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The development plan of an oil field in the Sarawak Basin, South China Sea includes extended reach wells. Drilling experience in exploration wells highlighted the issue of wellbore instability in the region. A wide range of wellbore instability-related problems has been experienced, ranging from tight hole (remedied by reaming) to overpull and stuck tool, and a range of drilling fluid designs has been implemented. There was considerable concern regarding wellbore stability, including coal bed instability, its effect on drilling performance and the potential for this to severely impact both schedule and budget. Subsequently, a wellbore stability study, with an emphasis on extended reach wells planned for the field development, was conducted. The aim of the study was to develop a strategy to maintain mechanical and time-dependent stability of the extended reach wells to ensure the well construction encounters minimum wellbore instability-related problems. With the availability of additional well data and the need to push the technical limit of the drilling campaign, a follow-up review study was conducted and the additional information was used to refine the critical mud weight contour plots for a range of wellbore enlargements. This paper describes the outline of the study, the approach adopted in the development of the strategies and the utilisation of the results from the two studies to develop recommendations for the mud weight programs. The drilling experience, material properties, typical in-situ stresses and formation pressure in the field are presented and discussed. Based on the gathered drilling experience, composite logs and laboratory tests conducted on downhole core materials, dominant failure mechanisms for the key formations and coal beds were identified. An empirical approach was adopted for the prediction of lower bound mud weights for wellbore enlargements ranging form 10% to 40% of wellbore diameter. Examples of critical mud weight contour plots and pore pressure change tables which form part of the strategy for designing optimal drilling fluid program are presented. |
