Ian Bryden

Contributions to Atmospheric Methane by Natural Seepages on the UK Continental Shelf

Natural gas seepages occur on the United Kingdoms continental shelf and although published reports suggest that they are very rare, the petroleum industry has identified, but not publicly reported, many more. There is also very little data on the flux of gas from seabed seepages, and even less on the contribution of seepages to atmospheric concentrations of gases such as methane. n nPotential gas source rocks include Quaternary peats as well as petroliferous source rocks such as the Carboniferous Coal Measures and the Upper Jurassic Kimmeridge Clays. There are also other organic-rich sediments which are potential source rocks. Together these cover a considerable part of the U.K. continental shelf. n nAnalogue seismic reflection (pinger) profiles acquired during the British Geological Surveys regional mapping programme have been reviewed to identify water column targets including fish and plumes of gas bubbles. The ability to distinguish targets is critical to an assessment of the distribution of gas seepages. Both theoretical predictions of target identity and the habits of shoaling fish have been investigated in order to identify a method of distinction. n nData from seabed seepages and measurements of seepage rates have been used to establish likely ranges of gas flux rates and the sizes of gas bubbles. The likelihood that a rising bubble will survive and escape into the atmosphere is determined primarily by bubble size and water depth; methane, the principal constituent of seepage gas, is relatively unreactive and sparingly soluble. n nThe studies have enabled a new estimate of the distribution of gas seepages on the U.K. continental shelf, and of the contribution to atmospheric methane levels. The results suggest that natural gas seepages are significantly more important as a source of methane than had hitherto been established. It is estimated that between 120,000 and 3.5 mtonnes of methane per year come from a continental shelf area of about 600,000 km2. This represents between 2% and 40% of the total United Kingdom methane emission. It is suggested that similar contributions arise from other continental shelf areas worldwide, and that geological sources of atmospheric methane are more significant than is generally acknowledged.

Tidal current resource assessment

Abstract This paper outlines present thinking on the determination of accessible tidal current resources within channels and other potentially exploitable locations. The fundamental principles behind tides and tidal currents are briefly discussed and the implications of temporal and spatial variations on the evaluation of the resources considered in the context of artificial energy exploitation. The thinking behind the flux approach to resource estimation is presented and an example based on the Pentland Firth is considered. The impact of energy extraction on the flow patterns is considered in both one and two dimensions and the principles required for three-dimensional analyses are presented in a generic form.

Choosing and evaluating sites for tidal current development

Abstract Tidal energy has been used since Roman times, although the use of tidal currents as a possible industrial energy source is a more modern concept. Unlike wind power, however, there is still no consensus on the most appropriate technology for resource exploitation. Similarities with wind power can cause errors of interpretation, especially when attempting to assess resource potential. The energy availability estimates that have been produced have, until now, only taken account of the apparent current flow speeds before the extraction of energy. As demonstrated in this paper, this may not be appropriate, especially for large energy extraction rates, which alter the underlying hydraulic nature of the flow environment. In a simple hypothetical channel linking two infinite oceans, a maximum extraction of 10 per cent of the apparent raw kinetic flux would appear to be acceptable. The limitations on sea loch type environments may, however, be less restrictive.

Contributions to atmospheric methane by natural seepages on the U.K. continental shelf: [Mar. Geol. 137 (1997) 165–189]

Abstract Natural gas seepages occur on the United Kingdoms continental shelf and although published reports suggest that they are very rare, the petroleum industry has identified, but not publicly reported, many more. There is also very little data on the flux of gas from seabed seepages, and even less on the contribution of seepages to atmospheric concentrations of gases such as methane. Potential gas source rocks include Quaternary and Tertiary peats as well as petroliferous source rocks such as the Carboniferous Coal Measures and the Upper Jurassic Kimmeridge Clays. There are also other organic-rich sediments which are potential source rocks. Together these cover a considerable part of the U.K. continental shelf. Analogue seismic reflection (pinger) profiles acquired during the British Geological Surveys regional mapping programme have been reviewed to identify water column targets including fish and plumes of gas bubbles. The ability to distinguish targets is critical to an assessment of the distribution of gas seepages. Both theoretical predictions of target identity and the habits of shoaling fish have been investigated in order to identify a method of distinction. Data from seabed seepages and measurements of seepage rates have been used to establish likely ranges of gas flux rates and the sizes of gas bubbles. The likelihood that a rising bubble will survive and escape into the atmosphere is determined primarily by bubble size and water depth; methane, the principal constituent of seepage gas, is relatively unreactive and sparingly soluble. The studies have enabled a new estimate of the distribution of gas seepages on the U.K. continental shelf, and of the contribution to atmospheric methane levels. The results suggest that natural gas seepages are significantly more important as a source of methane than had hitherto been established. It is estimated that between 120,000 and 3.5 mtonnes of methane per year come from a continental shelf area of about 600,000 km 2 . This represents between 2% and 40% of the total United Kingdom methane emission. It is suggested that similar contributions arise from other continental shelf areas worldwide, and that geological sources of atmospheric methane are more significant than is generally acknowledged.

Chapter 14 – Tidal Energy

Publisher Summary nThis chapter deals with tidal energy. There are two fundamentally different approaches to exploiting tidal energy. The first is to exploit the cyclic rise and fall of the sea level using barrages and the second is to harness local tidal currents in a manner somewhat analogous to wind power. Many industrial, commercial, and public bodies have suggested that there is a high degree of synergy between the development of a tidal current generation industry and the offshore oil and gas industry. This offers the intriguing prospect of a new renewable industry developing in partnership with the petroleum industry and could, perhaps, result in accelerated development, as a result of the availability of expertise and technology, which would otherwise have to be developed from scratch. However, the high capital costs associated with tidal barrage systems are likely to restrict development of this resource in the near future. Tidal current systems may not presently have the strategic potential of barrage systems but, in the short term at least, they do offer opportunities for supplying energy in rural coastal and island communities.