Work Package 2

Estimation of impact and modelling of a potential CO2 leak

The European Regional Sea Ecosystem Model Phase diagram from CO2 in seawater The carbonate system of seawater

Overall aim

The objective of this work package is to develop and integrate existing models (physical, biogeochemical and ecological) of the shallow marine sediments and seawater (Figures 1 to 3). This will give us the ability to predict the impact of a CO2 leak in a variety of situations.

We have reconstructed existing and newly developed models to inform the design of the controlled release to minimize the amount of CO2 we need to release and to ensure that impacts are controlled so the gas would reach the sea bed during the controlled release but not fracture the sediments as it passes through. These models were developed from information obtained from the project and also results from tests gathered in other research to predict a range of hypothetical leak scenarios, with acceptable accuracy.

Further calculations, using measurements made from samples of the sea floor sediments, indicated that CO2 released at a depth of 10 metres below the sea bed would reach the sea floor within 10 days of the start of the release. This would be achieved with a release rate of 90 kilogrammes of CO2 per day. Bubbles of gas at the sea floor were estimated to be 5 to 25 millimetres across (less than an inch). The bubbles were calculated to dissolve and become smaller as they rise through the sea water.

The predicted change in pH of the sea water (measure of acidity/alkalinity) was also calculated.

The tasks for Work Package 2 are as follows:

Task 2.1 Experimental and theoretical study of CO2 behaviour in seawater (Heriot-Watt University).
The flow of CO2 droplets and bubbles has been investigated through field experiments in collaboration with Dr. Peter Brewer from the Monterey Bay Aquarium Research Institute (MBARI). These experiments will determine the resistance of CO2 bubbles in water (also known as the drag coefficient (Cd)) and their rate of movement (also known as the mass transfer rate (Ke)). This will be carried out at a depth of 50 to 450 m which represents conditions found in UK seas. The data have been used to reconstruct the existing models and develop new models as necessary. This task is in collaboration between UK & USA researchers. The results from the QICS research on the rise and size of released bubbles have been combined with results from the Monterey Bay Aquarium research.

Analysis of the QICS field data is in progress. Observations of bubble size and bubble velocity were captured by video survey (Figure 4). Individual rising bubbles were measured against a ruler for size and velocity.

Task 2.2 Development of a small-scale model to predict CO2 dispersion in CO2 enriched seawater (Heriot-Watt University, National Oceanography Centre Liverpool, Plymouth Marine Laboratory)
The two-phase plume model has been designed and developed to predict the CO2 changes and distribution from a variety of CO2 leakage rates and CO2 bubble/droplet sizes. The outputs from this small-scale model (a few kilometres in size and a few days in time) served as the inputs to larger scale models. The models developed were used to predict the dispersion of CO2-enriched seawater and provide information on the changes that the CO2 undergoes (amount over time). This is the main input to the biogeochemical and ecosystem impact models (Task 2.4).

Task 2.3 Modelling of CO2 dispersion in sediments. (Heriot-Watt University).
CO2 dispersion (with and without water formation) in sediments has been investigated by numerical models developed using the Lattice Boltzmann Method (LBM). The dispersion of the CO2 along channels within the sediment has also been calculated. This model was used together with calculations of small-scale flows of CO2 in the marine sediment. The process of CO2 and water interactions was modelled at a small scale (mm to cm) to represent the pore size of the sediment and special attention has been paid to how CO2 dissolves within sediments. The results from this pore-scale model were then used to populate models on a much larger scale. This model has been used with the model from Task 2.2 to simulate the entire dynamics of CO2 changes through shallow sediments into seawater by simulation of the QICS released CO2 plume linked through to the initial bubble size model.

Task 2.4 Biochemical & ecological models of impacts from increased CO2. (Plymouth Marine Laboratory, National Oceanography Centre Southampton, Scottish Association for Marine Sciences)
The goal here is to develop the existing models of seafloor and sea water processes (ecological and physiological) based on the results from Work Packages 3 and 4. For example we will incorporate the understanding of how nutrient cycling and mixing of sediments vary with CO2 and/ or pH and the timescale of exposure. The model will indicate the degree of impact relative to exposure, which will feed into Task 6.4 (impact indicators).

Task 2.5 Integration of small scale and regional "fluid flow" models (National Oceanography Centre Liverpool, Heriot-Watt University)
Studies have shown the importance of the movement of the sea in the spread of leaked CO2 , especially in strongly tidal regions like the North Sea. The small scale dispersion models developed in the previous tasks will be input into a regional scale "fluid flow" model of the whole of the North West European Shelf (including all on-shelf UK waters) (Figure 5). The model will be used to investigate the dispersion of CO2 from a range of possible storage sites and also at the experimental site in a sea loch, under a variety of tidal, seasonal and weather conditions on time scales of 1 day to 1 year.

Both the regional- and small-scale models developed here will be used jointly to generate a number of differing leak scenarios under a number of different conditions.