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Mechanical properties of North Sea Tertiary mudrocks; investigations by triaxial testing of side-wall cores

L. Wensaas, P. Aagaard, T. Berre and E. Roaldset
Mechanical properties of North Sea Tertiary mudrocks; investigations by triaxial testing of side-wall cores (in Clay minerals in the modern society, D. C. Bain (editor))
Clay Minerals (March 1998) 33 (1): 171-183


In the North Sea Tertiary section, wellbore instability problems are frequently reported in Palaeocene-Early Oligocene smectite-rich mudrocks. Analysis of the mechanical properties of these Tertiary mudrocks is generally hampered by the lack of suitable core material. This study represents an attempt to study the geomechanical behaviour of mudrocks by triaxial tests of side-wall cores obtained from the borehole wall. The tests performed include measuring the changes in pore pressure during shearing and undrained shear strength in specimens initially consolidated to in situ effective stress levels. The coefficients of permeability (kf), estimated from the consolidation time behaviour range from 2.6 X 10 (super -11) to 2.4 X 10 (super -12) m/s. The tested cores behaved like slightly overconsolidated to normally consolidated materials with an initial near constant volume (elastic behaviour) for low deviatoric load followed by an increasingly contractant behaviour approaching failure. Compared with results from onshore analogues, the strength properties of the investigated mudrocks appear to be related to their content of expandable clay minerals. A wellbore stability chart to forecast adequate drilling fluid pressures for future wells has been developed by the use of linear (Mohr-Coulomb) failure criteria based on the peak strength data. It is demonstrated that side-wall cores can provide satisfactory test materials for rock mechanical analysis, and their use may serve to improve our knowledge of the rock mechanical behaviour of typically troublesome mudrocks for which no conventional cores are available.

ISSN: 0009-8558
EISSN: 1471-8030
Serial Title: Clay Minerals
Serial Volume: 33
Serial Issue: 1
Title: Mechanical properties of North Sea Tertiary mudrocks; investigations by triaxial testing of side-wall cores
Title: Clay minerals in the modern society
Author(s): Wensaas, L.Aagaard, P.Berre, T.Roaldset, E.
Author(s): Bain, D. C.editor
Affiliation: University of Oslo, Department of Geology, Oslo, Norway
Affiliation: Macaulay Land Use Reserch Institute, Aberdeen, United Kingdom
Pages: 171-183
Published: 199803
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
Meeting name: Rosenqvist symposium
Meeting location: University of Oslo, NOR, Norway
Meeting date: 19960519May 21, 1996
References: 32
Accession Number: 1998-075277
Categories: Engineering geology
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch map
N59°15'00" - N59°30'00", E02°00'00" - E02°19'60"
Secondary Affiliation: Norwegian University of Science and Technology, NOR, Norway
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland
Update Code: 199824
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