| Summary: | An injected CO2 or greenhouse gas (GHG) stream will dissolve into formation water forming carbonic acid, especially in the case of low salinity reservoir storage. Industrial GHG streams from coal combustion, cement or streel production may contain accessory gases including N2, Ar, SOx, NOx, or O2, several including SOx and NOx form stronger acids. The dissolved GHG stream will have reactivity to some rock forming minerals, potentially modifying porosity or water chemistry, or mineral trapping CO2. The Precipice Sandstone in the Surat Basin, Australia, is a low salinity target reservoir for CO2 storage. The Surat CCS project proposes to a demonstration-scale injection test of 60,000 t per year of a GHG stream captured from black coal PCC into the quartz rich Lower Precipice Sandstone. The dissolved GHG stream is expected to interact with the Lower and Upper Precipice Sandstone and sink to interact with the underlying Moolayember Formation of the Bowen Basin. However little was known about the lithologies of the Moolayember Formation which unconformably underlies the projects proposed injection site. Drill cores from the Moolayember Formation were sampled and characterized: indicating a high proportion of carbonate cements, potentially reactive clays, feldspars, and trace amounts of sulphides and coal. Complex carbonate assemblages and secondary textures indicate this may already be a valuable example of a natural analogue of CO2 alteration. Kinetic geochemical modelling was performed to predict the CO2 or CO2-SO2-NO reactivity of several lithologies. This indicted that after 30 years, low concentrations of SOx and NO (100 ppm) had minimal effect on the pH which was buffered to similar values relative to pure CO2 for these reactive mineralogies. The pH was instead controlled by the carbonate mineral content, which buffered acidity. Reaction of clay and feldspar rich lithologies resulted in dissolution of K-feldspar, chlorite, and plagioclase to form kaolinite, smectite, and additionally siderite mineral trapping CO2. SO2 was predicted to be mineral trapped as pyrite or alunite. The net predicted mineral volumes or porosities however did not change significantly. Experiments at reservoir conditions and further characterization are planned to validate and improve model predictions.
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