Fracture Initiation From Inclined Wellbores in Anisotropic Formations

Abstract

Summary This paper presents a wellbore stability design code that can be used to optimally drill, complete and stimulate inclined wells. The design code was developed for the most general case where the borehole, in-situ stress, and rock property coordinate frames are totally independent of each other. It essentially consists of a three-dimensional linear elastic stress analysis in anisotropic formations. Although this particular paper is only limited to tensile failure, based on the minimum normal stress theory, the numerical model is also able to consider shear failure by involving a generalized three-dimensional anisotropic yield criterion. The design code enables the selection of the most optimum borehole orientation and wellbore fluid gradient to avoid fracture initiation. It is also useful in conducting sensitivity studies, particularly when the data is scanty. A remarkable finding by exercising such code was that fracture initiation is strongly dependent on the formation an isotropy; a contradiction to the current industry thinking. Introduction Sedimentary rocks, because of their depositional environments, have a laminated structure with directional properties which are best described as transversely isotropic. Rahn showed that the moduli of elasticity of a foliatedrock can vary by about 50%, if measured normally and parallel to the beddingplane. Shale, a common sedimentary rock which accounts for 75% of the drilled sections in the world and causes 90% of the wellbore stability problems, is a transversely isotropic material. Coal, another sedimentary r6ck commonly encountered during drilling of shallow formations, is an orthotropic material which has also been known to cause considerable borehole instability problems. Fracture initiation occurs when the stress at the borehole wall, or its vicinity, goes into tension and exceeds the rock's tensile strength. This usually happens when there is an excessive wellbore pressurization such as during hydraulic fracturing stimulations or during highly overbalanced drilling operations. Traditionally, fracturing due to tensile splitting of the rock was predicated using the linear elastic stress distribution around a wellbore in homogenous and isotropic media. However, in view of the inadequacy of simple isotropic models to describe the behavior of real rocks, a more complex model describing more realistically the rock response, needed to be developed.