Review of Offshore Monitoring for CCS Projects

Publication Overview

This report reviews offshore monitoring practice for CO₂ storage projects in terms of tool capabilities, logistical practicalities and costs. The focus is on large-scale ‘commercial’ storage monitoring and draws together published experience from existing large offshore CO₂ storage sites as well as monitoring research at experimental test sites and in areas of natural CO₂ seepages. The strengths and limitations of monitoring techniques, strategies and methodologies are discussed, and relevant experience from onshore sites are also included. Monitoring over the full life-cycle from pre-injection (baseline) through injection and post-injection phases to transfer of responsibility to the competent authority is considered. The review draws on selected examples of current or planned monitoring practice.

Publication 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 Sleipner, Snøhvit and also at the pilot-scale K12-B project in the offshore Netherlands, and conclusions regarding the efficacy of key technologies are starting to emerge. 3D seismic surveys have been highly effective for tracking CO₂ plume development in Sleipner and Snøhvit reservoirs. Measurement of downhole pressure was crucial in establishing non-conformance at Snøhvit. A combination of 3D seismic and downhole pressure / temperature monitoring at Snøhvit has demonstrated the benefit of complementary techniques.
• 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..
• Developments in geophysical techniques, such as the P-Cable seismic system for higher resolution 3D data collection in the overburden, have been demonstrated successfully and effective integration of these shallow subsurface technologies with the seabed monitoring data can help to understand shallow migration processes.
• Controlled release sites such as QICS 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.
• Monitoring strategies need to be devised to cover large areas, typically tens to hundreds of km2 and also achieve accurate measurement and characterisation possibly over lengthy periods. Limited spatial coverage could lead to the risk that anomalies remain undetected or are only detected after a lengthy period of time. Ameliorative measures might then be harder to implement.
• 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.
• Assessment of the results from both the operational (predominantly deep-focussed) and research (predominantly 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.