A series of triaxial extension tests have been performed on high porosity chalk, and a failure criterion has been determined. In extensional failure the chalk may undergo large deformations at constant stresses (ideal plastic behaviour). The failure mechanism is shear failure, except at very high stresses where also compaction (pore collapse) may be important.


Une serie d'essais triaxiaux d'extension a ete effectuee sur une craie de haute porosite, et des conditions critiques de rupture sont deterrninees, La rupture dans des conditions d'extension est caracterisee par des grandes deformations sous des contraintes constantes (comportement plastique ideal). La mode de rupture est surtout rupture par cisaillement, sauf a des contraintes elevees ou compactation (pore collapse) aussi peut jouer.


Eine Serie triaxialen Extension Versuchen ist in hochporosen Kreide durchgefurt, Eine Kurve des Kritischen Bedingungen ist festgestelt. Unter kritischen Bedingungen wird die Kreide unter Konstanter Spannung deformirt (idealer Plastische Vervormungs-Bedingungen). Die Fehlermechanismus wird von Schubspannung bestimmt. Bei sehr hohen Spannungen wird auch Zusammenfall der Poren der wirksame Mechanismus.


Compaction and well stability problems associated with chalk reservoirs in the North Sea have led to extensive research on the properties of high porosity chalks. However, most of the laboratory work so far refers to compressive type tests. Very little has been reported on extension tests, although this type of tests is closer to the conditions existing around wells and perforation cavities. This paper presents the results from a series of triaxial extension tests on high porosity chalk. After compression in hydrostatic loading, the cylindrical samples were allowed to expand axially at constant radial confining pressure. The samples were saturated with equilibrium water and tests were run at drained conditions. The chalk chosen for this work came from the quarry of Lixhe near Liege in Belgium. This Upper Campanian chalk has mechanical properties similar to reservoir chalks, and it is available in sufficient quantities.


2.1 Test equipment The test cell used in this work was made by GeoDesign, France, and later modified for extension tests. This type of cell is hydraulically operated so that no external loadframe is needed. The cell is run with two pumps (Gilson 307), one for the confining pressure and one for the axial circuit. The cell is designed for cylindrical samples with 37 mm diameter. Maximum sample length for compressional tests is 75 mm. The maximum stroke of the axial piston is 20 mm. The axial piston assembly consists of three parts. The lower part is connected to the sample and the sleeve. The middle part is constructed so that the confining pressure in the cell is applied downwards to balance the upward push. This assures hydrostatic loading of the samples if no pressure is applied in the axial circuit. The upper part is designed to give downward push in compressional tests. In the solution chosen, the lower piston part is made with a groove containing a spring loaded split retainer ring. This ring will hook up into a cylindrical housing attached to the middle piston. By the end of the experiment, the lower part is again made free, by a release ring in the housing that is forced downwards to compress the retainer ring. This is achieved by release pins that will be forced downwards when the housing comes close enough to the ceiling of the cell. The release point can be set by choosing proper length of the release pins. The modified cell is illustrated in Fig. 1. Details of the connector system is given by Kaarigstad (1995).

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