Work Package 5

Monitoring leakage

Schematic of an active acoustic monitoring system for tracking bubble release Sensor equipment to be deployed on the sea bed A multi-parameter sea bed 'lander' prior to deployment

Overall aim

To evaluate techniques and protocols for monitoring a geological CO2 storage site.

Significant research and monitoring experience has been gained from existing and proposed sub-seafloor storage sites. There is a need now to expand our storage monitoring and improve how it is done.

Similarly, modelling of a CO2 leak into the marine environment has largely focused around "catastrophic" release of relatively large amounts of CO2, but we need also to look at how a more prolonged slow release leakage would affect the marine environment. Early research has shown it to be just as significant as a larger scale release.

The ultimate goal is to develop cost efficient but effective monitoring protocols that will detect early signs of a leak or assure the public that no leakage is occurring.

Task 5.1: Below surface and seafloor monitoring (National Oceanography Centre, Heriot-Watt University).
Seismic reflection surveying is a sophisticated technique routinely used for oil and gas exploration. It has been successfully used at an existing CO2 storage site to provide accurate images of the injected CO2. This technique has not yet been applied to shallow, CO2-rich sediments. This task presents an excellent opportunity to assess seismic imaging techniques as a method of "monitoring field testing" with shallow, CO2-rich sediments where the emission flow rate is a known, but relatively low, volume. High-resolution CHIRP imaging is being used to characterize the geophysical response of CO2-charged sediments and the development of micro-fractures as pathways for CO2 ascent within unconsolidated sediments. International collaborative partners are also using controlled source electromagnetic arrays to geophysically image sub-surface fluids.

Similarly, a variety of parameters including pH, dissolved oxygen and CO2, and sediment temperature, are being used to assess changes within sediment cores from the release site in the terms of sensitivity and time-lag of response after CO2 release. At the seafloor a wide range of seafloor instruments are being deployed including acoustic hydrophone recording (Figure 1), seafloor photography, chemical sensors (Figure 2) to record the processes of seafloor CO2 emission at the release site.

Task 5.2: Water monitoring (National Oceanography Centre, Scottish Association for Marine Sciences).
Water monitoring technology and survey techniques (Figure 3) of proposed and operational CO2 storage sites will have to address issues of routine baseline monitoring and specific monitoring of known leakage sites, if they occur.

Such an approach will require different survey methods nested at different scales. For site specific monitoring as at the QICS site, sensors (for chemical analysis) were installed in the water, samples were taken and instruments were deployed to measure the dispersion of released CO2 during typical tidal cycles. The sensitivity and time-lag response of these different sensors around the release site will be also assessed.

We have completed most of the sampling of the sea-water in the QICS study area and at a nearby site beyond the effect of the released CO2. We used a combination of instruments fixed on the seabed, such as our current meters, discrete sea-water samples collected using collection bottles and sensors mounted on battery-powered miniature submarines. To date we have collected over 120 samples for analysis of dissolved oxygen, nutrients (nitrate, nitrite, silicate, phosphate and ammonia), dissolved methane and carbon dioxide, trace metals, dissolved inorganic carbon and total alkalinity. These samples were collected during 6 sample campaigns and cover 5 different water depths at 3 or more stations within the test area. In addition to the collection and analysis of these samples we deployed two recording water current meters, each with a suite of sensors for temperature, oxygen and conductivity at the entrance to Ardmucknish Bay and in the area of the release. The analysis of all of this data is ongoing and the results will be published.

Task 5.3: Biological indicators (Plymouth Marine Laboratory, University of Southampton).
In year 3 of the project a workshop will be run to combine the biological data generated in the assessment of ecosystem impacts (Work Package 4) and assess the potential for biological responses to detect and monitor leakage from CCS sites.

A report will be generated containing recommendations for the use of biological indicators in future CCS monitoring programmes.