Engineering a Fit-for-Purpose Bit Doubles Performance and Durability in Abrasive and Impact-Prone Formations

Abstract

The Umm Gudair 12.25-in. vertical drilling performance section involves several technical challenges for engineering an application-specific PDC bit. The interval consists of two formations that require contradictory bit design principles. The abrasive Zubair formation incorporates sandstones interbedded with hard shale streaks, followed by dual Ratawi intervals of hard shales that are succeeded by compacted limestone. The initial sand interval abrades the cutting structure, resulting in slower rates of penetration in the deeper carbonates. The premature dulling of cutters then causes lower carbonate drilling efficiency. Typically, the section is drilled at 35 feet per hour (fph) and requires 2 bits to complete. After reviewing offsets with faster than average rates of penetration that successfully completed the interval in the same application, a design compromise was achieved. The new optimized bit design is based on a seven-bladed bit with sixteen-mm cutters. Subsequent design iterations optimized the cutter selection and the cutting structure to maximize durability in the sands while remaining aggressive enough to drill the lower carbonates. An iterative design-and-simulate process then followed, simulating bit stability against formation strength data and optimising drilling fluid flow dynamics. The design was adjusted repeatedly based on the simulation results until a balance in aggressiveness, stability and flow was reached. After the design process concluded, optimal drilling parameters for each formation were drawn up from offset wells, and the new design was field tested. Drilling stability improved in the sand interval while cutting structure integrity was sustained into the deeper carbonates, resulting in improved rates of penetration in both intervals. After several runs, the results demonstrated that significant gains in rates of penetration were made; reaching over 90 feet per hour (fph) in some cases, almost doubling current rates of penetration. The post-run dull condition of the bits tended to be virtually new (1-1-WT), and in one instance the bit re-ran on a second well, drilling the interval at 53fph. This paper demonstrates a successful engineering design process for such a challenging section in the Umm Gudair Field. It then highlights the technologies featured in this bit and their value in drilling this application. Finally, field test results are analysed and the improvements in drilling performance and time and monetary savings to the operator are quantified.