Fibre optic applications for dissolved carbon dioxide monitoring of marine geologic sequestration sites.
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Abstract Marine-based Carbon Capture, Utilisation, and Storage (CCUS) has the potential to reduce the rate at which CO2 is added to the atmosphere thereby contributing to the mitigation potential for anthropogenic climate change. There has, however, been public and regulatory resistance to the technology due to the the possibility of leakage, which could occur within a 5 km radius of the injection site. The risks of marine CCUS could be better characterised using distributed sensor networks to monitor possible leakage areas. Laboratories at the Universities of Toronto and Victoria demonstrated that Long Period Grating Fibre Optic Sensors (LPGs) can detect CO2 in laboratory conditions. These sensors have the potential for cost-eﬀective, distributed seaﬂoor deployment in sensor networks, but certain in situ deployment challenges must be overcome to move beyond the laboratory testing stage. In situ, LPGs will be susceptible to interference from non-target species and to biofouling. As such, sensor signal may not be uniquely responsive to CO2, and other substances or factors could contribute to noise, or could even mimic a seep or leak. Furthermore, sediment dynamic and pore-water water column exchange of total alkalinity and DIC may inﬂuence both background variability and signal in the marine context. The key research contributions of this thesis are as follows: (1) a characterisation of the eﬀect of biofouling on the response of LPGs for both single and multi-species bioﬁlms,(2) a deﬁned set of concentrations of carbonate and non-carbonate species in application environments of interest and an estimate of the likelihood of interference with LPG signal as sensitivity improves, (3) a characterization of the impact in back-ground variability in the key parameters of the carbonate system, (4) a discussion of the signal-to-noise ratio (SNR) for LPGs used in monitoring seaﬂoor geologic seques-tration sites, and (5) an evaluation of the technology readiness level of LPGs. Both laboratory experiments and ﬁeld studies were conducted to ascertain the eﬀect of bioﬁlm growth on sensor response. These studies showed that biofouling would likely contribute increase the central wavelength of the attenuation band within the ﬁrst 30 h of LPG immersion in seawater. Hydrochemical modelling work using Phreeqc 2.18 revealed that carbonate species and non-carbonate species were likely to be present in concentrations below the detection limits of oﬀ-the-shelf LPGs. Finally, the contribu-tion of these hydrochemical and biological variables to noise was calculated, and the type of signal escaping CO2 would generate was used to estimate SNR under diﬀerent seepage and leakage scenarios. The estimated SNRs showed signiﬁcant variability for seeps and leaks of the same scale under various pressure and temperature conditions. More work is needed to test LPGs under temperature and pressure conditions likely to be experienced in seaﬂoor environments, and also in settings where hydrate coat-ings may form ﬁlms that prevent the LPG from contacting the ﬂuid to be measured.