The Sirte basin is one of the main producing basins in Eastern Libya. The lower cretaceous reservoirs are typically sealed with evaporite sequences; salt and anhydrite. Wellbore instability in the 8½" section can be due to the Sirte & Rachmat shales, coupled with the requirement for high mud weights to stabilize the evaporite sequences. This has led to the typical well being drilled with an 8½" hole down to the top of the reservoir and the reservoir then being drilled with a 6" hole.

The use of a High Performance Water-Based Mud (HPWBM) has allowed a change in the typical drilling practice. These two sections can now be drilled as a single 8½" section. The superior wellbore stability provided by the HPWBM has allowed the mud weight required to stabilize the shales and the evaporites to be reduced significantly. This has had two benefits-the well can be drilled as a single 8½" hole, allowing larger completions to be used, and the option to drill a sidetrack is available if the reservoir quality is poor.

This paper presents field data to demonstrate the ability of the HPWBM to change the typical drilling practice. Case histories are provided which show the evolution of the drilling practice with the introduction of the HPWBM. The performance of the mud system in these harsh environments is also demonstrated with reference to wells drilled in other parts of Libya where extremes of temperature and density are prevalent.

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