EFFECTS OF POISSONS RATIO AND CORE STUB LENGTH ON BOTTOMHOLE STRESS-CONCENTRATIONS

Core disk fractures are produced by the concentration of in situ stres ses at the wellbore bottom hole cavity. In order to better understand core disking, these stress concentrations were calculated using detail ed three-dimensional finite element modelling of a buttomhole geometry with a variety of core stub lengths. Biaxial compression applied perp endicular to the wellbore axis induces high tension at the root of the core stub, This tension is reduced when a uniaxial compression direct ed parallel to the wellbore axis is applied, A state of incipient cove disking consequently depends on the relative magnitudes of these in s itu stresses. Hypothetical incipient failure curves derived from the m odelling are in good agreement with early experimental results, and in dicate 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 obser vations. The magnitude of the sti ess concentrations depend strongly o n Poisson's ratio and the stress concentrations are higher in material s with small Poisson's ratios. The length of the core stub influences the magnitudes of the concentrated stresses with tensions increasing t o a maximum for normalized core stub lengths of 0.25, Additional hypot hetical failure curves for differing core stub lengths suggest that co re disk thickness can aid in the estimation of in situ stress magnitud es. (C) 1997 Published by Elsevier Science Ltd.