Impure CO2 storage reactions of sandstone, mudstone and carbonate cemented cores: Experimental CO2 SO2 NOX O2 reaction metal mobilisation and fate

CO2 geological storage can be part of the solution to reduce carbon emissions to the atmosphere. An understanding of the geochemical processes occurring during CO2 storage is needed to reduce risk. Drill cores from a low salinity reservoir site proposed for CO2 storage, and the overlying and underlying formations, were characterised for minerals by QEMSCAN, total metals and porosity. Elements including Li, Ba, Sr, K, Mg, V, Zn, REE, Fe, Pb, P, and S were relatively elevated in the Moolayember Formation underlying the reservoir. Synchrotron XFM showed the main host of Mn was siderite, with Rb in K-feldspar, Zn and Cu in sphalerite and chalcopyrite, and As in pyrite in coal pores associated with coal laminations. Drill cores are reacted at reservoir conditions with synthetic formation water and an impure CO2 stream composition of CO2-SOx-NOx-O2 expected to be injected at the site. Elements released were dependant on mineral content, with quartz rich reservoir, lower Precipice Sandstone core reactions resulting in dissolution of trace carbonates, alteration of sulphides and monazite, and variable elevated dissolved Pb, and U. Dissolved Co, Ni, Ca, Zn, Li, Rb, and U were released at relatively elevated concentrations from the mudstone. For carbonate cemented upper Precipice Sandstone or Moolayember Formation core strong dissolution of calcite and ankerite, with corrosion of siderite, Fe-rich chlorite, and sulphides or monazite were observed after reaction. Dissolved elements including Ca, Mg, Mn, Sr, and Ba increased in experiments from the reaction of calcite, siderite, and ankerite. Generally dissolved Fe, Pb, Cr, Cu, Co etc. increased from dissolution, and subsequently decreased in concentration with adsorption and precipitation processes. The fast mobilisation of elements including Fe and Pb are consistent with the release of metals from carbonate dissolution and desorption. The presence of O2 and NOX in the gas stream results in Fe-(oxyhydr)oxide precipitation especially where Fe has been rapidly mobilised from dissolution of siderite and Fe-chlorite. This acts as a sink for Fe and provides new adsorption sites for sequestering a proportion of the trace metals. These processes are applicable to other CO2 storage sites and potential leakage indicators in overlying drinking water aquifers. The findings are also more broadly applicable to subsurface energy storage such as compressed air renewable energy storage, CO2 enhanced recovery, geothermal, natural gas or hydrogen storage.

Automated summary

CO2 geological storage is a potential solution for reducing carbon emissions. It is important to understand the geochemical processes involved in CO2 storage to minimize risks. This study analyzed drill cores from a proposed CO2 storage site and identified elevated levels of certain elements in the underlying formation. The research also demonstrated how different mineral contents can result in the release of different elements. The findings have implications for CO2 storage sites and potential indicators of leakage in drinking water aquifers. They are also relevant to subsurface energy storage in various forms.