Modelling large-scale CO2 leakages in the North Sea

A three dimensional hydrodynamic model with a coupled carbonate speciation sub-model is used to simulate large additions of CO2into the North Sea, representing leakages at potential carbon sequestration sites. A range of leakage scenarios are conducted at two distinct release sites, allowing an analysis of the seasonal, inter-annual and spatial variability of impacts to the marine ecosystem. Seasonally stratified regions are shown to be more vulnerable to CO2release during the summer as the added CO2remains trapped beneath the thermocline, preventing outgasing to the atmosphere. On average, CO2 injected into the northern North Sea is shown to reside within the water column twice as long as an equivalent addition in the southern North Sea before reaching the atmosphere. Short-term leakages of 5000 tonnes CO2over a single day result in substantial acidification at the release sites (up to -1.92 pH units), with significant perturbations (greater than 0.1 pH units) generally confined to a 10 km radius. Long-term CO2leakages sustained for a year may result in extensive plumes of acidified seawater, carried by major advective pathways. Whilst such scenarios could be harmful to marine biota over confined spatial scales, continued unmitigated CO2emissions from fossil fuels are predicted to result in greater and more long-lived perturbations to the carbonate system over the next few decades

Insights and guidance for offshore CO2 storage monitoring based on the QICS, ETI MMV, and STEMM-CCS projects

Carbon Capture and Storage (CCS) is a collective term for technologies that allow society to unlock the benefits of energy intensive processes like fertiliser production and combustion of fuels (fossil or biologically sourced) without releasing the CO2 to the atmosphere. Hence, CCS could assist in accelerating decarbonisation while society pursues a just energy transition. This paper aims to summarise the learnings of three research projects that all investigated aspects of marine monitoring for CCS from a CO2 storage operator’s perspective. The QICS (Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage), ETI MMV (Energy Technologies Institute Measurement, Monitoring and Verification of CO2 Storage), and STEMM-CCS (Strategies for Environmental Monitoring of Marine CCS) projects collectively represent over twelve years of dedicated research to assess environmental impacts and to develop technologies for detection, location, and quantification of potential leakage from offshore geological storage of CO2. Each project used controlled releases in representative environments to test their methods and technologies. QICS as the first of the three projects, focused on the understanding of sensitivities of the UK marine environment to a potential leak from a CO2 storage complex and tested technologies to detect such emissions. The ETI MMV project brought together research and industry partners to develop and sea trial an operational, integrated and cost-effective marine monitoring system for geological CO2 storage. As a commercial project, these results have never been published before and this paper shares for the first-time insights from this work. In February 2020, STEMM-CCS, completed its quest to test techniques for environmental monitoring over a marine CO2 storage site in the UK North Sea, further improved near seabed leakage characterisation capabilities, and delivered a first marine CCS demonstration level ecological baseline. This paper aims to summarise some of the key insights from the three projects and provides references where available for the interested reader. The key finding of all three projects is that the impacts of small to medium CO2 leakages from large-scale storage are limited and localised. Technology capabilities exist for integrated marine CO2 storage monitoring and their performance has been benchmarked at controlled release trials. Even small leakages of 10− 50 L/min can be detected at unknown locations in a large area of interest. Finally, the first important steps towards automated monitoring data analysis have been made, including automated leakage signal detection from Side Scan Sonar data (ETI MMV project) and automated species identification from marine biology images (STEMM-CCS project). Some remaining challenges include missed/ false alerts because of large variations in the background signal, the cost of monitoring large areas over long periods, and making real-time decisions based on big data. Continued work to reduce the cost of marine monitoring technologies and advancing automation of data processing and analysis will be important in order to support safe and efficient offshore CCS deployment at large scale

Monitoring of offshore geological carbon storage integrity: Implications of natural variability in the marine system and the assessment of anomaly detection criteria

