Impact of subsurface fluid flow on sediment acoustic properties, implications for carbon capture and storage

Abstract

Geological Carbon Capture and Storage (CCS) is a promising climate change mitigation technology, which allows the reduction of anthropogenic carbon dioxide (CO2) emissions into the atmosphere. Although CCS is considered to have a significant potential in tackling climate change, several uncertainties remain, including the efficiency and permanency of carbon sequestration, and notably risks of CO2 leakage from the storage reservoir. A better understanding of fluid flow activity within the sedimentary overburden and the identification of the best monitoring techniques are crucial for increasing societal confidence in sequestration.

This thesis reports findings from two different offshore CCS projects: a controlled sub-seabed CO2 release experiment completed in Ardmucknish Bay, Oban (Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage, QICS), and a multidisciplinary research project conducted in the vicinity of Sleipner CCS site, in the Central North Sea (Sub-seabed CO2 Storage: Impact on Marine Ecosytems, ECO2).


During the QICS project, a borehole was drilled from land, allowing 37 days of CO2 release in unconsolidated marine sediments. Analysis of the time-lapse high- resolution seismic reflection data reveals development of acoustic anomalies within the overburden and water column, caused by CO2 fluxing in the vicinity of the injection site. The impacts of CO2 injection on sediment acoustic properties are investigated, where changes in seismic reflectivity, seismic attenuation, acoustic impedance and P-wave seismic velocity are detected on high-resolution seismic reflection data. CO2 migration within the overburden is interpreted to be controlled by sediment stratigraphy and injection rate/total injected volume throughout the gas release, and by the sediment stratigraphic geometry post-release. Seismic quantification of the gaseous CO2 indicates that most of the injected CO2 is trapped below a stratigraphic boundary, located at 4 m depth below the seafloor, or dissolved, throughout the gas release. These observations are in agreement with seabed gas flux measurements by passive hydroacoustics and water column bubble sampling, which suggest that only 15% of the injected CO2 emerges at the seabed, towards the end of gas release.

Within the scope of the ECO2 project, increased fluid flow activity is detected along, and in the vicinity of a seabed fracture, the Hugin Fracture. Although there is no evidence of anthropogenic CO2 leakage in the Central North Sea from the current dataset, biogenic and thermogenic gas leakage at the Hugin Fracture suggest a well-established hydraulic and structural connection. The origin of the Hugin Fracture is proposed to be controlled by an E-W transtensional stress regime, and differential compaction above a buried tunnel valley system.

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ORCID for Jonathan Bull: orcid.org/0000-0003-3373-5807

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