The application of noble gases and carbon stable isotopes in tracing the fate, migration and storage of CO2

Storage of Carbon Dioxide in Saline Aquifers: Physicochemical Processes, Key Constraints, and Scale-Up Potential

CO₂ storage in saline aquifers offers a realistic means of achieving globally significant reductions in greenhouse gas emissions at the scale of billions of tonnes per year. We review insights into the processes involved using well-documented industrial-scale projects, supported by a range of laboratory analyses, field studies, and flow simulations. The main topics we address are (a) the significant physicochemical processes, (b) the factors limiting CO₂ storage capacity, and (c) the requirements for global scale-up.Although CO₂ capture and storage (CCS) technology can be considered mature and proven, it requires significant and rapid scale-up to meet the objectives of the Paris Climate Agreement. The projected growth in the number of CO₂ injection wells required is significantly lower than the historic petroleum industry drill rates, indicating that decarbonization via CCS is a highly credible and affordable ambition for modern human society. Several technology developments are needed to reduce deployment costs and to stimulate widespread adoption of this technology, and these should focus on demonstration of long-term retention and safety of CO₂ storage and development of smart ways of handling injection wells and pressure, cost-effective monitoring solutions, and deployment of CCS hubs with associated infrastructure.

420,000 year assessment of fault leakage rates shows geological carbon storage is secure

Carbon capture and storage (CCS) technology is routinely cited as a cost effective tool for climate change mitigation. CCS can directly reduce industrial CO₂ emissions and is essential for the retention of CO₂ extracted from the atmosphere. To be effective as a climate change mitigation tool, CO₂ must be securely retained for 10,000 years (10 ka) with a leakage rate of below 0.01% per year of the total amount of CO₂ injected. Migration of CO₂ back to the atmosphere via leakage through geological faults is a potential high impact risk to CO₂ storage integrity. Here, we calculate for the first time natural leakage rates from a 420 ka paleo-record of CO₂ leakage above a naturally occurring, faulted, CO₂ reservoir in Arizona, USA. Surface travertine (CaCO₃) deposits provide evidence of vertical CO₂ leakage linked to known faults. U-Th dating of travertine deposits shows leakage varies along a single fault and that individual seeps have lifespans of up to 200 ka. Whilst the total volumes of CO₂ required to form the travertine deposits are high, time-averaged leakage equates to a linear rate of less than 0.01%/yr. Hence, even this natural geological storage site, which would be deemed to be of too high risk to be selected for engineered geologic storage, is adequate to store CO₂ for climate mitigation purposes.

Exploring pore water biogeochemical characteristics as environmental monitoring proxies for a CO2 storage project in Pohang Basin, South Korea

Biogeochemical parameters of pore waters, including dissolved organic matter, nutrients, sulfate, alkalinity, and chloride are explored as convenient and sensitive proxies to monitor the CO₂ geological storage sites. Five sites for a CO₂ storage project in the Pohang Basin of the East Sea in South Korea were investigated for the pre-injection biogeochemical conditions of these sites. Higher dissolved organic carbon (~36 mg L^-1), chromophoric and fluorescent dissolved organic matter, nutrients, and alkalinity were observed in a fluvially affected acoustic blanking site with geological faults. A general increasing downcore trend of measured DOM parameters, nutrients, and alkalinity with depth was found at the acoustic blanking site affected by riverine runoff with significant correlations among the parameters (R²: ~0.4-0.8), highlighting the impact of geological features and external inputs on the downcore biogeochemical properties. The results presented in this study suggest that DOM could be utilized as a robust and complementary biogeochemical parameter.

Inherent Tracers for Carbon Capture and Storage in Sedimentary Formations: Composition and Applications

Inherent tracers-the "natural" isotopic and trace gas composition of captured CO2 streams-are potentially powerful tracers for use in CCS technology. This review outlines for the first time the expected carbon isotope and noble gas compositions of captured CO2 streams from a range of feedstocks, CO2-generating processes, and carbon capture techniques. The C-isotope composition of captured CO2 will be most strongly controlled by the feedstock, but significant isotope fractionation is possible during capture; noble gas concentrations will be controlled by the capture technique employed. Comparison with likely baseline data suggests that CO2 generated from fossil fuel feedstocks will often have δ(13)C distinguishable from storage reservoir CO2. Noble gases in amine-captured CO2 streams are likely to be low concentration, with isotopic ratios dependent on the feedstock, but CO2 captured from oxyfuel plants may be strongly enriched in Kr and Xe which are potentially valuable subsurface tracers. CO2 streams derived from fossil fuels will have noble gas isotope ratios reflecting a radiogenic component that will be difficult to distinguish in the storage reservoir, but inheritance of radiogenic components will provide an easily recognizable signature in the case of any unplanned migration into shallow aquifers or to the surface.