North Sea gas has provided Britain with a regular and plentiful supply since the mid–late 1960s when the first discoveries were made up to 50 km offshore in the southern North Sea. However, UK supplies from the North and Irish seas peaked in 1999, since when production has fallen by around half.
Declining production rates resulted in the UK becoming a net importer of gas in 2004 and peak production during the winter months is no longer sufficient. By 2015 production will have dropped by two-thirds and the UK will be importing around 70 per cent of its requirements, rising to an estimated 80–90 per cent by 2020 (National Grid, 2007; DTI, 2006a).
The UK is in need of increased gas storage volumes to smooth out sudden demands in supply during, for example, cold spells or if daily supplies are lost due to technical difficulties or other reasons.
Storage also has economic importance in that gas does not have to be bought during the main winter periods or at times of high demand when it is more expensive.
When compared to above ground storage tank facilities, underground gas storage provides the significantly greater and cheaper storage volumes required and when carefully constructed and monitored, added levels of safety. UGS is thus seen as a way of managing gas supply, particularly during periods of peak demand and as stand-by to cover for interruptions to imports.
The best solution for increasing UK gas storage volumes lies in constructing new underground facilities, which most typically will be in depleted hydrocarbon (oil and gas) fields or specifically designed solution mined caverns in halite (rocksalt) deposits, both in the onshore and offshore areas (Figure 1).
A third option, currently used in France and Germany, is to inject gas into aquifers. However, the technique is expensive, requiring far more investment to prove the extent and gas tightness of caprock and storage horizons not previously pressurised to those levels likely during storage.
The UK requires developments both offshore for strategic storage and to assist with imports, but also, because it takes time for gas to flow through the National Transmission System (NTS), onshore in order to provide more flexible responses to the requirements of both industrial and domestic users. Each underground storage type offers different benefits in terms of storage and deliverability and ideally a mix of storage facilities are required to provide a balanced portfolio of long term strategic storage and shorter term rapid response storage.
UGS has been operating worldwide for over 90 years and there are 630 facilities of different types in operation.
The techniques are, however, relatively new in the UK. There are only seven operational UGS facilities in the UK at:
Another 20 or so are at various stages in the planning process, but face lengthy delays (Figure 2).
The potential areas for the development of underground gas storage (UGS) onshore in Great Britain are limited to where suitable geological strata or trapping structures are present. In areas with potential a number of projects are currently operational, under development and construction, or are planned (Figure 2).
However, local communities close to proposed facilities oppose UGS applications. The opposition is illustrated by the Preesall cavern storage and depleted Welton Oilfield proposals.
In UGS, the need to contain the gas in the storage horizon is of paramount concern both for economical and safety reasons and there are many stages involved in the planning, construction and safe operation of any proposed UGS facility. To support the UGS industry, research into the areas most likely to attract proposals and the types of rock and structures available is needed, much like the emerging carbon capture and storage industry. There is also the need for research to show that natural gas storage in geological formations is safe and ensure gas tightness for each and every proposal and its location.
Before gas storage can begin, exhaustive studies on the gas tightness of the storage horizon are required as part of the Control of major accident hazards (COMAH) process.
This will include a full technical assessment of the possibility of leakage and identify potential migration pathways on a case by case basis, which will be very different for salt cavern storage compared to depleted oil or gas fields. Pressure cycling and the associated thermal effects also need to be fully taken into account in assessing likely rock responses to the stresses of gas storage.
The natural gas storage business will, therefore, play an important role in future UK energy supply and production and the Government is encouraging oil and gas operators and storage developers to invest in storage schemes.
In December 2006, the energy minister at the time, Lord Truscott, announced that £10 billion was to be spent on gas storage infrastructure in the period 2005–2010. Most of this will be spent on UGS space, but investment is being held up by the objections and doubts raised over the geological suitability of most proposals. Research in these areas could open up significant areas of work with potential developers in the geological appraisal and characterisation of sites or regions for gas storage.
For further information please contact Dr Jonathan Busby.