Abstract

This paper discusses an anisotropic model for assessing the mechanical stability of a deep borehole subjected to an internal wellbore pressure and a far-field stress tensor. The model consists of the three-dimensional analysis of stress concentrations around an inclined borehole due to the far-field stress tensor, combined with internal wellbore pressurization, flow and thermally induced stresses to give an approximate non-coupled solution. The model utilizes a generalized three-dimensional anisotropic strength criterion for assessing borehole collapse which is one manifestation of shear failure. Parametric studies indicated that the stability of the wellbore is significantly influenced by high degrees of rock anisotropy, high in-situ stress differentials and excessive wellbore cooling. Pore pressure and porous elastic constant also affect the stability but these effects are less pronounced.

Introduction

In recent years, the petroleum industry has experienced a rapid surge of horizontal well completions; primarily due to the advances in well drilling technology which have resulted in substantial reduced costs. The emergence of horizontal well technology also provided the industry the impetus for a large number of new and diverse applications, among others, to optimize production from naturally fractured reservoirs and to improve volumetric recovery from non-blanket hydrocarbon reservoir.

However, borehole instability can become a major concern in horizontal wells and problems have been reported more frequently when the inclination increases. These instabilities resulted in loss of time and sometimes of equipment with estimates running into the $100 millions per year world-wide. The stability of a borehole is therefore of fundamental importance to the industry.

Recently, concerted efforts have been directed towards solving rock mechanics problems associated with borehole stability. The increased demand for such analyses has arisen from the need to better understand the potential failure mechanisms. This need, which stems mainly from economic considerations, has been encouraged by the tremendous improvements in the predictive analytical method. The usual assumption of rock, in-situ stress and rock strength isotropies found in most earlier models have been deemed to be inadequate in describing failure in actual field conditions. Sedimentary rocks, because of their depositional environments, have laminated structure with directional properties which are best described as transversely isotropic. In addition, in tectonically active areas, the in-situ stresses can also exhibit large anisotropic characteristics.

This paper discusses a model for assessing the potential mechanical instability of a well that is based on an analytical approach of the classical rock mechanics analysis of an anisotropic homogeneous body, bounded internally by a cylindrical surface. The model is based on the concept of generalized plane strain linear elasticity, and all plastic and time-dependent effects are neglected. The model consists of a three-dimensional analysis of stresses around a borehole due to far-field stress tensor, combined with internal wellbore pressurization, flow and thermally induced stresses to give an approximate non-coupled solution.

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