineral content and the injected gas mix were the main controls on metal mobilisation to formation water. Further work should investigate new gas mixtures that may be expected in storage hubs, from blue hydrogen or from direct air capture.