Investigating the role of chemical and geochemical tracers for CO2 transport and storage
Changes in the atmospheric concentration of greenhouse gases and aerosols alter the energy balance of the climate system. CO2 is the most significant anthropogenic greenhouse gas. The primary source of the increased atmospheric concentration of CO2 since the preindustrial period is from fossil fuel exploitation. As the global need for energy is currently met by combustion of fossil fuels it is imperative that a method of reducing the levels of CO2 being emitted is used. Carbon capture and storage (CCS) is the combination of CO2 capture from large point sources, with the transport of CO2 to a suitable geological storage site where it can safely be contained. Geological CCS technology has the potential to a make a significant contribution to a low carbon technology future. As with any technology, it is imperative to identify techniques that could be used to form part of the monitoring programme. In this thesis, the role of chemical and geochemical tracers are investigated during the transport and storage of CO2. For the first part of this research, a review of the natural gas and CO2 pipeline network in North America and United Kingdom has been compiled from published literature and historical experience. Using this information, research was carried out to determine why odourising has been suggested for CO2 pipeline transport and what benefit it would add. Based on experience from natural gas, it is concluded that high pressure pipelines of CO2 through sparsely populated areas could have odourant added, but will gain little safety benefit. However, adding odourant to CO2 gas phase pipes could aid detection of leaks as well improve public assurance and should be considered in more detail. For the second part of this research, a specially constructed flow cell was designed and built to investigate how noble gases could be used as effective early warning tracers for CO2 migration in storage sites. From this equipment, experimental breakthrough curves for noble gases and SF6 travelling through a sample of Fell sandstone in relation to CO2 over a pressure gradient range of 10,000 – 50,000 Pa were generated. Although noble gases are described as conservative tracers, comparing the breakthrough curves over a range of pressure gradients show that they do not behave as simply as previously assumed. These results were then modelled using a one dimensional advective dispersion transport equation to fit curves to the experimental outputs using two different modelling approaches. A statistical approach can derive the input parameters for an analytical approach, which is needed to understand the dispersivity behaviour of the tracers. A set of values for the dispersivity of noble gases, SF6 and CO2 through porous media is presented in this research. Using a baseline value approach, initial arrival times for krypton and xenon from this research suggest that they could be used as a means of detecting CO2 migration. While helium, neon and argon appear to be unsuitable as early warning tracers for initial detection of CO2, this suggests that they can be used as part of mixture to fingerprint individual CO2 storage sites that may be in close proximity to one another. Results from the experimental and modelling analysis, identify a system where preferential paths exist depending on the change in pressure gradient. The different transport channels progress from a Darcy linear flow regime to a non-linear laminar flow. These results propose an explanation for the patterns observed from tracers in large-scale reservoirs but the output values obtained are limited by scale-dependence and would not be suitable for direct upscaling.