Characterization of major overburden leakage pathways above sub-seafloor CO2 storage reservoirs in the North Sea (CHIMNEY)

Lead Research Organisation: University of Southampton
Department Name: School of Ocean and Earth Science


Industrial emissions of carbon dioxide (CO2), including fossil fuel power generation, are recognised as a likely agent of global climate change and acidification of the oceans, but most economies will remain dependent on these technologies for the next few decades. Carbon dioxide Capture and Storage (CCS) has been identified as an important way of reducing the amount of CO2 added to the atmosphere. CCS is seen as making a key contribution to reducing mankind's greenhouse gas emissions by 80-95% by 2050 and keeping climate change derived temperature increases below 2 degrees C, as outlined in European Commission policy. In addition, CCS is considered an important way of reducing the cost of mitigation measures around the continued use of fossil fuels. Offshore storage of CO2 in depleted oil and gas reservoirs and saline aquifers is the option of choice for most European nations, and there is currently one operational storage complex (Sleipner, Norway), and several other commercial scale demonstration projects are in late stages of development (e.g. ROAD-Netherlands, Peterhead and White Rose-UK), and expected to be in full operation by 2020.
A key element of CCS offshore is that there is confidence that the risks of any leakage are understood. The location and potential intensity of any possible CO2 leakage at the seafloor are critically dependent on the distribution of fluid (dissolved and gaseous CO2) pathways in the rocks overlying the reservoirs in which the CO2 is stored, and on the ability of these pathways to transmit fluid (termed permeability). Recent studies of the structure of marine sedimentary rocks in the North and Norwegian Seas have revealed that near-vertical structures, which resemble chimneys or pipes, cross-cut the sedimentary sequence. These structures may be pathways for fluid flow. Natural fluids from deeper rock layers have migrated through these structures at some point in geological time. If CO2 leaking from sub-seafloor storage reservoirs reaches the base of these structures, and if their permeability is sufficiently high, they could act as CO2 leakage pathways towards the seafloor and overlying water column. To provide a reliable prediction of potential seafloor seep sites, the degree to which these pathways are continuous and especially their permeability needs to be better understood.
In this project (CHIMNEY) we will collect new data over a chimney structure within the North Sea by using a ship to make novel measurements with sound waves. We will use several different marine sound sources to make images of the chimney, using receivers at the sea surface, and also record the sound arrivals on sea bed instruments known as ocean bottom seismometers. By looking at the sound travel paths through the sub-surface from a range of directions and frequencies we will obtain information about fractures/fluid pathways in the chimney as well as the surrounding rocks. We will calibrate and understand our marine seismic results using laboratory studies of materials (synthetic rocks) that mimic the sub-surface rocks. By understanding the propagation of sound through synthetic rocks with known fluid pathways we can understand the results of the marine experiment. We will also drill into the chimney and collect core samples which we will analyse for core geology and fluid chemistry. A computer model of the sub-surface chimney will be constructed combining the results of the seismic experiment, rock physics, and chemistry. We will work with companies involved in CCS to build realistic computer models of fluid flow that tell us about the potential of leakage from chimney structures generally within the North Sea that are relevant to Carbon Dioxide Capture and Storage.

Planned Impact

Impact Summary

a. Who could potentially benefit from the proposed research over different timescales?

Identified user groups for the new knowledge that we will create:

(1) Policymakers and regulators, including Department of Energy & Climate Change and the European Commission
(2) CCS Operators
(3) General public, NGOs and fisheries organisations.

b. How might the potential beneficiaries benefit?

Policymakers and regulators
We will produce 3D models of the overburden and produce realistic leakage scenarios for structures relevant to Carbon Capture and Storage sites in the North Sea. This information is required for compliance with the EU CCS Directive and other international CCS agreements (e.g. London Protocol).

CCS Operators
Operators of CCS systems will benefit from the generation of new methodologies to constrain reservoir overburden CO2 permeability and potential near-surface fluid flow paths for relevant example structures in the North Sea. We will generate new tools to constrain leak risks for future CCS projects, developed through collaboration with stakeholders

General Public, NGOs and fisheries organisations
Acceptance of CCS, especially by local communities, depends on many factors, and confidence in the physical security of stored CO2 is a major issue. In this respect, clear and accessible communications designed for specific audiences are critical in ensuring that accurate information about sub-seabed CO2 storage is available to those who need it.

Specific activities for engagement with these target groups are covered in our Pathways to Impact document.


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Böttner C (2019) Pockmarks in the Witch Ground Basin, Central North Sea in Geochemistry, Geophysics, Geosystems

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Falcon-Suarez I (2021) Core-scale geophysical and hydromechanical analysis of seabed sediments affected by CO2 venting in International Journal of Greenhouse Gas Control

Description Cruise JC152 collected the major data sets for the CHIMNEY (Characterisation of major overburden leakage pathways above sub-seafloor CO2 storage reservoirs in the North Sea) NERC highlights research programme. The cruise investigated the Scanner and Challenger pock marks in the North Sea, which were previously known to be the locations of vigorous Methane venting. The cruise objectives were to collect data that could be used to constrain the geometry and internal structures of fluid flow "Chimney" structures with the eventual aim of determining the current permeability of the sub-surface. The cruise successfully completed two anisotropy experiments over the Scanner and Challenger pock marks by shooting various seismic sources into a grid of 25 and 7 ocean bottom seismometers respectively. Five different seismic sources (Bolt airguns, GI guns, Squid surface sparker, Duraspark surface sparker, and Deep Tow Sparker) were recorded by the ocean bottom seismometers, and an acoustic recorder deployed c. 25 m above the seabed. Multichannel seismic reflection profiles were collected with GI guns and both surface sparker sources, and single channel seismic reflection profiles were collected with the Deep Tow Sparker source. Single and multichannel echo sounder data were collected along all seismic profiles. We observe significant anisotropy associated with the chimney structure and we are determining the physical causes of this by integration of rock physics observations and theory with the marine geophysical observations.
Exploitation Route The key finding describes what happened in data collection.
Sectors Environment

