Wellbore stability in shale is a crucial issue for drilling in all kinds of environments. The analysis of time-dependent wellbore stability in shales has largely concentrated on the influence of fluid chemistry and filtrate invasion into the formation to predict compressive failure using continuum models. Another possible mechanism for time-dependent behavior is stress corrosion cracking (subcritical crack growth), an important mechanism for the growth of natural tensile and shear fractures. Using discrete element method (DEM) to simulate grain-scale processes, we apply the concept of time-dependent cracking to hole enlargement for vertical wellbores. We use a published example from the North Sea to demonstrate the application of this new modeling approach to hole stability in shale. Laboratory results on rocks indicate a wide range of susceptibility to stress corrosion cracking related to rock petrology and contact fluid chemistry. Using laboratory calibrated rock properties, we run two-dimensional simulations of vertical wellbore stability in shale, where hole enlargement is tracked through time. As a result of stress corrosion cracking, the numerical models show a time-dependent failure history, with an initial stable period of varying duration (influenced by mud weight, rock properties and in situ stress), followed by a brief period of combined shear and tensile failure, ending with stabilization at an enlarged, elliptically-shaped geometry. Time to failure increases with increasing mud weight. Enlarged hole shape changes from elliptical to roughly circular with decreasing stress anisotropy. This new modeling approach for time-dependent wellbore failure can be readily constrained with straight-forward fracture mechanics tests on rock samples, and has the potential to also be applied to time-dependent, intermittent sand or fines generation during production.

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