| Background to the study
Since the inception of CO2 injection into the Sleipner gas field in 1996 there has been considerable progress in monitoring offshore geological storage sites. There have also been recent developments, in-situ experiments, large-scale tests, and reviews on monitoring techniques for offshore monitoring applications. Some of these developments have occurred outside of the CCS sector. This is in addition to the deep monitoring for Statoil’s Sleipner project in the North Sea and Snøhvit project in the Barents Sea.
In addition to technology developments there has been a corresponding series of regulations and related objectives which are designed to ensure that CO2 storage in offshore reservoirs can be retained within secure repositories without detrimental environmental effects. As with onshore CO2 geological storage, the objectives for offshore monitoring include: CO2 geological storage performance, baseline studies, leakage detection, and flux emission quantification. There are advantages and disadvantages of offshore monitoring compared to onshore. There is better and more consistent seismic coupling to the geology because of the water contact, there are less access issues in terms of landowners and infrastructure. In addition, emissions at the seabed can be both ‘seen’ and ‘heard’ as bubble streams. On the other hand, there are the challenges of working in a more remote and hostile marine environment.
Sub-seabed geologic storage sites will have large spatial seafloor extent and large overlying ocean volumes (with potentially dispersed and localised emission sources) which provides a monitoring challenge. One requirement of any offshore leakage monitoring strategy development is to ensure wide area monitoring combined with sensitive detection thresholds. Potential CO2 leakage may have precursor fluid release of chemically-reducing sediment pore fluids and aquifer brines (each of which has a unique chemical signature). New marine sensor and existing underwater platform technology such as Automated Underwater Vehicles (AUVs) and mini-Remotely Operated Vehicles (Mini-ROVs) and seabed landers are under development to 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 and seabed. Such chemical and physical monitoring systems may also provide tractable and robust methods for quantifying leakage loss beyond just detection.
Developments in geophysical techniques, such as the P-Cable seismic system for higher resolution 3D data collection in the overburden, have been demonstrated successfully in the last few years and effective integration of these shallow subsurface technologies with the seabed monitoring data can help to understand shallow migration processes.
Deep-focussed monitoring of reservoir and overburden has proved successful offshore, notably at Sleipner and Snøhvit. This study has reviewed and assessed the performance of these monitoring technologies and methodologies tools, and how new or modified tools might contribute to monitoring capability.
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