Drilling experience in K-shale in the Malay Basin of Peninsular Malaysia highlighted the issue of wellbore stability in the formation. A wide range of drilling problems has been experienced, ranging from sloughing shale to tight hole and stuck pipe. Subsequently, a major collaborative project between CSIRO Petroleum and PETRONAS Research & Scientific Services Sdn Bhd was conducted to address the problems. The aim is to develop a technical framework of drilling fluid design and consolidate into a methodology for overcoming wellbore instability-related problems in the shale.

This paper describes the outline of the project and the approach adopted in the development of the methodology. The approach is based on drilling fluid-shale interaction and proven rock mechanics principles which include an extensive laboratory testing program on downhole core specifically cut for the study. Based on the gathered drilling experience and laboratory tests conducted on downhole core material, dominant time-dependent failure mechanisms of the shale were identified. Examples of critical mud weight contour plots and design charts of pressure change (due to drilling fluid-shale interaction mechanisms) which form part of the consolidated methodology for designing optimal drilling fluid are presented. Comparisons between predicted mud weights determined from the contour plots and mud weights used in one of the fields showed an overall good agreement between the mud weights, drilling experience and hole size. Counter to operational expectations, the results from the laboratory study showed that some water-based muds, through correct fluid design, are capable of preventing time-dependent instability in the shale.

The methodology provides a pragmatic approach for managing shale instability problems in drilling extended reach wells in the Malay Basin as well as other regions worldwide. The approach would result in an increase in drilling efficiency and a reduction in drilling cost.

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