Effects of Poisson's ratio and core stub length on bottomhole stress concentrations

Abstract

Core disk fractures are produced by the concentration of in situ stresses at the wellbore bottomhole cavity. In order to better understand core disking, these stress concentrations were calculated using detailed three-dimensional finite element modeling of a bottomhole geometry with a variety of core stub lengths. Biaxial compression applied perpendicular to the wellbore axis induces high tension at the root of the core stub. This tension is reduced when a uniaxial compression directed parallel to the wellbore axis is applied. A state of incipient core disking consequently depends on the relative magnitudes of these in situ stresses. Hypothetical incipient failure curves derived from the modeling are in good agreement with early experimental results, and indicate that the core disks produced under a combined state of vertical and uniform horizontal farfield stresses result from tensile fracture. A Mohr-Coulomb shear mechanism cannot explain the experimental observations. The magnitude of the stress concentrations depend strongly on Poisson's ratio and the stress concentrations are higher in materials with small Poisson's ratios. The length of the core stub influences the magnitudes of the concentrated stresses with tensions increasing to a maximum for normalized core stub lengths of 0.25. Additional hypothetical failure curves for differing core stub lengths suggest that core disk thickness can aid in the estimation of in situ stress magnitudes.