ABSTRACT:

A novel model to predict Failure Geometry Stabilization (FGS) of breakout in shales has been developed and quantitatively validated against breakout width and depth generated in six large scale polyaxial block tests conducted on two outcrop shales under different stress regimes, and observed in five study wells. The stabilized breakout geometries of the tests were laser scanned and contoured for use in validation of the FGS model. The model is able to predict breakout initiation, progression and stabilization of the failure geometry through removal of failed cells based on developed criteria. It was discovered that breakout width stabilization is governed by equivalent plastic shear strain threshold whilst breakout depth stabilization is governed by equivalent plastic shear strain and volumetric strain thresholds. “First-of-its-kind” correlations for shales were subsequently developed for equivalent plastic shear strain and volumetric strain thresholds against elastoplastic hardening parameters based on the polyaxial block test and triaxial test results, and more than sixty simulation cases on wells in various fields. In addition, a pragmatic methodology for application of the novel FGS model for planned wells has been developed. It predicts tolerable stabilized breakout geometry that will not pose any significant drilling problems and hole cleaning issues, and optimizes mud weight program and casing setting depth.

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