The Stability of Inclined and Fractured Wellbores

Abstract

SummaryThe theory of linear-elastic fracture mechanics is used to develop an analytical solution for a wellbore in an isotropic elastic medium when drilled inclined to a general state of 3D far-field stress, and attached to which are an arbitrary number of N straight and axially aligned fractures. Axial alignment refers to the special case where the borehole axis is the existing common interface of all planes defined by the fracture faces. The solution uses a familiar load decomposition and coordinate-transform scheme. Within the fracture-mechanics context of the analysis, this scheme translates into a set of three fundamental subproblems comprising a uniaxial stress problem, together with an in-plane mixed mode (Modes I and II), and a Mode III antiplane-crack/cavity-interaction problem.The overall solution is obtained by superposing the solutions to these subproblems. A numerical example is presented to demonstrate its usefulness in the stability analysis of inclined and fractured wellbores. Attention has been directed toward the underlying significance that the results would bring in fundamental understanding of lost-circulation events. For this purpose, the criteria for a possible extended margin of the mud weight that secures stable states of a fractured wellbore are recognized and quantified. These criteria include the wellbore-wall refracturing and the existing fractures propagation.