A case study on time-dependent bedding plane failure

Time-delayed wellbore failures are often observed in shale formation. In some cases, cavings indicating bedding plane failure are observed a few days after initial drill-out with a gradual increase, eventually leading to wellbore collapse. Possible causes of time-delayed wellbore failure include pore pressure diffusion, and its impact can be assessed by traditional poroelastic modeling in which coupled hydraulic-mechanical processes are taken into account. Externally applied loads induce pore pressure that is dissipated according to a diffusion law. During recently conducted wellbore stability analysis targeting a particular field within a strike-slip regime, we found: (1) time-dependent bedding plane failures are significant for highly inclined wells subparallel to the maximum horizontal stress direction (bedding plane failure region gradually increases with time), (2) the Kirsch solution and the assumption of no induced pore pressure (elastic nonporous model) does not give proper mud weight limits to avoid bedding plane failures, and (3) failure regions and modes predicted by the poroelastic model are different from those based on the elastic nonporous approach, under the given situation. Results given by the poroelastic model are apparently consistent with filed observations, demonstrating pore pressure diffusion as a possible cause of observed time-delayed bedding plane failure.