Quantification of a subsea CO₂ release with lab-on-chip sensors measuring benthic gradients

Abstract

We present a novel approach to detecting and quantifying a subsea release of CO₂ from within North Sea sediments, which mimicked a leak from a subsea CO₂ reservoir. Autonomous lab-on-chip sensors performed in situ measurements of pH at two heights above the seafloor. During the 11 day experiment the rate of CO₂ release was gradually increased. Whenever the currents carried the CO₂-enriched water towards the sensors, the sensors measured a decrease in pH, with a strong vertical gradient within a metre of the seafloor. At the highest release rate, a decrease of over 0.6 pH units was observed 17 cm above the seafloor compared to background measurements. The sensor data was combined with hydrodynamic measurements to quantify the amount of CO2 escaping the sediments using an advective mass transport model. On average, we directly detected 43 ± 8% of the released CO₂ in the water column. Accounting for the incomplete carbonate equilibration process increases this estimate to up to 61 ± 10%. This technique can provide long-term in situ monitoring of offshore CO₂ reservoirs and hence provides a tool to support climate change mitigation activities. It could also be applied to characterising plumes and quantifying other natural or anthropogenic fluxes of dissolved solutes.

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ORCID for Matthew Mowlem: orcid.org/0000-0001-7613-6121

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