Constructing the 12 1/4-in. direction hole section through approximately 3,000 ft of difficult interbedded lithologies (Mutriba-Lower Buran) in northern Kuwait presents a number of distinct challenges. In the upper portion of the hole section, a PDC bit must penetrate medium to hard carbonate and shale formations with compressive strength ranging between 9-12 kpsi with peaks up to 30 kpsi. Next, a challenging abrasive sand with compressive strength between 6-9 kpsi requires a RSS/PDC assembly to reach total depth (TD). The operator experimented with several different bit designs attempting to efficiently achieve directional objectives and improve borehole quality with limited success. Issues with baseline designs included lack of cutting structure durability and low rate of penetration (ROP).

To accomplish the operator’s objectives, the engineering team analyzed all relevant offset data and concluded an existing 12 1/4-in. six-bladed bit with 16 mm cutters would serve as the starting point for an optimization effort. The bit’s design data was fed into a FEA-based modeling system. Formation characterization software was then utilized to select the appropriate rock samples to simulate the field formations in the laboratory. Multiple simulations were run to determine the best combination of technologies to achieve the objectives. A new 12 1/4-in. directional design (616 type) would include premium cutters that can withstand impact in the interbedded carbonate/shale section and remain sharp while drilling the lower sand formations to TD. The bit also features a torque limiting feature in the blade top and TSP inserts in gauge to ensure hole quality. Next, a series of simulations were performed to observe how different RPM and weight on bit (WOB) values would affect vibration and torque levels. The results were plotted to create a smooth drilling parameter window to maximize the new bit’s ROP potential.