Abstract

This article summarizes the results of numerous laboratory tests, aimed at describing the poro-mechanical behaviour of chalk from Lezennes in northern France. This type of material makes up the reservoir rocks in the North Sea and fluid-skeleton coupling is frequently to be found in them. They partly enable subsidence which can adversely affect the stability of oil-rigs to be described. The chalk tested in this case is highly sensitive to the hydrostatic component of the load which can bring about an irreversible collapse in the porous structure. Our study uses the macroscopic approach to porous material initially developed by Biot [Biot, 1947] in the field of the thermodynamics of open continuous media. Adopted in France by Coussy [Coussy, 1991], this approach uses the rocks to bring out the essential notions of plastic, elastic or effective stress, which lead to elastic or irreversible deformation in the rock skeleton. Our tests are carried out in drained or undrained loading which firstly allows for a characterization of the isotropic compressibility of the material and its matrix by simultaneous measurement of deformation and of the variation in the contained fluid mass. Secondly a further triaxial test is proposed. This test enables a conclusion to be drawn as to the relevance of using Terzaghi effective stress hypothesis--wellknown in soil mechanics--for such a type of chalk tested at high confining levels.

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