Written by Thomas Wild, PhD Student in Geology, University of Aberdeen – I would like to thank the UKCCSRC for providing the funds enabling me to attend this conference, and look forward to working closely with them in the future
This is a summary report of the Storage Technical Session on Monitoring & Verfication held at the UKCCSRC Biannual meeting – CCS in the Bigger Picture – in Cambridge, 2-3 April 2014
Pore-scale imaging of capillary trapping and reactive transport in sandstone and carbonate rocks – Dr Branko Bijeljic – Imperial College London
This study comes off the back of recent advances in x-ray imaging techniques which allows the imaging of solids and fluids as the voxelised representation of pore space. This study looked at categorizing heterogeneity of flow and transport reactions.
He observed very different reactions and flow properties within different reservoir lithologies. Flow velocities varied greatly due to the heterogeneity of the media, which included sandstones and carbonates. This direct imaging highlighted changes in porosity, permeability and velocity induced by reaction and demonstrates a crucial tool for all future site characterisations.
Through use of micro-CT imaging of reservoir lithologies he concluded that significant volumes of CO2 can be trapped through capillary action, and that this method of reservoir imaging will be extremely beneficial for design and monitoring of storage sites. It did however highlight the variability caused by heterogeneity that will need to be taken into consideration with all site selection.
This project aims to assess the impacts of CO2 leaking, such as the mobilization of heavy metals or pollutants resulting from a drop in Ph. To do this Giorgio has built a rig to simulate a CO2 leak through the seafloor and into the water column. This lab study has been used in conjunction with study through scuba diving and use of unmanned subsea vehicles at natural CO2 seeps from field areas in Kagoshima Bat, Japan and Panarea, Italy.
He observed a fast local response to a sudden gas leak at the sea floor, with a sharp drop in Ph, and then a gradual recovery as the area went back towards equilibrium. He did also record an increase in alkalinity at times to CaCO3 dissolution/enhanced weathering of the sediment. The structure of the sedimentary layers controls the diffusion of the gas and the behaviour of the emitted bubbles. This again highlights the need for detailed analysis of the composition and structure of the subsurface at all proposed sites.
One very interesting point to note was that the gas emitted from the shallow-water submarine volcanic vents contained up to 98% CO2 at origin, however the chemistry changed drastically as the bubbles moved up through the water column and the gas diffuses. This result has major implications for monitoring and detection of sub-sea leaks as CO2, as the gas may not be detectable at the surface, and highlights the need for the development of reliable detection technologies.
In 2011 accusations of CO2 leakage were made at the site of an EOR operation at the Weyburn operation in Canada. This presentation covered the results of a subsequent investigation by the BGS, in conjunction with continual site monitoring since 2001. Significant seasonal variability of measured CO2 was found, which was seen to be well within the range of values measured at surrounding properties and control sites. Subsequent isotope studies of the CO2 determined that the chemical signature was consistent with a shallow biogenic origin and showed no evidence of CO2 leakage.
This presentation highlighted the importance of establishing a baseline survey at any site selected for CO2 storage, over a long enough timescale, so that natural variability can be accounted for. The importance of effective communication of results was also seen to be crucial, as the picture of a dead rabbit next to a pond, is much more likely to stick in the public’s mind, than a graph showing the results of carbon isotope analysis. Helping the public to understand the risks and uncertainties involved, but also the rigorous site analysis and monitoring that goes on is crucial if this technology is to gain wide public acceptance.
Observations of multiphase flow, trapping and reactive transport for the CO2-brine system at reservoir conditions: Implications for capacity estimations and monitoring in the UK – Dr Sam Krevor, Imperial College London
Study of multi-phase flow requires analysis of a samples capillary pressure, relative permeability and capacity for residual trapping. He stressed the importance of careful experiment design for the measurement of CO2-brine flow properties.
His main takeaway was that for the most part, the multiphase flow properties were easy to interpret within the existing Darcy-an’ framework of multiphase flow theory. He did note however that high CO2 density led to some variations in relative permeability which has yet to be well constrained.
An important point made was that the volume of residual trapping in not dependant on the effectiveness of the seal rock, but on the volume of CO2 that enters the pore spaces. He concluded that residual trapping is likely to be a first order affect, with 10-30% of the pore space saturated, and is likely to be an important trapping mechanism.
Potential environmental impacts of CO2 leakage: results from the ASGARD facility – Prof Michael Steven, University of Nottingham
Michael opened with the theory of Murphy’s law. Gas leakage from a site is unlikely, however if something can go wrong then it will, and this was the premise for his study looking at the effect of CO2 on plant growth and development at ASGARD (Artificial soil gassing and response detection.)
He cultivated a number of different plants, to which CO2 was then delivered to the system through injected into the soil, simulating a gas leak. He recorded that there was visual plant stress within 7-14 days. An important result of this study was that a relatively small change of just 5-10% CO2 was enough to have a significant impact upon plant survivability, with an increase of 20% CO2 being enough to kill 50% of seedlings planted.
Future work will look at developing a ‘stress index’ which will be able to categorically say if deterioration was caused by CO2 or something else. He is also hoping to look at the potential for using plant responses for remote sensing of gas leaks. So he asks the question: ‘Can plant stress response be measured and used to locate gas leaks?’ This has the potential to be used as a first order site monitoring/assessment technique and would help pin down the exact location of leaks in a field area.