L'expérience du Gaz de France en matière de réservoirs souterrains de gaz comprimés

Chef du Département Réservoirs souterrains de la Direction des Etudes et Techniques nouvelles, Gaz de France, Paris

Résumé

1. UNDERGROUND STORAGE OF NATURAL GAS. In order to modulate its gas supplies, Gaz de France has had to set aside substantial reserves of gas. With gas-heating on the uptrend, temperature variations give rise to large time-variations in gas supplies (Figure 1). Figures 2 and 3 show annual and weekly variations in supplies drawn from certain typical reservoirs. Two techniques are used in France to constitute reserves of natural gas. By making use of porous rocks (sand, sandstone, limestone etc.), reservoirs in groundwater aquifers can be set up. These are equivalent to deposits of natural gas (Figure 4). A more recent technique consists in following out cavities in salt deposits. Figure 5 shows a cross-section through a salt-deposit storage cavity at Tersanne. II. APPLICATION OF GAS TECHNIQUES TO COMPRESSED AIR STORAGE. In order to ensure power supplies during periods of peak demand, compressed air is withdrawn from storage for a few hours per day, the reservoir being topped up again overnight. Reservoir " breathing cycle" thus covers 24 hours instead of a whole year in the case of natural gas. The performances of compressed-air and natural-gas reservoirs are quite dissimilar. If Q is the outflow in m2/day at NTP, Vu the volume of air or gas withdrawn during one cycle (reservoir breathing value) and Vc the buffer volume i.e. the volume of air or gas which remains in the reservoir to ensure correct operation, then the time variations of these variables are as shown in the table. The following points are of special interest: (a) Aquifers. Buffer gas volume is larger by a factor of 10 to 20. This makes operation easier by avoiding water-coning in the wells and by increasing well yields. Setting up wells is also made easier by the relative size of the compressed-air bubble. Also of interest are the modifications to well equipment which increase potential well yields. (b) Salt-cavity reservoirs. If operation with gas expansion is used, the buffer volume should be increased to avoid excessive temperature and pressure variations during withdrawal. III. ECONOMIC ASPECTS. The investigations carried out by Gaz de France with a view to equipping aquifers or salt cavities for compressed-air storage have led to an estimation of the average cost of the underground portion of the equipment. To equip an aquifer for the supply of 1 Mm3/hr of 15-atm. compressed air during five or six hours per day would cast approximately 50 F per m3/hr (NTP). The cost of equipping equivalent performance salt cavities operating with gas expansion would be about half the above figure. IV. CONCLUSIONS. Natural-gas storage techniques are well suited to storing compressed air for peak-hour power supply. Cavities hollowed out in salt deposits would appear to afford the best means of storage.