Characterization of major overburden leakage pathways above sub-seafloor CO2 storage reservoirs in the North Sea (CHIMNEY)
The NERC-funded Characterisation of Major Overburden Leakage Pathways above Sub-sea floor CO 2 Storage Reservoirs in the North Sea (CHIMNEY) project is investigating the risks of leaks from storing carbon dioxide (CO 2 ) under the seabed. Academics from the University of Southampton, the University of Edinburgh, and the National Oceanography Centre (NOC) are working together to understand more about the hazards involved in the storage of CO 2 in depleted oil and gas reservoirs and saline aquifers in the North Sea. The project has been designed to complement the 2017 Strategies for Environmental Monitoring of Marine Carbon Capture and Storage (STEMM-CCS) experiment.
Background
Industrial emissions of carbon dioxide (CO 2 ), 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 CO 2 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 Celsius, 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 CO 2 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 CO 2 leakage at the seafloor are critically dependent on the distribution of fluid (dissolved and gaseous CO 2 ) pathways in the rocks overlying the reservoirs in which the CO 2 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 CO 2 leaking from sub-seafloor storage reservoirs reaches the base of these structures, and if their permeability is sufficiently high, they could act as CO 2 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.
Further details are available on the CHIMNEY project page .
Participants
The following institutions are involved in the CHIMNEY project:
- National Oceanography Centre, Southampton (NOCS) - Project Principal Investigator: Professor Jonathan Bull
- University of Edinburgh
- GEOMAR - Helmholtz Centre for Ocean Research, Kiel
- Lawrence Berkeley National Laboratory
- CGG
- Applied Acoustics
Research details
Four Work Packages have been funded and are described in brief below:
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Work Package 1: Novel broad-band seismic anisotropy experiment over a North Sea chimney in order to understand its structure and origin, derive fracture geometry and topology, and ascertain if fractures are open or closed. Additional high-frequency seismic reflection imaging. 10 days of ship-time funded to supplement STEMM-CCS experiment. BODC is only likely to receive data from this work package.
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Work Package 2: Rock physics experiments on synthetic samples to constrain chimney and surrounding rock permeability.
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Work Package 3: Geochemical characterisation of chimney material and pore fluids. Core samples from the North Sea chimney will be used to assess hydraulic connectivity, water-rock reactions that affect permeability and the longevity of fluid flow, and to constrain reactive transport modelling.
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Work Package 4: Flow and reactive transport modelling to integrate outcomes of the three other WPs and do scenario modelling.
Fieldwork and data collection
Data will be collected over a chimney structure within the North Sea using a ship to make new and unusual measurements with sound waves. Several different marine sound sources will be used 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, information will be obtained about fractures and fluid pathways in the chimney as well as the surrounding rocks. The results will be calibrated and analysed via 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 the results of the marine experiment can be understood. A computer model of the sub-surface chimney will also be constructed combining the results of the seismic experiment, rock physics, and chemistry.
