Frequently Asked Questions - Geology
Where can gas be stored?
The Health and Safety Executive states: “storage of gas onshore and offshore is only possible in certain areas with the required geology and geological structures. These are present in a limited number of locations in Great Britain. Suitable salt strata are thought to occur in the Wessex, Cheshire and West Lancashire basins and along the Yorkshire North Sea Coast.” 26
How big is the Preesall development?
For salt cavity storage the size of a facility will depend on the thickness, depth and lateral extent of the salt beds. The amount of above ground infrastructure that is required to support such facilities is not usually very extensive. 27 The ultimate capacity at Preesall will be determined by the salt, science, and the HSE.
What is so special about the Preesall salt?
The Preesall halite salt, alternatively known as rock salt, was laid down some 200 million years ago. The depth of the salt bed deepens from the east, the high ground, to the west, under the river. The shallowest area was worked by the Romans and subsequently by salt activities over the last century. The deepest salt has not been mined and is ideal for the creation of new gas storage caverns.
The Preesall project will create multiple caverns in the halite salt to hold up to 60 bcf (see Glossary) of natural gas.
The deeper Preesall salt is about 400 meters underground (to the top of the salt) which allows the gas pressure in the caverns to be in the same pressure range as the NTS pipeline. This feature optimises the process of flowing gas back and forth between the caverns and the NTS. The salt depth also lowers the energy cost of washing the caverns, lowers the cost of cavern infrastructure and lowers the operating cost of compressing the gas to inject into the caverns. Deeper salt caverns are correspondingly harder to construct and more expensive to operate.
What are the properties of salt which make it safe for underground gas storage?
Rock salt (halite) exhibits unique physical properties and mechanical behaviour. Halite beds are nonporous and are known to provide a natural seal to gas. 28 Halite with its extremely low permeability and general viscoplastic deformation is viewed as an ideal storage medium.29
When subjected to stress halite has an ability to flow by crystal plastic deformation processes. This means it has a low susceptibility to fracturing and even if fracturing were to occur, the halite flows, thereby allowing any fractures and cracks to seal.30
The British and European Standards on Underground Gas Storage require a series of properties and measurements for the design of caverns. Canatxx have ensured the salt matches the British and European Standards by taking borehole samples and subjecting them to rigorous laboratory stress and strain tests and/or in situ tests.
The tests and models are designed to demonstrate that the cavern will be mechanically stable and capable of containing gas under the proposed permitted operating conditions, “using acknowledged geological methods and databases.” 31
Will there be any subsidence?
The subsidence calculations predict that subsidence will be too small to be reliably measured.
Studies of the subsidence associated with a cavern maintained at minimum operating pressure were undertaken for Canatxx by cavern design experts. In the design of a gas storage cavern, a pressure range is defined within which the caverns should operate at given depths and geostatic pressures. The upper (higher) pressure limit is designed to prevent over-pressurisation that could cause fracturing of the rocks. The lower operating pressure is designed to stop the inward movement of the cavern walls due to salt creep. 32
The subsidence estimates for these conditions were calculated using industry recognised modelling methods and covered a range of possible cavern sizes and designs.
Data collected by ICI and Canatxx on the modern caverns at Preesall verify the theoretical figures generated by the studies.
26 - DBRR “Gas Storage in your
Area - Your Questions Answered” July 2007 http://www.berr.gov.uk/files/file40536.pdf
27 - DBRR “Gas Storage in your Area - Your Questions Answered” July 2007
http://www.berr.gov.uk/files/file40536.pdf
28 - Health and Safety Executive, “An appraisal of underground gas storage technologies and incidents, for the development of risk assessment methodology”. Prepared by the British Geological Survey for the Health and Safety Executive 2008 P. 18
29 - Health and Safety Executive, “An appraisal of underground gas storage technologies and incidents, for the development of risk assessment methodology”. Prepared by the British Geological Survey for the Health and Safety Executive 2008 P. 19
30 - Health and Safety Executive, “An appraisal of underground gas storage technologies and incidents, for the development of risk assessment methodology”. Prepared by the British Geological Survey for the Health and Safety Executive 2008 P. 20
31 - Health and Safety Executive, “An appraisal of underground gas storage technologies and incidents, for the development of risk assessment methodology”. Prepared by the British Geological Survey for the Health and Safety Executive 2008 P. 32
32 - Health and Safety Executive, “An appraisal of underground gas storage technologies and incidents, for the development of risk assessment methodology”. Prepared by the British Geological Survey for the Health and Safety Executive 2008 P. 100