The authors have previously presented an anisotropic poroelastic constitutive model for application to wellbore stability modelling. To obtain the required parameters, triaxial test apparatus has been developed, which is specially designed for low permeability shales. The development of the test apparatus is described. A series of consolidated undrained triaxial tests and novel tests, referred to as H tests, was carried out on an anisotropic shale. The test procedures and the measured elastic and poroelastic parameters are presented and discussed.
Precedemment, les auteurs avaient presente un modèle constitutif de la poro-elasticite anisotrope en vue d'application à la modelisation de la stabilite des forages. A fin d' obtenir les paramètres recherches, un equipment d' essais triaxiaux a ete conçu et developpe pour I'etude du schiste à faible permeabilite. Une description de la mise au point de l'equipment d'essais est donnee. Une serie d'essais triaxiaux non-draines consolides ainsi que de nouveaux essais designes tests H out ete realises sur du schiste anisotropique. Le papier presente la methode d'essai utilisee et discute les paramètres elastiques et poro-elastiques obtenus.
Die Autoren haben bereits ein anisotropisches poroelastisches konstitutives Modell fuer Anwendung zur Bohrlochstabilitaetsmodellierung vorgestellt. Um die gesuchten Parameter erhalten zu koennen, wurde dreiachsiger Versuchsapparat, der speziell fuer niedrig-permeabele Tone geeignet ist, entwickelt. Die Entwicklung des versuchsapparates ist beschrieben worden. Eine Reihe von konsolidierten, undraenierten dreiachsige Versuche, die als H-Versuche bezeichnet worden sind, wurde an einem anisotropischen Ton durchgefuehrt. Die Versuchsprozedur und die gemessenen elastischen und poroelastischen Parameter sind dargestellt und diskutiert worden.
Wellbore instability, a major cause of drilling problems in the oil and gas industry, occurs mainly in shale formations. Since shales are fairly porous and saturated in buried formations, pore pressure behaviour should be incorporated in the mechanical constitutive model. In addition, the laminated texture of shales results in anisotropy of stress-strain relationship as well as of strength properties. This complex mechanical behaviour of shales has not been studied extensively although some studies of pore pressure behaviour (e.g. McLean 1989) and anisotropy effects (e.g. Aadnoy 1987) for this application have been conducted. The authors have previously presented a constitutive model for anisotropic rocks based on the theory of poroelasticity (Aoki, et al, 1993 & 1994) which was extended from the theory for isotropic porous rocks (Biot 1941; see also Detournay and Cheng 1993). Experimental data for the anisotropic poroelastic parameters in the constitutive model are unavailable in the literature, to the best present knowledge of the authors. A research study has been conducted to develop the test procedures and the test apparatus required to determine the poroelastic parameters of transversely isotropic low permeability shales for application to wellbore stability modelling. This paper describes the requirement and development of the test apparatus, specially designed test procedures and test practices, and presents a discussion of the test results. This study forms part of the research program on wellbore stability conducted at the Australian Petroleum Cooperative Research Centre, CSIRO Division of Petroleum Resources (Tan, et al. 1994).
Shales encountered during drilling of oil and gas wells have extremely low permeabilities, in the microdarcy to nanodarcy range. However, they are of relatively high porosity. In the in-situ state, the shales are saturated with ground water and subjected not only to the ground pressure (in-situ stresses) but also to ground water pressure (pore pressure). They may additionally be at a considerably higher temperature than that occurring at the surface, because of the geothermal gradient. To obtain the mechanical properties of the shales, triaxial compression test apparatuses have commonly been used to simulate the in-situ stresses, pore pressures and the loading conditions. This test technique would be suitable provided that the pore pressure changes can be monitored accurately. The shale around a wellbore is likely to deform in an undrained condition shortly after being drilled due to its low permeability and hence, such a condition is required for the loading stage of a triaxial compression test. This type of triaxial test is commonly referred to as a consolidated undrained test.