The design of efficient monitoring programmes required for the assurance of offshore geological storage requires an understanding of the variability and heterogeneity of marine carbonate chemistry. In the absence of sufficient observational data and for extrapolation both spatially and seasonally, models have a significant role to play. In this study a previously evaluated hydrodynamic-biogeochemical model is used to characterise carbonate chemistry, in particular pH heterogeneity in the vicinity of the sea floor. Using three contrasting regions, the seasonal and short term variability are analysed and criteria that could be considered as indicators of anomalous carbonate chemistry identified. These criteria are then tested by imposing a number of randomised DIC perturbations on the model data, representing a comprehensive range of leakage scenarios. In conclusion optimal criteria and general rules for developing monitoring strategies are identified. Detection criteria will be site specific and vary seasonally and monitoring may be more efficient at periods of low dynamics. Analysis suggests that by using high frequency, sub-hourly monitoring anomalies as small as 0.01 of a pH unit or less may be successfully discriminated from natural variability – thereby allowing detection of small leaks or at distance from a leakage source. Conversely assurance of no leakage would be profound. Detection at deeper sites is likely to be more efficient than at shallow sites where the near bed system is closely coupled to surface processes. Although this study is based on North Sea target sites for geological storage, the model and the general conclusions are relevant to the majority of offshore storage sites lying on the continental shelf

Controls on near-bed oxygen concentration on the Northwest European Continental Shelf under a potential future climate scenario

Dissolved oxygen concentrations in the ocean are declining on a global scale. However, the impact of climate change on oxygen in shelf seas is not well understood. We investigate potential future changes in oxygen on the northwest European continental shelf under a business as usual greenhouse gas emissions scenario (Representative Concentration Pathway RCP8.5). Regions of the European shelf are thermally stratified from spring to autumn, which can cause oxygen depletion in sub-pycnocline waters. A transient climate-forced model simulation is used to study how the temperature, salinity and concentration of near bed dissolved oxygen change over the 21st century. In warming and freshening water, the oxygen concentration declines in all shelf regions. The climate change signal emerges first in salinity, then in temperature and finally in near bed oxygen. Regions that currently experience oxygen depletion (the eastern North Sea, Celtic Sea and Armorican shelf) become larger in the future scenario and oxygen depletion lasts longer. Solubility changes, caused by changes in temperature and salinity, are the dominant cause of reducing near bed oxygen concentrations in many regions. Until about 2040 the impact of solubility dominates over the effects of the evolving ecosystem. However, in the eastern North Sea by 2100, the effect of ecosystem change is generally larger than that of solubility. In the Armorican Shelf and Celtic Sea the ecosystem changes partially mitigate the oxygen decline caused by solubility changes. Over the 21st century the mean near bed oxygen concentration on the European shelf is projected to decrease by 6.3%, of which 73% is due to solubility changes and the remainder to changes in the ecosystem. For monthly minimum oxygen the decline is 7.7% with the solubility component being 50% of the total

Introduction to the STEMM-CCS special issue

This special issue brings together a selection of papers resulting from the STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project. STEMM-CCS was an ambitious, four-year, project on offshore geologic carbon dioxide storage funded under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 654,462). The aim was to deliver new insights, guidelines, and tools for the monitoring of CO2 storage at putative offshore Carbon dioxide Capture and Storage (CCS) sites. CCS is an important potential mitigation strategy to reduce anthropogenic CO2 emissions. Although CO2 leakage from geological reservoirs is considered unlikely, there is a regulatory need and societal expectation to undertake appropriate monitoring to provide assurance that leakage is not occurring. Should leakage be suspected, the capacity to detect, attribute, monitor, and quantify potential CO2 leaks from sub-seafloor CCS reservoirs will be critical. In addition, it is important to predict and understand potential environmental impact from a range of leak scenarios, such that mitigation can be enacted if necessary. Regulatory and legislative bodies need assurance that potential storage leaks can be rapidly detected and quantified. Operators need to be able to detect and quantify, but also must have the ability to attribute leaks to a specific reservoir, potentially within a field of different storage systems and operators. Additionally, quantification techniques will be vital for storage operators if a carbon tax credit system were to be implemented. STEMM-CCS has successfully demonstrated solutions to these issues, though further development will be required to increase regulator and operator confidence. Further, these solutions will also underpin efforts to gain social licence to sequester CO2 in sub-seafloor geologic storage. Prior to any CO2 storage at a site, an environmental assessment needs to be carried out in order to identify any site-specific risks and characterise natural environmental variation sufficiently to allow the efficient detection of environmental anomalies and impacts throughout the lifetime of the storage site. One of the challenges for regulators is to understand what information is required to sufficiently characterise an environment, while for operators, it is obtaining that baseline environmental data cost effectively. STEMM-CCS has not only shown how to establish and interpret baseline data using traditional surveying and sampling but has shown that modelling and other numerical analysis can be used that negate the need for intensive surveying and/or enable targeted surveying, thus balancing the needs of regulators, the public and industry. These approaches can also be utilised for continued monitoring once storage commences

