ABSTRACT:

To face the challenges of oil production today, the mechanical behavior of rocks is to be studied under very specific conditions. In particular, thermo-hydro-mechanical couplings are important concerns in geomechanics of the drilling and reservoir engineering. In the Mining Engineering department of the Faculté Polytechnique de Mons (Faculty of Engineering, Mons - Belgium), we developed a new triaxial system to reproduce great depth conditions. Starting with a triaxial cell equipped with pore fluid provisions, a heating jacket and temperature control system, we designed and built a complete automated triaxial device comprising : a stiff loading frame, hydraulic equipments to control the stresses and pore fluid pressure, an electronic interface with a microcomputer, and we also developed a software to drive the tests and process the results. The equipment is designed for confining stresses and pore pressures up to 70 MPa and temperatures up to 232°C. This paper describes this apparatus and also reports the results of tests performed on a porous rock for validation.

1. INTRODUCTION

From a technical point of view, wellbores are deeper than ever before and depths of few thousands of meters are very common. Moreover, a high interest is also shown in HP/HT (high pressure and high temperature) oil fields where the pressure can exceed 100-150 MPa and the temperature can reach values much higher than 100°C (below 200°C in most cases) [1]. Even if we initially only consider conventional reservoirs, rocks are submitted to very different states of stresses, pressures and temperatures in comparison with surface conditions. Stress measurements in underground workings [2] indicate values of few tens of MPa at a few thousands of meters. As far as the formation pressure is concerned, according to case references in European oilfields [3], the reservoir pressure generally does not exceed a value of about 70 MPa in most cases. And finally it is well known that, as a consequence of the geothermal gradient, the temperature can easily reach 90°C at 3,000 m. A reliable modeling of the rock behavior, accounting for all those parameters, is essential. That is why it is important to reproduce depth conditions in laboratory in order to characterize the evolution of rocks behavior and properties.

2. DEVELOPMENT OF THE TRIAXIAL SYSTEM

2.1. Objectives

The triaxial system was developed in order to study the behavior of rocks at great depth. More than ten years ago, a true triaxial testing device was developed in our laboratory to study the effect of confining pressure on rocks [4-7]. This equipment can reproduce complex confining states by applying three different principal stresses on the faces of cubic samples. This device is therefore of high interest to study great depth conditions even if the control of pore pressure is not implemented; in fact, samples can be submitted to stresses of about 500 MPa in each direction. In order to extend the study to thermo-hydro-mechanical effects, new developments were therefore necessary. A modification of the true triaxial cell to include a control of the pore fluid and temperature was impossible to realize. The laboratory then acquired a triaxial cell equipped with pore fluid provisions and a heating jacket to develop a new conventional triaxial testing equipment.

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