ABSTRACT
When analyzing borehole stability, two failure mechanisms are generally considered, namely, failure due to either tensile or compressive (shear) stresses. These mechanisms are produced respectively by either too high or too low drilling fluid pressure. Mechanical and hydraulic properties of rock masses and in particular sedimentary rock masses may show a considerable degree of spatial variability. This paper describes procedures for the evaluation of the limits for the internal pressure associated with a target probability of failure taking into account both the spatial variability of hydraulic and mechanical properties and the simple variability of the initial pore pressure and insitu stresses. The analysis is performed with a finite element program that incorporates coupled fluidmechanical effects and elasto-plastic behavior of the rock mass. In this way, the proposed borehole stability analysis can be set into a reliability based framework as described in the paper. Examples are shown and conclusions are drawn regarding the effect of spatial variability on the borehole stability. Available stochastic data from the literature were used in the analyses. The analysis is carried out both in 2 and 3D. When analyzing the stability of boreholes, two mechanisms leading to instability
1 INTRODUCTION
The numerical analysis of geomechanical problems of petroleum related problems poses considerable challenges. One of the questions involved relate to the scale of problems in hand. Generally, problems can be considered as being at the reservoir scale or the borehole scale. Traditionally, reservoir simulation has been carried out at the reservoir scale and basically involved in the past, the analysis of flow phenomena. In recent years however, a growing interest appeared in the analysis of geomechanical coupling at that scale. At the same time, the interest in the simulation of detailed coupled phenomena at the borehole scale grew. The present paper focuses at the latter problems. Simulations using continuum based formulations and pore-scale formulations are considered. Some specific details of the implemented formulations are described in a companion paper in this volume (Vargas et al., 2012). For the continuum based analyses, finite element implementations were carried out. For pore scale simulations, discrete element method coupled with lattice-Boltzmann procedures were implemented. The paper will also focus on some particular problems namely borehole stability and sand production problems.
