Noble Gases as Monitoring Tracers in CCS: A Case Study with CO2 from the Waste-to-Energy Plant Klemetsrud, Norway

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To design monitoring schemes for CO2 storage sites, the composition of the injected CO2 needs to be characterized. Noble gases are one of the environmental tracers, which could be cost-effective since they are naturally inherent in the CO2 and the storage reservoir fluids. The elemental, e.g. He/Ne, He/CO2, or isotopic, e.g. 3He/4He ratios of a gas are potentially diagnostic fingerprints which can be used to discriminate gas sources in the environment, e.g. deep crustal versus shallow, biogenic gas. The waste-to-energy (WtE) plant Klemetsrud, which generates 460000 tCO2/a, was investigated as a future source of captured CO2 for storage in the up-coming large-scale CCS project ‘Longship’, in Norway, aimed to be operational by 2024. In 2019, a demonstration capture facility at the WtE plant was set up to establish the feasibility of future large-scale CO2 capture. We sampled the main gas streams on-site and present the analysis of noble gases in (I) the exhaust of the waste incineration, i.e. the flue gas stream with high concentrations of CO2, (II) the capture outlet, i.e. the CO2-depleted gas stream and (III) the captured, concentrated CO2 gas stream. Noble gas concentrations are depleted in the captured CO2 after the amine treatment by several orders of magnitude compared to the flue gas. Isotopic ratios are air-like. This behavior has been observed at other CO2 capture plants. However, absolute concentrations of noble gases measured at Klemetsrud are significantly lower for all species, except He, which is within the range of previously measured concentrations. Further, we apply phase partitioning calculations to investigate the application of inherent noble gas in CCS monitoring schemes. The low noble gas concentrations, and their associated elemental and isotopic ratios, would only be preserved when the injected CO2 remains almost pure and does not mix or interact with fluids already present in the reservoir (e.g. native gas, formation water). Interaction of the CO2 with the formation water will lead to noble gas stripping that overprints the highly depleted composition of the captured CO2 with a noble gas elemental and isotopic ratios characteristic of deep crustal fluids. This characteristic signature, especially when coupled with the distinct major gas composition of leaked CO2 (e.g. extreme CO2/CH4 ratios) may form an effective tracer to discriminate a CO2 leakage from other gas sources at the seabed (e.g. shallow biogenic, dissolved air or hydrate decomposition). To take advantage of this low cost tracer, baseline geochemical surveys should include inventories of the background dissolved noble gas load in the formation water of storage reservoirs, in other (hydrocarbon bearing) formations along the migration route and in existing seabed gas seeps overlying the storage site.