Maintaining wellbore stability remains a challenging task in the oil/gas well drilling industry, especially for wells drilled through naturally fractured shale formations. Field evidences show that some wellbores that are stable immediately after drilling may collapse after several days of exposure to the drilling mud, resulting in significant financial losses. Thus, a better understanding and prediction of the time-dependent wellbore instability behavior in naturally fractured shale formations is of high demand. In this paper, we present a poro-elasto-plastic finite element model for predicting the progressive evolution of damage zone around the wellbore drilled in naturally fractured shale formations. The Mohr Coulomb model has been employed to capture the plastic behavior of the naturally fractured shale formations, while strength reduction caused by the penetration of drilling mud into the formation is also included in the model. Permeability enhancement due to stress damage before peak strength has been experimentally tested and an empirical relationship between the permeability enhancement and the damage state has been obtained and incorporated into the model. As an example application, the model has been applied to a typical wellbore drilled in the Bohai oilfield of China, where serious wellbore instability problems have been frequently encountered when the fractured shale formations of Dongying group is drilled through. The influences of drilling fluid density and plugging capability of the drilling fluid on the time-dependent wellbore stability were investigated. Some suggestions for maintaining long term wellbore stability in these naturally fractured shale formations have been provided.

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