Increased fluid flow activity in shallow sediments at the 3 km Long Hugin Fracture in the central North Sea

The North Sea hosts a wide variety of seafloor seeps that may be important for transfer of chemical species, such as methane, from the Earth's interior to its exterior. Here we provide geochemical and geophysical evidence for fluid flow within shallow sediments at the recently discovered, 3-km long Hugin Fracture in the Central North Sea. Although venting of gas bubbles was not observed, concentrations of dissolved methane were significantly elevated (up to six-times background values) in the water column at various locations above the fracture, and microbial mats that form in the presence of methane were observed at the seafloor. Seismic amplitude anomalies revealed a bright spot at a fault bend that may be the source of the water column methane. Sediment porewaters recovered in close proximity to the Hugin Fracture indicate the presence of fluids from two different shallow (<500m) sources: (i) a reduced fluid characterized by elevated methane concentrations and/or high levels of dissolved sulfide (up to 6 mmol L−1), and (ii) a low-chlorinity fluid (Cl ∼305 mmol L−1) that has low levels of dissolved methane and/or sulfide. The area of the seafloor affected by the presence of methane-enriched fluids is similar to the footprint of seepage from other morphological features in the North Sea

Detection and impacts of leakage from sub-seafloor deep geological carbon dioxide storage

Fossil fuel power generation and other industrial emissions of carbon dioxide are a threat to global climate1, yet many economies will remain reliant on these technologies for several decades2. Carbon dioxide capture and storage (CCS) in deep geological formations provides an effective option to remove these emissions from the climate system3. In many regions storage reservoirs are located offshore4, 5, over a kilometre or more below societally important shelf seas6. Therefore, concerns about the possibility of leakage7, 8 and potential environmental impacts, along with economics, have contributed to delaying development of operational CCS. Here we investigate the detectability and environmental impact of leakage from a controlled sub-seabed release of CO2. We show that the biological impact and footprint of this small leak analogue (&lt;1 tonne CO2 d?1) is confined to a few tens of metres. Migration of CO2 through the shallow seabed is influenced by near-surface sediment structure, and by dissolution and re-precipitation of calcium carbonate naturally present in sediments. Results reported here advance the understanding of environmental sensitivity to leakage and identify appropriate monitoring strategies for full-scale carbon storage operations

Geochemical tracers for monitoring offshore CO2 stores

Chemical tracers are proposed as an effective means of detecting, attributing and quantifying any CO2 leaks to surface from geological CO2 storage sites, a key component of Carbon Capture and Storage (CCS) technology. A significant proportion of global CO2 storage capacity is located offshore, with some regions of the world having no onshore stores. To assure regulatory bodies and the public of CO2 storage integrity it is important to demonstrate that robust offshore monitoring systems are in place. A range of chemical tracers for leakage have been tested at onshore pilot CCS projects worldwide, but to date they have not been trialled at injection projects or CO2 release experiments located offshore. Here, for the first time, we critically review the current issues surrounding commercial scale use of tracers for offshore CCS projects, and examine the constraints and cost implications posed by the marine environment. These constraints include the logistics of sampling for tracers offshore, the fate of tracers in marine environments, tracer background levels, marine toxicity and legislative barriers – with particular focus on the Europe and the UK. It is clear that chemicals that form a natural component of the CO2 stream are preferable tracers for ease of permitting and avoiding cost and risks of procuring and artificially adding a tracer. However, added tracers offer more reliability in terms of their unique composition and the ability to control and regulate concentrations. We identify helium and xenon isotopes (particularly 124,129Xe), and artificial tracers such as PFCs and deuterated methane as the most suitable added tracers. This is due to their conservative behaviour, low environmental impact and relative inexpense. Importantly, we also find that SF6 and C14 are not viable tracers for CCS due to environmental concerns, and many other potential tracers can be ruled out on the basis of cost. Further, we identify key challenges that are unique to using tracers for offshore monitoring, and highlight critical uncertainties that future work should address. These include possible adsorption or dispersion of tracer compounds during ascent through the overburden, longevity of tracers over the timeframes relevant for CCS monitoring, the permissible environmental effects of tracer leakage, and tracer behaviour in seabed CO2 bubble streams and in dissolved CO2. These uncertainties directly affect the selection of appropriate tracers, the injection programme and concentrations necessary for their reliable detection, and appropriate sampling approaches. Hence offshore tracer selection and associated expense are currently poorly constrained. Further, there is limited experience of sampling for tracers in the marine environment; current approaches are expensive and must be streamlined to enable affordable monitoring strategies. Further work is necessary to address these unknowns so as to evaluate the performance of potential tracers for CO2 leak quantitation and provide more accurate costings for effective offshore tracer monitoring programmes

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

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

The amount of ice in Martian lobate aprons, derived from analogue experiments on the plasticity of ice-rock mixtures

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The amount of ice in Martian lobate aprons, derived from analogue experiments on the plasticity of ice-rock mixtures

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The rheology of ice-rock mixtures inferred from analogue models: application to the gravitational flow of martian superficiel formations

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Debris aprons on Mars: rock glaciers or “mud glaciers" ?

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