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 Pressure Swing Adsorption (PSA) technology will be put into service in July 1989. This paper discusses put into service in July 1989. This paper discusses the principles of these two methods and reviews the operating characteristics of each facility.
When operating an underground storage of gas in an aquifer, a more or less large part on the stored gas cannot be recovered. The gas trapped in the pores and the volume of free gas not recovered at the end of the withdrawal form the storage cushion gas whose purely mechanical role does not call for the chemical and calorific properties of natural gas.
In the case of seasonal modulation storage, the cushion gas can represent up to half of the total amount of gas stored and up to 70 % of the investment necessary to create the storage. This has prompted the search for a substitute gas which is prompted the search for a substitute gas which is less costly. Different inert gases have proved to be adapted substitutes.
The risks of inert gas dispersion within natural gas lead to a limit in the amount of inert gas that can enter into the cushion composition. Experimental studies along with field experience led to the conclusion that at least 20 % of the cushion gas can be replaced by inert gas without causing any inconvenience in field operations.
Given the planned goals, the inert-gas must not be reactive with the products and materials with which it comes into contact : surface equipment, well completion, reservoir rocks. Thus, inert gas must be essentially free of oxydating compounds, nitrogen oxides and sulfurated products. Furthermore, it must not constitute an explosive mixture with natural gas (gas mixtures bearing more than 8 % oxygen, like air, are therefore excluded). In general, nitrogen and mixtures of nitrogen and carbon dioxide meet these requirements.
Seasonal modulation storages are of limited capacity and injection operations on one site will not cover the life span of the producing plant. Given that the storage locations are scattered over France, inert gas production units developed for this application must satisfy the following requirements :
transportability from site to site at reasonablecost and delay,
adaptability to variable discharge pressures,
complete autonomy of the units : only operating power (electricity or fuel gas) is supplied from outside.