Background to the Study

Many regions of the world with offshore continental shelf areas offer significant potential for large-scale CO₂ storage.  These regions often have depleted oil and gas fields, deep saline aquifers or other permeable formations which are suitable for injected CO₂.  The gas must be safely stored and retained within predefined reservoirs but should any seepages occur they need to be detected so that mitigation measures can be implemented.  It is also important to be able to verify the pattern of CO₂ migration within reservoirs by comparison with predictive models.  IEAGHG contracted a consortium of leading UK research institutes, led by the British Geological Survey, to review monitoring techniques currently available for CO₂ geological storage.  The National Oceanography Centre, Plymouth Marine Laboratory, and the University of Southampton added their expertise to the study.

Summary

A range of monitoring techniques are available for CO₂ geological storage offshore, both deep-focussed (providing surveillance of the reservoir and deeper overburden) and shallow-focussed (providing surveillance of the near seabed, seabed and water-column).  Deep-focussed operational monitoring systems have been deployed for a number of years at the offshore Norwegian storage sites Sleipner and Snøhvit.  The efficacy of key technologies are starting to emerge.  Research based on 3D seismic surveys have been highly effective for tracking CO₂ plume development in Sleipner and Snøhvit reservoirs.  In the Snøhvit field a combination of 3D seismic and downhole pressure / temperature measurments at Snøhvit has demonstrated the benefit of complementary techniques.  This monitoring programme led to a switch in the injection strategy into an alternative reservoir.  Assessment of the results from both deep-focussed and shallow-focussed monitoring activities from Sleipner and Snøhvit indicates that many elements of the European storage requirements have been met at these large-scale sites which were both initiated before the CCS Directive was introduced.

Shallow-focussed monitoring systems are being developed and demonstrated.  New marine sensor and existing underwater platform technology such as Automated Underwater Vehicles (AUVs) and mini-Remotely Operated Vehicles (Mini-ROVs) enable deployment and observation over large areas at potentially relatively low cost.  Seafloor and ocean monitoring technologies can detect both dissolved phase CO₂ and precursor fluids (using chemical analysis) and gas phase CO₂. 

Controlled release experimental sites such as QICS (Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage) have proved to be useful test-beds for shallow seismic techniques and acoustic detection systems.  They can also reveal how CO₂ migrates through, and is partially retained by, unconsolidated sediments.

Developments in geophysical techniques, such as the P-Cable seismic system, have generated higher resolution 3D images of the overburden.  Successful integration of these shallow subsurface technologies, with the seabed monitoring data, can help to build a better understanding of shallow migration processes.  Search areas could be narrowed down by the integration of information from deeper-focussed monitoring such as 3D seismics, which can identify migration pathways, with shallow surface monitoring such as acoustic detection.

There has been significant progress in demonstrating monitoring of offshore CO₂ geological storage sites, and this report compiles and reviews the developments to-date.