This paper describes the fundamental concept and a mathematical model of the mud pressure penetration mechanism and the associated mud support reduction as the drilling mud invades the rock material. The variation of breakthrough pressure with cumulative pore size distribution of a Cretaceous shale from the North West Shelf of Australia has been quantified for a polyacrylamide, an ester-based and an oil-based mud systems. The results indicate that the improvement in the stability of reactive shale sections drilled with ester-based and oil-based muds may not be due solely to the muds inhibiting the hydration and loss of strength of the shale but also due in part to the comparatively better mechanical support provided by the muds.

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