Building part of the cushion of an aquifer underground gas storage by an inert gas produced at a much lower cost than pipeline gas, is an operation of high economic interest.

The paper presents the actual case history of the underground aquifer gas storage of SAINT-CLAIR-SUR-EPTE, FRANCE. Preliminary studies performed in 1978 and 1979 have shown the technical feasibility and economic interest of injecting a significant quantity of inert gas (20 % of the cushion gas) before filling up the storage with natural gas.

The inert gas injection actually began on the 6th of August 1979 to reach a total inventory of 60.2 106 m3(n) on the 14th of March 1982. The first natural gas injection began on the 19th of August 1981 as the total inert inventory was 50.4 106 m3(n), and the first withdrawal took place on the 15 of November 1983, with a total inventory (inert + natural gas) of 462.6 106 m3(n). It should be noted that the inert gas injected contained a small proportion of air. The storage was operated since with injection-withdrawal cycles, the total working capacity (247.6 106 m3(n)) was withdrawn during winter 1986-1987.

The concentrations of gas components obtained from chromatography were collected on a set of eleven observation wells, at a rather important frequency (once a month on 7 wells during the withdrawal period of winter 1986-1987), giving the natural gas and inert concentrations. The plotting of curves, as well as the mapping of concentrations of inert and natural gas permits to have a good control of the gases repartition inside the porous medium. The proportion of O2 and CO2 were also analysed on the same observation wells. The evolution with time of these two variables is interesting and will be presented In the paper.

As far as we know, for the first time, a storage with a cushion consisting of 20 % of inert gas has been successfully operated for several years. The synthesis of the field data and its interpretation is included in the paper.

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