ABSTRACT:

Improved laboratory techniques for measuring deformations of triaxial specimens of soft rocks are described, and the stress-strain behaviour of an uncementeded and a strongly cemented clay-shale during Ko-consolidation (loading and unloading) and subsequent shearing, are compared. The displacement measurements are made directly on the surface of the specimen, thus reducing the uncertainties connected with measurements made outside the rubber membrane. Internally measured vertical strains (i.e. measured directly on the specimen to avoid end effects) are compared with externally measured vertical strains. The Ko-values during loading were very similar for the two clay-shales, approximately 0.6. However, during unloading, while Ko for the uncemented material increased to 0.9 at an over consolidation ratio of 2.5, the value for the strongly cemented material decreased to 0.35.

RESUME:

L'article decrit des techniques de laboratoire ameliorees pour mesures les deformations de roches tendres en essai triaxial, et presente une comparison du comportement contrainte-deformation durant la consolidation Ko (chargement et dechargement) suivie d'un cisaillement pour une argile schisteuse non-cimentee et une seconde fortement cimentee. Les mesures de deplacement sont faites directement a la surface de l'echantillon, reduisant ainsi les incertituedes reliees aux mesures faites sur une membrane. Les deformations verticales internes (i.e. mesurees directement sur I'echantillon afin d'eviter les effets de longueur) sont cornparees aux deformations verticales externes. D'importantes differences sont notees entre les valeurs Ko pour des argiles schisteuses cimentees et non-cimentees.

ZUSAMMENFASSUNG:

Verbesserte Methoden zur Deformationsmessung an triaxialen Proben weicher Gesteine werden beschrieben. Es wird das Spannungs- Dehnungsverhalten eines nicht zementierten mit dem eines stark zementierten Tonschiefers wahrend Ko- Konsolidierung (Belastung und Entlastung) mit nachfolgender Scherung verglichen. Um Unsicherheiten verbunden mit Messungen auf der Membranauβenseite zu verringern, werden die Deformationsmessungen direkt auf der Probenoberflache vorgenommen. Intern (die Messungen werden direkt an den Proben vorgenommen urn Endeffekte zu vermeiden) und extern gemessene Vertikaldeformationen werden miteinander verglichen. Beide Tonschiefer hatten ahnliche Ruhedruckbeiwerte unter Belastung (ca. 0.6). Unter Entlastung stieg der Ruhedruckbeiwert des nicht zementierten Tonschiefers auf 0.9 bei einem Überkonsolidierungsverhaltnis von 2.5, wahrend der Ruhedruckbeiwert des stark zementierten Tonschiefers auf 0.35 abnahm.

1
INTRODUCTION

In connection with petroleum exploration and reservoir mechanics, the Norwegian Geotechnical Institute (NGI) is engaged in the experimental determination of geotechnical properties and stress-strain behaviour of hard soils and soft rocks. Reservoir cap rocks like clay-shales and reservoir rocks like sandstones and chalks are being studied (Berte et al., 1994). The large compaction of the Ekofisk reservoir in the North Sea and the resulting dramatic seabed subsidence and accompanying well problems, have led to intensive studies of the reservoir chalk and the stress strain behaviour of the overburden (Sulak, 1991, Gutierrez et al., 1994). The paper is focused on the laboratory measurement of strains and Ko-values when performing triaxial tests on small samples. Ko- the effective lateral stress coefficient at rest, is defined as the ratio between effective horizontal stress (or stress increment) and effective vertical stress (or stress increment) when no horizontal strain is allowed. This is commonly denoted as a uniaxial or one-dimensional strain condition. Ko is an important parameter for defining in-situ stress conditions. Besides, many reservoirs are very wide compared to their thickness, often disk-shaped, so an approximately one-dimensional deformation condition develops during pressure depletion and petroleum production. In the following deformation measurement techniques for triaxial testing at NGI are described. Furthermore, the stress-strain behaviour for an uncemented and a strongly cemented clay-shale are compared.

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