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Title: CO2 sequestration : the risk of leakage
Author: Davies, John Bleddyn
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2013
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The Utsira Sand is a major North Sea Saline aquifer. It is currently being utilised as a CO2 sequestration reservoir for the Sleipner project, and its large estimated storage capacity and proximity to suitable infrastructure ensures that it is an attractive prospect for further storage projects. As a forerunning CCS project it is the subject for scrutiny from environmental, industrial and governmental regulatory bodies, with the key concern being the confinement of the CO2 within the reservoir. Regulations are in place to ensure that operators monitor closely the migration of the CO2 plume within storage reservoirs and that they can demonstrate that the plume is behaving as modelled. Any unexpected behaviour of the plume requires escalation of the monitoring program and leakage would involve financial penalties in addition to remediation costs. Diligent site selection and a complete and robust understanding of the reservoir, adjacent units, and the overburden is therefore critical pre-injection. Geological features which have become apparent following CO2 injection into the Utsira Sand would suggest that the injection site was not completely understood before injection commenced. This study has contributed to a better understanding of the storage reservoir, specifically in regards to the deformed base of the Utsira Sand, which has been shown to be the result of sand remobilisation from depth in the form of sand intrusions. These intrusions, hosted within low permeability shales, represent permeable migration pathways from the deeper subsurface to the base of the Utsira Sand. Furthermore, their post-emplacement compaction has facilitated subsidence of the overlying strata. This movement, shown to affect strata up to the top of the Utsira Sand, has formed a ring of faults which projects obliquely upwards from the base of the Utsira Sand to the top of the storage reservoir. Amplitude anomalies within the overburden suggest that these zones of increased permeability have been exploited as paleo-gas migration pathways. Other amplitude anomalies within the succession overlying the Utsira Sand are also recognised. These include: high amplitude anomalies interpreted as gas accumulations within the Lower Seal, vertically focused ‘wipe out zones’ and zones of disrupted reflections interpreted as gas chimneys. The gas chimneys in particular, shown to extend to the seafloor, pose a significant leakage risk to sequestered CO2. On the basis of these observations, a plumbing system, from the Mid-Miocene to the seafloor, is presented. Potential CO2 leakage scenarios are proposed and discussed in respect to the storage ‘site’ and storage ‘complex’. It is suggested that potential future operators within this area should be aware of the features observed and appreciate the associated risk to stored CO2. It is also concluded that the viability of carbon sequestration technology as a climate change mitigation option requires that it represents an attractive proposition for those that operate the technology. Therefore it is considered that the regulation of carbon sequestration requires a finely measured balance between careful regulation and appropriate penalties for poor practice and flexibility in the interpretation of a ‘storage complex’ and ‘leakage.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QE Geology