Drilling mixed lithology formations of Shales, soft/hard Carbonates, and long Anhydrite intervals in a single section that comprises directional drilling from vertical to well landing with elevated mud weights and high overbalance represents numerous challenges including wellbore stability, tight hole, formation ledges, differential sticking, high torsional and lateral vibration levels, erratic drilling torque, poor rate of penetration, and failure to run the 7" liner to bottom. Controlling such formations with water-based mud system is difficult, even with large percentage of water-based lubricant, thus leading to multiple downhole tools failures and failing to drill this challenging section in one run.

Due to the fact that drilling this section falls under a lumpsum turnkey project and the section performance and cost are crucial to the well delivery, the drilling engineering and drilling fluids teams worked together to address the section challenges by replacing the water-based mud system with a customized oil-based mud system. All associated risks for this replacement including the spill control measures and environmental hazards were discussed and presented in a risk assessment form.

Wireline and logging-while-drilling formation characteristics logs in different field locations, offsets drilling performance, stuck pipe incidents, and tripability data were gathered and a rigorous lab testing was performed to formulate an efficient oil-based mud formulation with optimum bridging design to control the formation challenges whilst maintaining a competitive system cost compared to the previous water-based mud system.

The optimized bridging materials design in accordance with the very low lubricity coefficient of the oil-based mud system helped strengthening the weak formations, reduced the stuck pipe risks and intensely minimized the drilling vibrations thus helped in applying full drilling parameters, improved the rate of penetration well after well, and delivered the section in one run without downhole tools failure. In addition, the controlled filtration showed exceptional tripability compared to the old mud system and saved more than 8 hours to pull the directional assembly back to surface. The 7" liner was run smoothly to the planned depth with an improved section delivery time and operational costs.

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