Solutions Found to some Problems connected with the Construction of Natural Gas Storage Cavities by Dissolution in Salt
Authors: Donat G.F. 
Published: December 2007

Summary
In 1970 and 1971 GAZ de FRANCE initiated operation of an underground reservoir for storing natural gas in cavities created by dissolution in salt. The reservoir comprises two cavities with geometrical volumes of 90,000 m3 and 125,000 m3 respectively, in which the gas is stored at a maximum pressure of about 220 kg/cm2. This facility is located at TERSANNE, about 60 km south of LYONS.
The aim was to create, in a salt layer of limited thickness, cavities which would have the greatest possible volume in conjunction with a shape that would ensure a good probability of stability in the course of operational cycles probability of stability in the course of operational cycles involving a pressure drop from 220 to 80 kg/cm2.
The need to be able to control the dissolution process calls for the use of an inert blanket (fuel-oil) in order to be able to accurately direct the elevation of the roof of the cavity being leached and thus obtain the desired shape. It is therefore necessary to be able to position, and subsequently locate with great accuracy, the fuel-oil/brine interface in the leaching well. A novel method based on the use of radioactive tracers was developed and used with success for the purpose.
After each cavity had been leached, it underwent a leak tightness test before being filled with gas. For various reasons connected primarily with the pursuance of the dissolution and thermal variations it is impossible to be certain of the absence of leaks by merely observing the pressure at the head of the well. A test procedure was therefore devised and put into effect that enables meaningful results to be obtained in a comparatively snort space of time.
Finally, by analogy with storage reservoirs in aquifers, each well was provided with special equipment before the injection of gas. Without placing excessive limitation on productivity, this equipment makes it possible to protect productivity, this equipment makes it possible to protect the cementation against pressure changes and to ensure total safety in the event of a wellhead hitch (through the insertion of an automatic underground valve in the tubing line).
In connection with the creation of the first two underground gas storage cavities at TFRSANNE, GAZ de FRANCE devised a novel way of overcoming certain problems connected with the generation of cavities of predetermined shape and evolved a meaningful tightness test and ultimate equipment that protects the well cementation and provides safety underground in the event of a surface accident. Some of these solutions are described in the paper which follows. paper which follows

Introduction
For many years cavities obtained by dissolution in a salt mass have been used to store liquid hydrocarbons, and their use for the storage of gas under pressure is a more recent development. A number of facilities of this type exist in the United States and Canada, The only ones of any size in Europe are to be found in Armenia (USSR) and in France.
Two cavities with geometrical volumes of 90,000 m3 and 125,000 m3, respectively, in which the gas is stored at a maximum pressure of close on 220 kg/cm2, have recently been put into pressure of close on 220 kg/cm2, have recently been put into operation by GAZ de FRANCE at TERSANNE, 60 km south of the city of LYONS. Thus a volume of over 40 millions Nm3 of gas is stored at a depth of about 1400 meters. The cavities were created in a layer some 120 meters thick, containing salt charged with an average 12% of insoluble substances (clay and anhydrite). The salt cap is constituted by several hundred meters of clay (figure 1).
Before completing this brief description of the TERSANNE underground reservoir it should be noted that the gas is injected by two 700 hp piston-type compressor units and that it is possible, independently of reinjections, to draw off by expansion a total of about 30 millions Nm3 at the rate of more than 2 millions Nm3 daily, the minimum pressure that can prevail in a cavity having been set at 80 kg/cm2.