The extreme mobility of natural gas (low viscosity and specific gravity) as well as the very large and rapid pressure variations are two traits peculiar to the storage of gas in salt cavities. In order to reach the objectives set with utmost respect for the environment, Gaz de France has been led to examine with particular care the problems of cavity stability and well tightness. Brine reinjection into a cavity has confirmed the predictions and legitimized the approach adopted.
For over 10 years now, Gaz de France has been storing natural gas in underground cavities that it hollowed out by leaching in salt formations. The development program is being furthered at the Tersanne and Etrez sites in parallel with the extension and creation of aquiferous underground storages, another storage technique developed by Gaz de France. These developments as a whole must enable a storage capacity of 73.6 × 109 kWh (8 % of which in salt cavities) to be available by 1985, in a situation where the part of natural gas in the French primary energy requirement should be 16 %, in conformity with the imperatives fixed for Gaz de France by the French Government. The materialization and operation of salt cavity storage require the mastering and implementation of a number of specific techniques relating to among others:
Thermodynamic behavior of stored gas
The beginning of any project for the storage of hydrocarbons in salt cavities is the stability study. Two types of problems arise when designing a storage project: location and long term behavior. Rare is the case of a single storage cavity: usually, the storage project concerns for obvious economic reasons a large number of cavities that have to be appropriately positioned. Moreover to ensure the safety of persons and goods at the site as well as to safeguard the environment, the long term stability of this group of cavities has to be studied. To broach these two problems we turn to rock mechanics which bears in fact on the rheological behavior research of the rocky material within which the works will be set.
The cores taken out during survey drillings (or during well drilling) and carefully packed in paraffin for clay and salt are subjected to various laboratory tests. Simple and triaxial compression tests are made to determine the elastic characteristics (E Young\'s modulus and ν Poisson's ratio) as well as the material's yield strength. Moreover creep tests (distortion as a function of time with a constant load) and relaxation tests (stress as a function of time under constant distortion) are performed in order to obtain information concerning delayed behavior. The totality of these laboratory tests are synthesized in the form of behavior laws, that is, the determination of the non-elastic deformation progress speeds εnl and of the hardening variable vi as a function of the stresses. For certain cases these laws can be schematically represented by rheological models, analogic models resulting from the association of springs, shock absorbers and runners.