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

Allison Schaap,Dirk Koopmans,Moritz Holtappels,Marius Dewar,Martin Arundell,Stathys Papadimitriou,Rudolf Hanz,Samuel Monk,Matthew Mowlem,Socratis Loucaides

doi:10.1016/j.ijggc.2021.103427

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.

Highlights

Atmospheric carbon dioxide concentration has risen to >407 parts per million, a 47% increase over the concentration at the beginning of the industrial era (Friedlingstein et al, 2019)
The field site was located at a depleted oil and gas reservoir, the Goldeneye complex (58◦ 0′ 10.8′′ N, 0◦ 22′ 48′′ W, approx. 100 km off the coast of Scotland), which has been identified as a potential offshore CO2 storage facility
There were no detectable correlations between total alkalinity (TA) and current direction, current magnitude, or time of day

Summary

Introduction

Atmospheric carbon dioxide concentration has risen to >407 parts per million, a 47% increase over the concentration at the beginning of the industrial era (Friedlingstein et al, 2019). A requirement for this offshore CO2 storage is the ability to monitor and ensure reservoir integrity during and after the initial storage activity (Dean et al, 2020). This is necessary to satisfy regulatory requirements and to alleviate public concern around potential risks (Mabon et al, 2015). Any techniques used to monitor for emissions should be sufficiently sensitive that the emission is detected and quantified before the regulatory limit is breached, and before any damage can occur to the local environment or ecosystem Such monitoring techniques need to be developed and validated at low, known rates of CO2 release in a relevant environment, to provide confidence in their suitability

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Discussion

Conclusion