Abstract

The predominant drilling challenge in Oman's southern fields is efficiently penetrating the thick conglomerate sediments. The sedimentary section is encountered at depths ranging between 2500–3400m and is approximately 700–900m thick. The hard conglomerate sections have UCS values that range between 12–30 kpsi. The formations’ rounded clasts, pebbles, and/or boulders are deposited in a fine grained matrix and make rollercone (RC) drilling difficult. The major dulling issue is early gauge row cutting structure degradation that requires multiple bits/trips to complete the 12 ¼" hole section.

Over the last two years, a series of iterative design changes and subsequent field tests have led to optimized tungsten carbide insert (TCI) geometry and strategic placement of 100% diamond enhanced heal row inserts. A new-type of insert and layout geometry has also enhanced shirttail protection. The ever improving TCI design has significantly extended cutting structure life and improved borehole quality. New carbide grades, tougher inner row inserts and an advanced twin elastomer sealing system that protects the bearing have all extended bit life in high WOB conditions.

The field derived design iteration process has consistently improved performance over the last 12 years. In 1999 a typical rollercone used in Oman's conglomerate fields had a life expectancy of 10–20 hours. Currently, the operator can expect a rollercone bit to provide sustained ROP for 110–120 hours drilling in these same fields. The improved bit and motor technology has optimized drilling the build sections and dramatically reduced cost/ft by an average of 40% over the past two years. This paper will examine the bit development/design, parameter selection and the science that led to a significant increase in conglomerate drilling efficiency. Finally, the authors will outline new durability and ROP benchmarks and discuss reduced rig-time and resulting cost savings.

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