Abstract

This paper presents the findings of a laboratory investigation of the thermomechanical behaviour of anisotropic rock. The tests were performed on natural (Tournemire shale) using triaxial cells modified to control and to go to high temperature. The range of temperatures that were investigated is from 20° to 250°C (20, 100, 150, 200 and 250 C°), and the range of confining pressures is from 0 MPa to 20MPa (0, 5, 10, and 20 MPa). The influence of temperature on their mechanical behaviour was investigated for drained tests, and anisotropic elastic response and plastic deformation have been investigated. It seems that, the thermo-mechanical behaviour of the Tournemire shale is anisotropic and strongly depends on confining pressure and loading orientation at the applied temperature. Hydrostatic compressibility tests (in the perpendicular orientation =90°) allowed to present the thermal effect on the mechanical behaviour of this rock. In this range of temperatures, the deformability and strength of this rock were found to be strongly dependent of temperature.

Introduction

Much research, in recent years, realizes to study the thermo-mechanical behaviour of rocks because of the increasing of the geo-mechanical problems involving the thermal effects.

Many underground works, such as chemical and nuclear waste storage, oil boreholes, injection and production activity, are located in anisotropic rocks and based on the study of thermo-mechanical behaviour. The design and stability analysis of such structures require knowledge of the deformation and failure of these rock materials.

A large number of experimental investigations involving the laboratory tests have been performed. However, most of experimental data reported in the literature is obtained from the tests carried out between the ambient temperature (20 C°) and (100 C°), especially for the clay [1–9]. For a higher temperature, few experimental results are available in the literature; this may be due to the technical difficulties in the experimental works. (Chalaturnyk, 1996), [10], has carried out a detailed experimental investigation on McMurray formation oil sands, McMurray formation shale and Waterways limestone under two different temperatures (22 C° and 220 C°).

The group of sedimentary rocks, termed shales, represents a particular interest in oil industry. Experimental investigations are still necessary to have a better understanding of the thermo-mechanical behaviour of these materials. In the oil industry, the exploitation of heavy oil by the technical injection of vapour at high temperature, the rocks of the reservoir are subjected to coupled thermal and hydro-mechanical efforts. So It is necessary to study the thermo-mechanical behaviour of these materials subjected to variations of temperature in order to study the mechanical stability of the petroleum reservoirs. The oil companies, in their investigations, attach a great importance to the localization of the argillaceous benches in the sedimentary layers. Indeed, these layers can correspond to bed rocks of oils or gas (exits of a mud rich in matter organics deposited in anoxic or hypoxic conditions), but, especially, they represent excellent rocks of cover [11]. On other side, wellbore stability problems represent an extra drilling cost of up to 15% where 80% of such problems occur in shales. The shales are generally anisotropic in deformability and the failure mode is due to their sedimentary structure. This means that much more complex models should be used to describe their mechanical behaviour. Moreover, these rocks are of low porosity and permeability [12].

The object of this study consists in carrying out new experimental study of the thermo-mechanical behaviour of the saturated stiff shales subjected to high temperatures (until 250 C°) and to compressive stresses. The main aim was to carry out extensive laboratory experiments on the thermo-mechanical behaviour of Tournemire shale. The emphasis is given to investigating thermal effect on the elastic response, plastic flow and failure behaviour of the shale. Experimental results presented in this part provide a data base for the development of thermo-elastoplastic modelling and failure criteria.

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