In recognising the petroleum industry's need for a technology to design optimal drilling fluid (weight, type and chemistry) to solve and manage shale instability problems, a collaborative project has been undertaken to develop the technology which will provide the basis for state-of-the-ant drilling fluid design criteria. The key drilling fluid-shale interaction mechanisms included in the study are mud pressure penetration, chemical potential and swelling-hydrational stress.

Fundamental concepts, processes, models and a range of novel test equipment and procedures for laboratory simulation of the mechanisms were developed. The laboratory test results were used in the refinement and verification of the models. The verified models, which have been integrated in a wellbore stability analysis software, were used to conduct analyses to demonstrate the effects of shale and drilling fluid properties on time-dependent wellbore (in)stability.

The understanding of the drilling fluid-shale interaction mechanisms and the development of the capacity to model them enabled the development of drilling fluid design charts. These charts, together with a shale property database and property correlations, provide a pragmatic and effective approach for designing optimal drilling fluids to manage shale instability efficiently.

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