Impact of CO2 leakage from sub-seabed carbon dioxide storage on sediment and porewater geochemistry

Anna Lichtschlag,Matthias Haeckel,David Olierook,Kate Peel,Anita Flohr,Christopher R Pearce,Chiara Marieni,Rachael H James,Douglas P Connelly

doi:10.1016/j.ijggc.2021.103352

Abstract

Sub-seabed geological CO2 storage is discussed as a climate mitigation strategy, but the impact of any leakage of stored CO2 into the marine environment is not well known. In this study, leakage from a CO2 storage reservoir through near-surface sediments was mimicked for low leakage rates in the North Sea. Field data were combined with laboratory experiments and transport-reaction modelling to estimate CO2 and mineral dissolution rates, and to assess the mobilization of metals in contact with CO2-rich fluids and their potential impact on the environment. We found that carbonate and silicate minerals reacted quickly with the dissolved CO2, increasing porewater alkalinity and neutralizing about 5% of the injected CO2. The release of Ca, Sr, Ba and Mn was mainly controlled by carbonate dissolution, while Fe, Li, B, Mg, and Si were released from silicate minerals, mainly from deeper sediment layers. No toxic metals were released from the sediments and overall the injected CO2 was only detected up to 1 m away from seabed CO2 bubble streams. Our results suggest that low leakage rates of CO2 over short timescales have minimal impact on the benthic environment. However, porewater composition and temperature are effective indicators for leakage detection, even at low CO2 leakage rates.

Highlights

The rapid increase of atmospheric CO2 since the start of the indus trial revolution has led to global warming, ocean acidification and expanding oxygen minimum zones that are impacting global biodiver sity and ecosystem functioning (e.g., Gattuso et al, 2015; Nagelkerken and Connell, 2015)
The accidental leakage of CO2 from a sub-seabed storage reservoir is regarded as un likely (IPCC, 2005), the effects of potential leakage through wells, faults and fractures on the marine environment needs to be assessed in terms of ecosystem response, other consequences for the marine environment, and to identify geochemical parameters that should be monitored as indicators for a leakage from a CO2 reservoir
The element distribution in the sediments is consistent with early diagenetic processes, authigenic carbonate formation and ongoing alteration by reverse weathering leading to the formation of authigenic clays (Dale et al, 2021)

Summary

Introduction

The rapid increase of atmospheric CO2 since the start of the indus trial revolution has led to global warming, ocean acidification and expanding oxygen minimum zones that are impacting global biodiver sity and ecosystem functioning (e.g., Gattuso et al, 2015; Nagelkerken and Connell, 2015). While the distribution and extent of pH plumes in the water column has been modelled for different CO2 leakage scenarios (Blackford et al, 2020), very little is known about the fluid flow patterns and the effects of sudden high CO2 concentrations in near-surface sediments. These near-surface sediments play a crucial role for the state and health of the marine environment as they convert, store and release chemical com pounds. Leakage of CO2 from offshore CCS reservoirs might reduce biodiversity and might disrupt important ecological functions, with the impacts on indi vidual biological species differing across different taxonomic groups (Kim et al, 2016). Laboratory experiments on sed iment-CO2 reactions have demonstrated mineral dissolution and metal release from aquifer and near-surface sediments due to desorption and pH decrease, with release of potentially harmful substances, such as toxic metals, into the benthic environment at low pH (Ardelan et al, 2009; Kirsch et al, 2014; Ostertag-Henning et al, 2014; Payan et al, 2012; Wunsch et al, 2014)

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