ABSTRACT:

The behavior of Tersanne natural gas underground storage field in bedded rock salt formations is described in an analytical way and compared to available in situ measures. Evolution of surface subsidence and cavity volume loss is of main concern. Although the problem is three dimensional in nature, an axisymmetric model for a single cavity is used together with a superposition method previously validated. There is a reasonable agreement between in situ data and numerical values.

RESUME:

Le comportement du champ de cavites de Tersanne pour stockage de gaz naturel en sel gemme est decrit d'une façon analytique et compare à des mesures in situ disponibles. L'evolution de la subsidence en surface et la diminution de volume des cavites sont suivis en particulier. Bien que Ie problème soit tridimensionnel on utilise un modèle axisymetrique pour une cavite isolee en conjonction avec une methode de superposition valide avant. On trouve une raisonnable accord entre les valeurs in situ et ceux obtenues numeriquement.

ZUSAMMENFASSUNG:

Das Deformations und Spannungsverhalten Naturgasspeicher in salzigem Untergrund wird analytisch beschrieben und mit vorhandenen Messwesten veglichen. Dabei werden insbesandere die Absenkunz der Oberflache und der Volumen Schwund der Kavernen vesfolgt. Das dreidimensionale Problem wird fuer eine Kaverne axisymetrisch berechnet, zusammen mit der bekannten Superpositionsmethode. Die berechneten werte stimmer mit den Messergebnissen gut ueberein.

1 INTRODUCTION

The search for a good characterisation of long term behavior of leached rock salt cavities is justified by the need to minimise eventual damages resulting from the subsidence on the surface. In two locations in France, Tersanne and Etrez, a subsidence control has been performed during more than ten years. Simulation by numerical methods allows to perform some predictions on future evolution, and also to test specific numerical procedures. The issue is to obtain a good accuracy in the far field region, e.g. the surface subsidence. For the moment, only Tersanne case has actual in situ measures available. The subsidence data have been already subjected to numerical analyses (Nguyen-Minh et al, 1993), where a two step finite element procedure was used, by taking advantage of the two-layered medium and the elastic response of the cover-layer. Herein, such procedure in not needed as the far field region can be efficiently discretized with boundary elements and special infinite elements (Bettess, 1992). A special procedure is used to speed up numerical calculations (Menezes & Nguyen-Minh, 1993).

TERSANNE FIELD DATA

At Tersanne, located in France, subsidence has been measured continuously for more than ten years (Durup, 1990, 1991). The observation zone (Figure 1) covers an area of 15 Km2, where cavities are leached 1500 m depth, in a thick 650 m bedded salt layer, and are located 150 m beneath the clay cover. Beyond 2000 m depth there is a rigid sandstone base. There are fourteen cavities spaced 600 m in a hexagonal arrangement, each one with an equivalent volume of 268 000 m3. At Etrez there is the same number of cavities with the same equivalent number. We will admit that all the cavities are leached at the same time and that they have the regular distribution as seen in Figure 2. A quasi axisymmetric subsidence pattern can be observed on the surface at Tersanne (Figure 3). The volume loss at the surface to the total caverns volume loss shows out a 60% ratio (Figure 4). As rock salt is believed to behave as an incompressible material, one can suspect a possible dilatancy inside the rock mass, which could affect its tightness.

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