Ulrich Wolfgang Weber

Carbon capture and storage (CCS) may play a significant role in reducing greenhouse gas emissions. Noble gases are potential tracers to monitor subsurface CO2 storage sites and verify their containment. Naturally occurring noble gases have been used successfully to refute alleged CO2 leakage in the past. We present results from several sampling campaigns at two Norwegian CO2 capture facilities, the demonstration plant Technology Centre Mongstad (TCM) and the natural gas processing plant with CO2 capture and storage on Melkøya. The gas streams in the capture plants were monitored with a combination of on-site mass spectrometry and subsequently analysed discrete samples. This allows us to define the factors controlling noble gas concentrations in captured CO2, to monitor temporal variation of noble gas concentrations and finally evaluate the potential to use noble gases as inherent environmental tracers for labelling CO2 in storage reservoirs. At both sites, CO2 is captured using amine gas treatment. Noble gas concentrations in the gas streams were observed to decrease by several orders of magnitude during the processing. Isotopic ratios are air-like for CO2 captured after natural gas combustion at TCM and natural gas-like for CO2 captured from natural gas processing on Melkøya. Further, we detected a solubility trend caused by the amine solvent at TCM with higher solubility for heavier noble gases. We find that the relative concentrations of noble gases in the captured CO2 are defined by the gas from which the CO2 is captured and the design of the amine gas treatment process. Both factors were observed to cause temporal variation in the captured CO2. Using mixing and noble gas partitioning calculations we show that the significant depletion in noble gas concentrations, together with degassing of noble gas enriched formation water, means that the injected CO2 will inherit the noble gas signature of the storage formation, even following the injection of significant CO2 volumes. Any CO2 leaked from the storage formation is thus likely to have a crustal noble gas signature, characteristic of the storage site, which can be targeted for monitoring.

In the ICO2P project [1] a monitoring scheme for carbon capture and storage is developed with the aim to use noble gases as signatures for capture products. One approach to capture CO2 is amine-based temperature swing absorption. During such processes there is a potential for depletion of radiogenic noble gases from the fossil source, due to the addition of a gas phase void of radiogenic noble gases. The influence on the noble gas fingerprints of such techniques is investigated by single, analysed samples with full isotope resolution. This is combined with in-line, continuous noble gas measurements with a portable mass spectrometer [2]. This is a new approach that will provide knowledge of the uniqueness and temporal variations of noble gas fingerprints in the product stream from CO2 capture operations. The feasibility of such an on-site measurement system was successfully tested at a CO2 capture testing facility in Norway (TCM). Matching of the data with the reservoir source combusted at that time, forms the basis for setting up geochemical analytical schemes in mixing scenarios. Monitoring data from CO2 product streams are key in designing leakage detection schemes for future storage sites.