ABSTRACT:

Experimental study of the behaviour of a sandstone is firstly presented. Triaxial tests, reduced extension tests are performed. The obtained results show clearly progressive development of microcraks in the plane perpendicular to the minor principal stress. A conceptual damage model is proposed to describe the stress induced microcracking of the rock. This model will be applied to the analysis of an horizontal well. The numerical results show very cleary that the stress and strain distribution around the wellbore is strongly influenced by the development of microcracks. Comparisons with the classical elastic model and a pressure dependent elastic model will be presented.

RESUME:

Des resultats des essais triaxiaux et d'extension laterale effectues sur un gres montrent tres clairement Ie phonomène de microfissuration induite dans le plan perpendiculaire à la contrainte principale mineure. Un modele incremental est propose pour decrire ce type de comportement. Ce modèle est utilise pour I'analysis d'un puits horizontal axisymetrique. Les resultats numeriques permettent de montrer que la dIstribution des contraintes et des deformations autour du puits est fortement influencee par la procedure de microfissuration. Des comparaisons avec la theorie d'elasticite lineaire et un modèle elastique dependant de la pression de confinement seront presentees.

ZUSAMMENFASSUNG:

Zunachst wird eine experimentelle Sudie des Verhalten von Sandstein prasentiert. Die Resultate zeigen deutlich die progressive Entwicklung von Mikrorissen in der Ebene orthogonal zur Richtung der niedrigsten Hauptspannungen. Ein konsntitutives Gesetz, basierend auf dem Konzept der Materialschadigungs-theorien zur Beschreibung del' spannungsinduzierten Mikroribbildung in Fels wird Vorgestellt. Dieses Modell wird zur Analyse eines horizontalen Brunnens angewandt. Die numerischen Ergebnisse zeigen deutlich den Einfluss der Mikrorisse auf die Spannung undo Dehnunngsverteilung am Bohrloch. Vergleiche mit dem klassischen elasuschen Modell und einem druckabhangigen elastischen Modell werden gezogen.

1 INTRODUCTION

The problem of predicting the occurence and the extension of failure near a borehole has a great importance in petroleum operations (drilling, production, stimulation). This problem is directly related to the constitutive modelling of rock materials. The classical linear elasticity theory is now considered as to be valid in a very restricted applications. Improvement of more, realistic constitutive models is one of the most important research subjects in geomechanics during the last decades. The extended plasticity theory has been successfully used in soil mechanics. For rock materials, the plasticity theory has been also used for some rocks where the failure is given by shearing strain or the localization of this one. Let us mention the results presented by Cristescu (1987), Desai and Salami (1987), Shao and Henry (1991). But for other many rocks, the anelastic deformation is mainly due to the initiation and propagation of microcracks in the material. The classical damage mechanics theory has been extended to describe such behaviour. Because the damage in rock is generally anisotropic and induced under compressive stress, only a few of quantitative resultats are now available and more development is still necessary. It is obvious that the development of a constitutive law is a very complex work. It requires a good knowledge of mechanical behaviour of the rock and an appropriate process of validation. It is impossible to obtain an universal model which would be able to describe the behaviour of all rock materials. It is very useful to have specific constitutive laws for a certain range of applications. For the well bore stability problem in brittle rock, one of the main effects of creating an underground excavation is generally to result in a large tangential stress and a small radial stress.

This content is only available via PDF.
You can access this article if you purchase or spend a download.