Quantifying the precipitation and dissolution of CO2 within geological carbon storage analogues

Natural CO2 reservoirs that have contained CO2 for millennia hold the key to assessing the safety and viability of engineered CO2 storage. These CO2 reservoirs are critical in determining the long-term consequences of increasing the gross volume of CO2 in the subsurface over geological timescales. This study builds on published noble gas data from five natural CO2 reservoirs of the Colorado Plateau and Rocky Mountain provinces [1] by utilising published noble gas and ?13C(CO2) data from a further four CO2 reservoirs. This forms an extensive global dataset as two of the additional reservoirs are located in China and one each from Europe and the USA. Within the majority of these CO2 reservoirs there is a clear lowering of the CO2 component relative to the tracer 3He. This reduction of the CO2/3He correlates directly with an increase in groundwater-derived noble gases. This can only be explained if processes associated with groundwater interaction are responsible for reducing the magmatic CO2 component relative to 3He. We use a combination of the noble gas and ?13C(CO2) measurements to distinguish between and quantify the amount of CO2 drawdown within the individual reservoirs caused by dissolution of CO2 into the groundwater from that caused by precipitation processes. Our results illustrate that precipitation of CO2 gas as carbonates is a volumetrically a less significant mechanism of CO2 drawdown than CO2 dilution into the groundwater within these natural reservoirs. Additionally, evidence for ?13C(CO2) isotopic fractionation caused by precipitation processes is not readily apparent within the reservoirs that have a significant carbonate lithology.