Salt caverns redeployed for gas storage Apr 2011
Patrick Reynolds, Freelance Reporter
The UK lags behind several other European countries in gas storage capacity but the Government wants to quadruple the assets by 2021. Consultant Atkins has been studying the geological structures of a group of salt caverns in England for client EDF Energy to convert them to gas storage in the near future, reports Patrick Reynolds
A cluster of salt caverns near Warmingham in England are being flushed of residual brine following lengthy geomechanical studies by consultant Atkins to help EDF Energy convert them into gas storage assets.
Sonar survey image of a salt cavern dome

Sonar survey image of a salt cavern dome

The 10 caverns are about 240m deep and approximately cylindrical of 80m high and 50m in radius. They are spaced about 150m between their centres, and are among many caverns in the expanding salt mining and gas storage site located north of Crewe, in Cheshire.
British Salt developed the 10 caverns over some years and recently sold them to EDF Energy.
Salt caverns give an advantage over other types of natural underground voids by being comparatively easy to charge to operating pressures and deliver gas in relatively large quantities to demand. It is also possible to retrieve almost all the stored gas when the facility is decommissioned.
Dr Evan Passaris has led the geomechanical work by Atkins on the caverns and is a specialist in the field, having worked on underground storage schemes across Europe in Italy, Germany, the Netherlands and Denmark. In terms of scale those near Warmingham are an average 600,000m3-650,000m3 scale compared to larger 800,000m3 caverns he has worked on.
He said that salt caverns provide good underground gas storage facilities as the geological make-up of the thick-layered impervious salt formations create an ideal gas-tight container. The consultant has been studying the gas storage possibilities of the site since early 2008 when British Salt began examining the potential.
Brine-filled caverns formed by solution mining of salt are suitable for gas storage

Brine-filled caverns formed by solution mining of salt are suitable for gas storage

Converting the former salt caverns is a multi-stage process and has involved geomechanical assessments to determine various rock properties include density; deformational elastic parameters; constants concerning triaxial shear strength and uniaxial tensile strength; creep constants concerning the appropriate non-linear viscoplastic creep model; the coefficient of linear thermal expansion; thermal conductivity; and, specific heat capacity.
In terms of structural integrity, there are a number of factors to be examined as gas pressures in the caverns will vary and result in stress redistribution in the rock. The depth of the caverns and the thickness of salt rock left between the voids influences stress patterns and there is gradual convergence behaviour due to the creep characteristics of salt, which are also temperature dependant. The analyses, therefore, cover the combination of static, creep and thermal loadings.
During operation gas will be drawn equally from the caverns although they are not inter-linked. The operating pressure of gas in the Warmingham caverns will be within the 2.9MPa to 4.5MPa range.
As strategic plan and with the Britain's natural gas reserves falling, the UK Government wants the volume of underground gas storage in the nation to quadruple within the next 10 years. Dr Passaris said the UK consumes approximately 103 billion m3 of gas but stores less than 4% in comparison to 27% in France and 21% in Germany. The UK's storage volume, at less than 5 billion m3, equates to about 14 days supply while France has about three months and Germany and Spain can boast 77 days and 65 days, respectively.
Dr Passaris added that despite the drive for investment in new storage caverns, it will take time to establish the additional assets. It is not an overnight transformation.

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