An evaluation of viscous deformation of chalk on wellbore stability

Wellbore instability has become an increasing concern for horizontal wells, especially with the move towards using drilling innovations that the entire lateral sections are remained without casing during the lifetime of the reservoir and after plug and abandonment operations. We aim to understand the long-term effect of viscous deformation on wellbore stability for horizontal wells through advanced laboratory experiments and viscoplastic simulations. Two-stage single lateral hole tests were conducted on chalk samples, where a horizontal wellbore was drilled laterally at the center of a cylindrical specimen and loaded under triaxial condition with a constant stress ratio. The chalk samples were imaged at two stages in the creep phase using X-ray computed tomography CT(scanning) to precisely observe the breakout development of the damaged zones formed around the borehole. The results from the experiments and CT analysis confirmed model predictions. Results presented in this work clearly demonstrate that viscous deformation can trigger failure modes and induce breakout development around the wellbore in the creep phase.