Management of a Complex Cavern Storage Facility for Natural Gas
Authors: Bittkow P. , Riechel M. , Boor G. 
Published: December 2007

Abstract
The Epe Cavern Storage Facility, operated by Ruhrgas AG of Essen, Germany, has developed into one of the largest gas cavern storage facilities in the world. At present, 32 caverns are used; in future, about 50 caverns are intended. The isothermal working gas volume will increase from a current level of approx. 1.5 billion m3 to far in excess of 2 billion m3. A special management concept during both installation and operation is required for this large storage facility, taking into consideration the very different behaviour of each individual cavern as well as the thermodynamic properties of natural gas. New methods for the following aspects have been developed for optimum usage of the geological resources as well as for cost minimisation in connection with installing not only each individual cavern but also the overall facility and for minimisation of the operating costs:–Optimisation of the available pressure range Pmax/Pmin–Dimensioning of the caverns taking into consideration the geological conditions with respect to maximum cavity gain–Minimisation of convergence during storage operation–Optimisation of the entire process with respect to the maximum working gas volume and the maximum available withdrawal capacity.

Introduction
Regarding their structure, cavern storage facilities are, in particular, ideally suitable for short-term peak-shaving. The large volume of gas stored in a cavity permits high withdrawal rates due to unimpeded gas expansion. The pressure and temperature behaviour which is dependent on the properties of gas in a cavern, the dimensions of the cavern itself, the design of the production string as well as the surface pressure and temperature losses in the gathering lines and the station all determine the capacity of the cavern storage facility.
When gas is withdrawn from several caverns, the availability of working gas depends on different parameters, such as, for example, the cavern volumes, the cavern depths, their individual cavity convergence and the respective gas levels at different pressures and temperatures.
The greater the number of caverns involved, the more extensive the set of parameters to be taken into consideration. Operation of the currently 32 and in future 50 caverns in Epe (Fig. 1) therefore entails a comprehensive supportive engineering concept in order to cover all these above-referenced factors and ensure an optimised operating mode with respect to thermodynamic and rock-mechanical behaviour.
The salt deposit in which the storage facility has been installed has a surface area of approx. 7 km2. The caverns are located in a Werra-Salinar (Na 1) stratified salt deposit at a depth of between 1,100 m and more than 1,400 m. This deposit is structured by few tectonic elements and its thickness is between 250 m at its edge and more than 400 m at its centre. The caverns are leached by Salzgewinnungsgesellschaft Westfalen mbH (SGW) which uses the recovered brine in the chlorine industry. This means that the caverns are made available to Ruhrgas commensurate with brine production. Prior to leaching, these caverns are dimensioned to the size required for gas storage purposes.
This paper presents the components of a complex management system which covers the design, construction and operation of the storage facility as a complete package. This system has been developed by Pipeline Engineering GmbH (PLE) for Ruhrgas AG. Its main features are as follows:
P. 219^