In 2012 ZADCO commenced drilling operations from artificial islands in the Upper Zakum field. The field development is based on drilling extended reach wells up to 35,000 ft measured depth in three different reservoirs. More than twenty wells have been drilled from these islands so far, with the longest well reaching ~31,000 ft MD. Longer than 35,000 ft MD wells are planned to be drilled in the near future. The 9 5/8″ casing in 12 1/4″ open hole is set at a measured depth of ~9,000-17,000 ft with cement designed to reach surface. There are several challenges experienced while cementing these wells, due to the narrow pore and fracture pressure gradients. Key challenges include: ECD management, maintaining fluids density and rheology hierarchy, proper centralization, lost circulation, use of NAF drilling fluids and limitations to pipe movement opportunities. Based on ultrasonic imaging of initial wells, the overall desired cement bond was not as good as desired on initial wells. Several improved practices were applied to enhance the cement bond across reservoir sections. However, the log quality was still below expectations. Hence, a more robust solution was required to successfully cement the long maximum reservoir contact (MRC) wells.

These challenges were successfully addressed and mitigated through a step-wise approach. The cement slurry designs were optimized by adjusting rheologies and static gel strength, lowering fluid loss values, adding lost circulation material and using trimodal cement designs. Mud removal was further enhanced by using better cleaners in the spacer system to provide efficient cleaning and de-emulsification. Updated mud conditioning procedures and better centralizers was also implemented.

Cement bond log and interpretation techniques were improved by using ultrasonic measurements and flexural attenuation measurements and imaging the annulus through these tools to determine actual casing centralization. The log showed a significant improvement especially across the horizontal reservoir sections.

The use of these techniques has improved cement quality and enhanced zonal isolation of the producing zones in these horizontal MRC wells and will assist in maintaining the quality for future development of the Upper Zakum field.