RÉSUMÉ:

On a realise un essai in situ pour chercher la valeur d'equilibre a laquelle se stabilise la pression de la saumure contenue dans une cavite souterraine fermee. L'equilibre resulte de Paction du fluage du massif de sel et de la permeation de la saumure. Sa valeur est sensiblement inferieure a la pression lithostatique, ce qui exclut un risque de fracturation naturelle.

ABSTRACT:

A test has been performed on a deep brine-filled cavern, whose objective was to measure the equilibrium pressure reached when the cavern is closed. Such an equilibrium is reached when salt mass creep, which leads to cavern shrinkage, balances the permeation of brine through the cavern wall. This pressure is significantly smaller than the lithostatic pressure at cavern depth, alleviating any natural fracturation risk.

ZUSAMMENFASSUNG:

:Es wurde ein Feldversuch an einer tiefliegenden solegefiillten Kaverne durchgefiihrt mit der Zielsetzung, den Gleichgewichtsdruck nach dem Verschie,Ben der Kaverne zu messen. Ein solches Gleichgewicht wird erreicht, wenn das Kriechen des Steinsalzgebirges, das zu einer Volumenverringgerung der Kaverne fiihrt, dem Infiltrationsvolumen von Sole durch die Kavernenwand entspricht. Der sich einstellend Druck ist erheblich geringer als der lithostatische Druck an die Kavernentiefe, wiirde die Gefahr des Aufrei,Bens des Gebirges vermindern.

EXTENDED SUMMARY

When a brine-filled salt cavern is sealed and abandoned, its pressure builds up due to two main phenomena (Berest et al, 1979):

  • thermal expansion of the brine; and

  • cavern creep.

The former is by far the predominant phenomenon during a period of several years or dozens of years (depending on the cavern size). When thermal expansion can be disregarded, cavern creep leads to cavern shrinkage, at which point pressure builds up and, in turn, slows the cavern convergence rate, which becomes very small when brine pressure is close to lithostatic pressure (Wallner & Paar 1997).

With the possible exception of very deep caverns, the balance between cavern pressure and lithostatic pressure will only be achieved after several centuries. Due to the density difference between brine and rock salt, only an average balance can be achieved: in a high cavern, brine pressure will exceed lithostatic pressure by a significant amount at the cavern top (Wallner 1988). Fears have been expressed that this excess will lead to rock fracturing and/or a severe increase of rock permeability - in either case, leading to a large amount of brine seepage.

Theoretical considerations prove that this situation can be alleviated, provided that the salt exhibits (even very small) some permeability to brine.

In order to determine the effect of salt permeability on cavern pressure build-up, Gaz de France and Ecole Polytechnique, supported by the Solution Mining Research Institute (SMRI), have performed an in-situ test. The Ez53 cavern of the Etrez site in France was selected for this test. In Spring 1982, the relatively small cavern (7500 m3) was leached out in a bedded salt formation at an average depth of 950 meters. In 1996 a temperature measurement showed that thermal equilibrium had been reached (see Figure1) in this cavern.

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