4 th AEGC: Geoscience – Breaking New Ground – 13-18 March 2023, Brisbane, Australia 1 Assessing geochemical reactivity during CO2 geological storage: an example from the Surat Basin J K Pearce G K W Dawson S D Golding University of Queensland University of Queensland University of Queensland j.pearce2@uq.edu.au D Kirste G Southam D Paterson Simon Fraser University University of Queensland Australian Synchrotron F Brink V Rudolph Australian National University University of Queensland SUMMARY CO2 storage is part of the transition to lower emissions; however, it necessitates a geochemical assessment of potential site-specific impacts. Injected CO2 will dissolve into formation water and the resulting acidification can induce mineral dissolution and precipitation, alteration of porosity and permeability, or mobilisation of metals to groundwater. Reactivity can depend on several factors including the captured gas stream composition, and mineralogical content. We present a comprehensive assessment methodology with a focus here on understanding metal mobilisation. Drill core samples are characterised for minerals, poro-perm, and for metals via total digestions; sequential extractions; and synchrotron X-ray Fluorescence Microscopy. Drill cores are reacted at reservoir conditions with pure CO2 and the specific greenhouse gas stream, e.g., CO2-SOx-NOx-O2. Kinetic geochemical models are then history matched to experimental data that ultimately are inputs to a reactive transport model. The Surat Basin Precipice Sandstone is undergoing feasibility studies as a CO2 storage reservoir, and an example from a storage site assessment is presented here. The Evergreen Formation is the overlying cap-rock, and the Moolayember Formation underlies the reservoir. The lower Precipice Sandstone is quartz-rich while the upper Precipice Sandstone, and the Evergreen and Moolayember formations are mineralogically diverse with higher feldspar, clay, and carbonate content. Dissolved elements Ca, Mg, Mn, Sr, and Ba increased in experiments from reaction of trace amounts of carbonates. Generally dissolved Fe, Pb, As, Cr, Se increased and subsequently decreased in concentration indicating adsorption and precipitation. Kinetic reaction path modelling demonstrated that carbonate minerals and chlorite are the main minerals reacting. The presence of O2 and rapid Fe mobilisation results in the precipitation of Fe-oxyhydroxides that act as a sink for Fe and provide new adsorption sites for sequestering a proportion of the trace metals. In the longer term, CO2 mineral trapping as ankerite at the reservoir-seal interface additionally traps metals. Key words: carbon storage, CCUS, Surat Basin, geochemistry, Bowen Basin. INTRODUCTION Carbon dioxide geological storage traditionally involves the injection of a captured CO2 stream into a suitable reservoir underground such as a deep saline aquifer, depleted oil and gas reservoir or coal seam. In the case we discuss here CO2 is injected ~1-2 km deep into a sandstone reservoir. A seal or cap-rock complex with low porosity and permeability structurally prevents migration. In Australia several low salinity aquifers have been previously proposed as reservoirs, since these may have other uses in other areas it is important to understand any potential impacts to rock properties or water resources and if these stay within the expected areas. Appraisals for CO2 storage will be site specific and involve multiple components, this paper focuses on CO2 rock reactivity aspects. A demonstration scale CO2 injection trial was proposed in the Surat Basin in the Glenhaven area near Wandoan for the feasibility of geological storage (Figure 1). More recently larger scale storage is proposed in the southern Surat Basin west of Moonie (that is separate to proposed Moonie oil field EOR/storage). The Precipice Sandstone is the proposed reservoir, the Evergreen Formation is the seal, and the Moolayember Formation underlies the reservoir (Figure 1). Injected CO2 dissolves in formation water, and the resulting acidification can induce rock reactions. Industrial captured CO2 streams can contain other gases e.g. SO2, O2, NOx that can cause higher acidification. Storage hubs may contain a mixture of gas stream compositions e.g. from power plants, blue hydrogen production, direct air capture, cement, steel or ammonia production. Injected CO 2 can dissolve in formation water lowering pH: CO2 + H2O → HCO3 - + H +