Description We have demonstrated that we can measure the physical properties of a sub-surface fluid flow (chimney) structure by geophysical means. This was revealed at the 4th International Workshop on OffshoreGeologic CO2 Storage in Bergen in February 2020 in front of 150 practitioners, including leading experts from the petroleum industry, and policy advisors.
First Year Of Impact 2020
Sector Energy,Environment
Impact Types Economic,Policy & public services

Title Development of Seismic anisotropy techniques for application to marine fluid flow features 
Description We are developing seismic anisotropy techinques - P wave and S wave for application to fluid flow features in the marine sub-surface. 
Type Of Material Improvements to research infrastructure 
Year Produced 2018 
Provided To Others? No  
Impact Impacts will be reported as they develop 
Title Multibeam bathymetry data from James Cook 152 submitted to British Oceanographic Data Centre 
Description Multibeam bathymetry data has been submitted to BODC. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact The data was handed over to BODC in February 2018 
Description Collaboration with Applied Acoustics Ltd on Sparker systems 
Organisation Applied Acoustic Engineering Ltd
Country United Kingdom 
Sector Private 
PI Contribution We tested the Applied Acoustics Deep Tow Sparker system in the North Sea.
Collaborator Contribution Provision of equipment and technical support. Technical information
Impact The outcomes are currently being incorporated into publications.Technical insights were fed back to the company after the cruise.
Start Year 2017
Description Collaboration with Lawence Berkeley Labs (LBL) on reactive transport modelling 
Organisation Lawrence Berkeley National Laboratory
Country United States 
Sector Public 
PI Contribution Development of TOUGH-REACT applications to Carbon Capture and Storage
Collaborator Contribution Provision and advice on numerical modelling code TOUGH-REACT.
Impact Jon Bull visited LBL in October 2017, and his group is now using the code. We have benefited from interactions with the LBL group in developing how to apply the model to CCS.
Start Year 2017
Description Collaborative meetings with CGG Ltd 
Organisation CGG
Country France 
Sector Private 
PI Contribution We discussed how to determine anisotropy in the sub-surface during the forthcoming Cook Cruise in August-September 2017. CGG provided advice on OBS experiment design and will provide access to software for analysis of the resulting dataset.
Collaborator Contribution During the meeting the following was discussed: Issues regarding survey design: 1. Given limited number of seabed receivers, it might be better to have well-sampled 2D arrays than a 3D array. For example, to sample well a small number of azimuths, use an asterisk geometry, or half an asterisk, or even quarter of an asterisk. 2. Seabed instruments will need to be sited as accurately as possible. Useful to explore what current information might be available in real time (ADCP?). 3. Could we use a vertical hydrophone array to record the far-field source signature? 4. Do we know the sparker waveform and how variable it is from shot to shot? 5. We should avoid active rigs and shipping lanes (sources of noise). 6. We need to select a shot spacing that will limit spatial aliasing. 7. Useful to have frequencies below 10 Hz in the dataset - ideally down to around 2 Hz. 8. Need to make sure there is time between sparker direct arrival and reflections. 9. Possibility of doing modelling associated with survey design at CGG, but we need to decide first what we want to model. Issues around data analysis: 1. Scholte waves important - give good S wave resolution. Surface wave inversion works with receiver gathers. 2. Anisotropic reflection tomography works with common image gathers, so we need the receiver spacing to be close enough and/or the velocity structure to be known well enough that there is not spatial aliasing. Can do with up to orthorhombic symmetry, but normally invert for only a small number of anisotropic parameters. Can certainly do 3D TTI tomography. 3. Useful to know the response of the instruments, in particular if there is a resonance. 4. Can do imaging with multiples. Ways of working: 1. Software use would be via visiting CGG. Visitors should not need much supervision, so postdocs strongly preferred over PhD students. 2. Next interaction may be around modelling survey design, or feedback on our survey design once completed. Can interact by email or short meetings (1-2 hours) by VC.
Impact We have adapted our experimental plans in light of input from CGG Ltd.
Start Year 2016
Description Blog of CHIMNEY cruise JC152 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact We described James Cook 152 activities and motivations in the form of a series of blogs written by participants in the cruise.
Year(s) Of Engagement Activity 2017
Description Oral Presentation at UKCCSRC Meeting in Edinburgh - 14-15September 2016 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Oral presentation on the CHIMNEY project to experts in CCS at UKCCSRC Biennial meeting.
Year(s) Of Engagement Activity 2016
Description Presentation at Greenhouse Gas conference - GHGT-14 in October 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact CHIMNEY NERC grant work was presented together with the EU project STEMM-CCS at the premier Greenhouse Gas Control conference - GHGT-14.
Year(s) Of Engagement Activity 2018
Description UKCCSRC posters 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Policymakers/politicians
Results and Impact Poster presentations on CHIMNEY at UKCCSRC meeting in Cambridge
Year(s) Of Engagement Activity 2018