An efficient approach to wellbore stability analysis and management of shale instability by taking into consideration the dominant instability mechanism(s) has been developed. The mechanism(s) is dependent on the type of shale, in-situ stress environment and drilling fluid system used. These factors determine whether a drilling fluid program can be developed using a mechanical (stress-induced) wellbore stability analysis or complex time-dependent drilling fluid-shale interaction and thermal mechanisms need to be taken into account.

A range of wellbore stability analytical tools for efficient management of shale instability is presented. A pragmatic approach to use drilling fluid design charts together with a shale property database and property correlations for designing optimal drilling fluids to manage shale instability efficiently is described. The utilisation of the tools is demonstrated through a field case study in which strategies are developed to control wellbore instability in horizontal wells. Parametric analyses conducted demonstrate the effects of shale and drilling fluid properties on time-dependent wellbore (in)stability. They highlight the conditions in which the drilling fluid-shale interaction mechanisms are critical and need to be incorporated in the analysis.

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