Heterogeneity of impacts of high CO2 on the North Western European Shelf

The increase in atmospheric CO2 is a dual threat to the marine environment: from one side it drives climate change, leading to modifications in water temperature, circulation patterns and stratification intensity; on the other side it causes a decrease in marine pH (ocean acidification, or OA) due to the increase in dissolved CO2. Assessing the combined impact of climate change and OA on marine ecosystems is a challenging task. The response of the ecosystem to a single driver can be highly variable and remains still uncertain; additionally the interaction between these can be either synergistic or antagonistic. In this work we use the coupled oceanographic–ecosystem model POLCOMS-ERSEM driven by climate forcing to study the interaction between climate change and OA. We focus in particular on carbonate chemistry, primary and secondary production. The model has been run in three different configurations in order to assess separately the impacts of climate change on net primary production and of OA on the carbonate chemistry, which have been strongly supported by scientific literature, from the impact of biological feedbacks of OA on the ecosystem, whose uncertainty still has to be well constrained. The global mean of the projected decrease of pH at the end of the century is about 0.27 pH units, but the model shows significant interaction among the drivers and high variability in the temporal and spatial response. As a result of this high variability, critical tipping point can be locally and/or temporally reached: e.g. undersaturation with respect to aragonite is projected to occur in the deeper part of the central North Sea during summer. Impacts of climate change and of OA on primary and secondary production may have similar magnitude, compensating in some area and exacerbating in others

Modelling impacts and recovery in benthic communities exposed to localised high CO2

Regulations pertaining to carbon dioxide capture with offshore storage (CCS) require an understanding of the potential localised environmental impacts and demonstrably suitable monitoring practices. This study uses a marine ecosystem model to examine a comprehensive range of hypothetical CO2 leakage scenarios, quantifying both impact and recovery time within the benthic system. Whilst significant mortalities and long recovery times were projected for the larger and longer term scenarios, shorter-term or low level exposures lead to reduced projected impacts. This suggests that efficient monitoring and leak mitigation strategies, coupled with appropriate selection of storage sites can effectively limit concerns regarding localised environmental impacts from CCS. The feedbacks and interactions between physiological and ecological responses simulated reveal that benthic responses to CO2 leakage could be complex. This type of modelling investigation can aid the understanding of impact potential, the role of benthic community recovery and inform the design of baseline and monitoring surveys

Impact potential of hypersaline brines released into the marine environment for CCS reservoir pressure management

The environmental impact potential arising from the possible disposal of hypersaline brines into the ocean as part of reservoir pressure management for Carbon Capture and Storage is assessed using sophisticated high-resolution hydrodynamic models for the first time, investigating several industry guided scenarios. Although the characteristics of some brines in their undiluted form would have a high environmental impact potential, we find that dispersion in a hydrodynamically active region like the North Sea acts to dilute disposed brine rapidly, even in a worst case approach, such that the potential impact footprint (area exposed to environmentally damaging salinity or temperature) is small, measured in 10’s of meters depending on the release scenario and site specific data such as the hypersaline water contaminants along with in-situ conditions such as currents and mixing. The method of brine disposal has a significant influence on dispersal, such that brines released nearer the sea surface disperse more rapidly, compared with release at the seabed. Hence consideration of brine release height is recommended to further limit impact potentia

Impact and detectability of hypothetical CCS offshore seep scenarios as an aid to storage assurance and risk assessment

Carbon Capture and Storage has the potential to make a significant contribution to the mitigation of climate change, however there is a regulatory and societal obligation to demonstrate storage robustness and minimal local environmental impact. This requires an understanding of environmental impact potential and detectability of a range of hypothetical leak scenarios. In the absence of a significant body of real-world release experiments this study collates the results of 86 modelled scenarios of offshore marine releases derived from five different model systems. This synthesis demonstrates a consistent generalised relationship between leak rate, detectability and impact potential of a wide range of hypothetical releases from CO2 storage, which can be described by a power law. For example a leak of the order of 1 T per day should be detectable at, at least, 60 m distance with an environmental impact restricted to less than a 15 m radius of the release point. Small releases are likely to require bottom mounted (lander) monitoring to ensure detection. In summary this work, when coupled with a quantification of leakage risk can deliver a first order environmental impact assessment as an aid to the consenting process. Further this work demonstrates that non-catastrophic release events can be detected at thresholds well below levels which would undermine storage performance or significantly impact the environment, given an appropriate monitoring strategy