Micromechanical Study of Borehole Breakout Mechanism

Abstract

Borehole breakouts are zones of enlargements of the wellbore cross section that form as a result of rock failure at the wellbore wall during drilling. The analysis of borehole breakout is essential in addressing wellbore stability, well completion, and sand production problems. This research numerically investigates the effect of the material micro and macro-parameters on the failure mechanism and the geometry of the wellbore breakout. A three-dimensional discrete element method was used in the simulations. In this research, a systematic methodology was developed for calibrating material micro-properties. Next, the calibration performance was assessed by simulating thick walled cylinder tests and comparing the model results against laboratory measurements. The model was also verified against analytical solutions for a TWC sample under plane strain axisymmetric conditions. The numerical tool was then used in a series of parametric simulations of drilling conditions. These simulations allowed the relating of the breakout type to the sandstone’s micro-properties. To be able to relate the breakout type to the macro-mechanical properties, a series of triaxial testing simulations were conducted to relate the micro and macro-mechanical material properties together. The results showed that the geometry of the breakout was affected by micro-parameters such as the particle contact modulus, the parallel bond normal and shear strengths, the particle crushing strength, and the particle size distribution. In addition, it was found that the macro Young’s modulus, friction and dilation angles, and the uniaxial compressive strength also affect the type of breakouts. The triaxial testing simulations showed that: (a) Young’s modulus is not just affected by the particle contact modulus, but also the friction coefficient between particles and the percentage of bonded contacts, (b) the dilation angle is a function of the particle contact modulus, percentage of bonded contacts and inter-particle friction, (c) the friction angle is not only affected by the friction coefficient between particles, but also by bond strengths, (d) cohesion is not just affected by bond strengths and the percentage of bonded contacts, but also by the friction coefficient between particles, and (e) the post-peak modulus is affected by the percentage of bonded contacts and inter-particle friction.