Failure Mechanisms in Thick-Walled Cylinder Tests: Numerical and Laboratory Analyses on the Role of Plasticity and Borehole Size on Near Cavity Deformations

Understanding the factors affecting wellbore stability is crucial for safe and cost effective drilling and completion operations. Thick-wall cylinder (TWC) laboratory tests have been widely used to calibrate wellbore stability models. Although the TWC test is meant to simulate the wellbore geometry, the influence of the finite wall thickness makes the results difficult to interpret. To account for these problems, elastoplastic models were calibrated to laboratory tests using the MCC material model. These calibrated models were then combined with finite element analysis to investigate failure mechanisms. Three mechanisms have been postulated: Runaway Instability (RAI), when the strains at the borehole grow uncontrollably; Negative Rate of Work (NRW) of plastic strains which is interpreted to cause borehole spalling; and Considere instability, a catastrophic failure which occurs when global equilibrium conditions cannot be maintained. To investigate the effect of wall thickness and rock properties on TWC failure mechanisms, a suite of core samples was tested using both multistage triaxial and TWC tests. The Modified Cam-Clay model was calibrated to test results for the multistage triaxial and TWC tests for the three rock types (incipiently cemented Miocene Sandstone, Bentheimer Sandstone, Mancos shale, and Austin Chalk). The MCC calibration parameters are stress dependent, meaning the parameters change depending on the mean stress of the experiment run. A more complicated ‘’double yield surface’’ was required in order to describe the most ductile rock type.
A dependence of TWC test results on wall thickness was observed. The ultimate strength at which the TWC collapses increases with increasing wall thickness. Failure mechanisms around the borehole were investigated using high resolution micro CT scanning. The three different failure mechanisms (spalling, borehole closing, and catastrophic) were all observed. The MCC model was used to further investigate stresses and deformation present interior to the wall of the TWC specimen. It was observed that failure occurs in thick-walled cylinder tests when a particular near cavity displacement gradient is achieved for each rock type, this displacement gradient is constant for each rock type independent of wall thickness.