West Kuwait's 22-in. section comprises a vertical hole through 3,500 feet of interbedded carbonates varying significantly in compressive strength, and drilled commonly with minimal or no fluid returns. The section is typically drilled with roller-cone tungsten carbide insert (TCI) bits because large polycrystalline diamond compact (PDC) bits are extremely costly and require expensive performance motors to support their generated torque. PDC bits are also at risk of impact damage when drilling through the interbedded formations in this interval. Operators tend to apply higher drilling parameters while in the lower compressive strength intervals to achieve higher rates of penetration (ROP). Consequently, when the TCI bit enters the following harder formations with the same high operating parameters, it often suffers severe cutting structure damage. The result is reduced ROP. Greater weight on bit is then applied, causing further bit damage and possible sealed bearing failure.
An engineering project was launched to develop a TCI bit specifically for the interbedded carbonates of this section. The primary challenges of the project were:
Ensure the bit finishes the section in one run
Improve dull condition of bit coming out of hole
Surpass current field average ROP consistently
Maintain efficient cleaning at lower flow rates for drilling in complete fluid-loss scenario
Based on data and experience in drilling the application, an engineering process took place where several designs improvements with potential to improve performance were identified and trialled. Field engineers then worked onsite to identify how to drill the interval with optimal parameters for each sub-layer. The final design included:
Specialized TCI cutting structure for carbonate drilling
High impact-resistant insert geometry
Simulated hydraulic efficiency to improve hole cleaning
Improved high aspect ratio elastomer seals designed to endure longer runs
Specialized tungsten carbide to improve cutting structure durability
The engineering process yielded a design that successfully drills the complete section in one fast run - the fastest of the section - with an improved field average penetration rate to 63%, saving the operator more than 38% in associated cost for drilling the section. The application-engineered cutting structure enabled the use of lower drilling parameters than normal, thereby improving drilling efficiency and enhancing the post-drilling dull condition from the average of 3-3-BT to 1-2-WT.
The paper shows a case study in Kuwait demonstrating the engineering and results of designing a TCI bit matched to application.