Metal mobility in sandstones and the potential environmental impacts of offshore geological CO2 storage
Carruthers2016 Batch Experiment data.xlsx (69.78Kb)
Carruthers2016 SEP data.xlsx (42.50Kb)
Carruthers, Christopher Ian Andrew
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Geological carbon dioxide (CO2) storage in the United Kingdom (UK) will likely be entirely offshore, which may lead to the production and disposal into the sea of reservoir waters to increase storage capacity, or through CO2-Enhanced Oil Recovery (CO2-EOR). These produced waters have the potential to contain significant concentrations of trace metals that could be of harm to the environment. Batch experiments with CO2, warm brines, and reservoir sandstones were undertaken for this thesis to determine concentrations of 8 trace metals (arsenic, cadmium, chromium, copper, mercury, nickel, lead, zinc) which could be leached during CO2 storage in 4 UK North Sea hydrocarbon reservoirs. A sequential extraction procedure (SEP) was also used to determine the potential mobility of these metals under CO2 storage from mineral phases making up the reservoir samples. The results broadly showed that mobilised trace metal concentrations were low (parts per billion, ppb) in the batch experiments, with the exceptions of nickel and zinc. These metals were associated with carbonate and some feldspar dissolution, with other metals apparently desorbed from mineral surfaces, probably clays. The results of the SEP, however, were a poor predictor of actual mobility with respect to the batch experiments, although useful in determining the distribution of trace metals within the defined mineral phases (water soluble, ion exchangeable, carbonate, oxide, sulphide, silicate). In addition, fieldwork was carried out at Green River, Utah, to collect 10 CO2-driven spring water samples and 5 local aquifer rock samples. This area was used as a natural analogue for CO2-mobilised trace metals from sandstone aquifers. Trace metal concentrations in spring waters were very low (ppb) and batch experiments using Utah rock samples, spring water collected from Crystal Geyser, and CO2 confirmed very low mobility of these metals. The SEP was repeated for the Utah reservoir rocks, but again was not a reliable predictor for actual mobility, other than to confirm that overall bulk concentrations of trace metals was low. Comparison of trace metal concentrations from the batch experiments with data from UK North Sea oil and gas produced waters shows that overall, concentrations mobilised in batch experiments are within the range of concentrations across all North Sea fields reporting their data. However, on a field-by-field basis, some CO2 mobilised concentrations exceeded those currently produced by oil and gas activities. Furthermore, average batch experiment trace metal loads are higher than average oil and gas produced waters, and in some cases exceed international guidelines. Therefore, while the majority of trace metals have low mobility and therefore low environmental impact, this should be assessed on a case-by-case basis. Regular monitoring of dissolved constituents in produced waters carried should also be carried out, particularly in the initial stages of CO2 storage operations, with remedial action taken as required to reduce the environmental impact of offshore carbon capture and storage.