Drained triaxial tests, including stress path studies, have been conducted on a selection of sedimentary rocks to investigate strength behaviour with reference to wellbore instability. The tests were undertaken using 'state-of-the-art', servo- controlled equipment, and specimens were subjected to conditions of elevated temperatures, stresses and pressures. From data obtained, established failure models were compared to alternative models developed from the critical state concept. Stress distributions around a wellbore have been computed, using finite element solutions, to investigate the influence of different peak strength criteria.


The occurrence of mechanical instabilities, which can result in complete or partial collapse of wellbores, remains a continuing problem in the drilling of deep petroleum boreholes. Instabilities may occur during or shortly after drilling, or may develop later during the operation of the well. Such time dependency can be introduced by creep or swelling behaviour, or by a gradual increase in the effective stresses around the wellbore as pore pressures are reduced on production. Amongst the most important factors governing the integrity of a borehole at any time is the load bearing capacity of the rock immediately around it. Similarly, of vital importance in predictive modelling or back analysis of wellbore instability, is the availability of appropriate and reliable strength data, and the selection of criteria which can describe the peak strengths of the materials. Pan and Hudson (1988) have reviewed a number of strength criteria, any of which could be applied to the problem of wellbore instability, but it is the long-established criteria, including those by Drucker-Prager and Mohr-Coulomb, which remain those most commonly used in wellbore applications. however, many such criteria were developed to describe not only the strength behaviour a sedimentary rocks, but also of igneous and metamorphic rocks, soils and engineering materials. Consequently, they are often inadequate for describing the peak strength of weak rocks, such as those in which wellbore instabilities often occur. One such criterion, and an alternative semi-empirical criterion, are considered in this paper to describe the behaviour of some sedimentary rocks tested under simulated petroleum environments. Several individual and joint programmes of research have been undertaken at Imperial College (Santarelli et al. 1986, Nakken et al. 1989) to investigate many aspects of the mechanical behaviour of intact sedimentary rocks in petroleum applications. This paper focuses data from two such projects (Marsden, Wu), in which strength behaviour of sedimentary rocks in wellbore environments, and the application of critical state concepts to borehole instability, have being investigated. The work has included a large number of high quality laboratory tests, including conventional and stress path triaxial studies, at room and elevated temperatures. The sedimentary rocks investigated were a limestone, a shale and several sandstones with varying degrees of anisotropy. The tests, were undertaken using a 'state of- the-art', closed-loop, servo-controlled rock mechanics test facility, capable of high quality thermo-hydro-mechanical acoustic testing under conditions of elevated temperature and pressures, for a wide range of petroleum, civil and mining engineering applications.

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