ABSTRACT:

The objective existing in-situ stress field and the physical mechanical properties of rock are closely related to the borehole stability in petroleum engineering. However, in present engineering design, rock mass is simply treated as isotropic material. This method may be acceptable when the well is shallow, but for deep rock engineering, with the increase of drilling depth, the anisotropic properties of rock mass become stronger and need be considered. This paper sets out to further understand the actual situation of the well site taking anisotropy into account. It investigates the effect of the rock anisotropy in the stability and the stress distribution at the wall of various well bores. The closed-form solution for stress distribution in transversely isotropic materials was used on the basis of rock experiments. A program was developed to compute the stress components at the inclined borehole wall whereby sensitivity analyses were performed to analyze the effect of the elastic moduli, stress anisotropy, arbitrary orientation of the borehole, the in-situ stress and the bedding inclination. Tensile and shear failure criteria were considered for the borehole stability analysis.

1 INTRODUCTION

Borehole stability means wellbore, drilled to get hydrocarbons from underneath, keeps its regular size and shape during drilling process. It's a core technology guaranteeing drilling safety and high well quality (Jin & Chen 2012). According to statistics, cost on borehole instability worldwide is more than 6 billion dollars annually. Therefore, actively developing study on borehole stability is of significant economic consideration (Chen et al. 2008). Harold (1940) initialed borehole stability mechanics study on wellbore stress distribution. However, Santarelli et al. (1992), Franck et al. (2003), Helstrup et al. (2004), Germanovicha et al. (2000), Chen et al. (2005), Huang RZ et al. (1995), and Jin et al. (2004) have already assumed that surrounding rock and its stress distribution are isotropic, which is acceptable for shallow rock engineering. With the increase of drilling depth, encountered geological body presents various crossed structural planes (joint plane, fracture plane or bedding plane), and surrounding rock shows strong anisotropic characteristics. Generally coal seam gas reservoirs consist of sedimentary rocks with smooth laminar structures. Under normal conditions, the elastic modulus perpendicular to the bedding plane is slightly greater than that in the direction parallel to the bedding plane, and Poisson's ratio as well as other mechanical parameters, also vary between these two orientations.

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