| Popis: |
Drilling experience in Alab and Samarang fields in the Sabah Basin highlighted the issue of wellbore stability in the region. A wide range of drilling problems has been experienced, ranging from tight-hole to overpull and stuck pipe, and a range of drilling fluid designs has been implemented. Subsequently, a wellbore stability study was conducted to address the problems in the planned extended reach Alab wells. The aim is to develop a strategy to maintain mechanical and time-dependent stability of the extended reach wells to ensure well constructions encounter minimum wellbore instability-related problems. This paper describes the outline of the study and the approach adopted in the development of the strategy. The drilling experience, material properties, typical in-situ stresses and formation pressure in the Sabah Basin are presented and discussed. Based on the gathered drilling experience, caliper and composite logs, and laboratory tests conducted on downhole core materials, dominant time-dependent failure mechanisms were identified. Mud weight stability profiles and examples of critical mud weight contour plots which form part of the strategy for designing optimal drilling fluid program are presented. Comparisons were made between predicted mud weights determined from the contour plots and mud weights used in the field together with associated drilling experience and hole size in a post-drilling review. The comparison showed a good agreement for a low angle well but inconsistent for two high angle wells. The reasons for the inconsistencies and means of reducing the uncertainties of input data required for wellbore stability study are presented and discussed. |
