Numerical Modeling of Wellbore Instability (TENSILE Failure) Using Fracture Mechanics Approach

Abstract

When a well is drilled, the equilibrium in-situ stress is changed. In order to support the stress relief induced by the drilling and to prevent hydrocarbon influx into the cavity, the borehole is filled with a fluid. These operations create new stress configurations. The main point in wellbore projects is the definition of the drilling fluid density to keep the wellbore stable. The lower bound to the fluid density is the collapse stress that is the limit to shearing. The upper bound is the fracture stress that limits the tensile failure. The fluid densities between these limits is named safe mud weight window. Conventional wellbore stability analysis usually considers the effects of shear or tensile failure using failure criteria that are modeled based on the strength of the formation. This thesis uses numerical finite element method techniques to simulate cracking phenomena that can lead to instability of well configurations within and between shale formations that are relevant to oil wells under pressure. The range of critical conditions associated with possible crack lengths are established by equating the computed crack driving forces to the ranges of published fracture toughness data reported in earlier studies. The ranges of pressures associated with upper mud weight drilling pressure are thus established and compared with the prediction from empirical theories.