In seasonal modulation storages of natural gas In an aquifer, a varying fraction of the injected gas, the cushion gas, cannot be recovered. This cushion gas may represent more than half the total gas volume, and up to 70 % of the initial investment for the storage facility. Experimental studies, backed up by field experience, have shown that at least 20 % of the cushion gas can be replaced by a less expensive inert gas.

Different methods are available to produce an inert gas which meets the specifications required. Recovery of natural gas air combustion products (mixture of 88 % N2 and 12 % CO2) and physical separation of air components (more or less pure N2) have been selected. For the specific needs of France inert-gas injection program, the production facilities must satisfy requirements of autonomy, operating flexibility and transportability.

The technique of recovering natural gas air combustion products has been experienced in two facilities. A nitrogen production unit using the Pressure Swing Adsorption (PSA) technology will be put into service in July 1989. This paper discusses the principles of these two methods and reviews the operating characteristics of each facility.

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