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Abstract

Roller cone bits have dominated the 8-1/2" steerable application in Abu Dhabi due to their proficient steerability, consistently achieving the required build rates in the soft Nahr Umr shale. In the vertical section, PDC bits often achieve up to four times the penetration rates of TCI bits. However, directional performance when using PDC bits to build angle was compromised by fluctuations in reactive torque resulting in poor tool face control and inconsistent build up rates. The overall result was poor penetration rate and performance compared to roller cone bits. The operator and a service company utilized a new design process and cross functional team approach to aggressively seek new steerable PDC technology to drill the curved section with controllable torque response and consistent directional behavior while achieving the full penetration rate advantage of PDC bits. This process was complemented by the Well Delivery Limit process (WDL) that was already established by the operator that focused on delivering high value wells with significantly reduced costs. The team analyzed the drilling operations from virtually every perspective using numerical models, laboratory drilling tests, and field testing. The key to the PDC solution was the team process that identified the relevant drilling problems and performance requirements in order to develop the needed technology. After 30 bit runs from November 2001 to June 2002, the team reduced drilling costs by $44.26 per foot for a total savings of $3,792,367 over 85,682 ft and 2,386 drilling hours. The average ROP for the section doubled from 18 ft/hr with TCI bits to 36 ft/hr. The current ROP record stands at 60 ft/hr over a 3,169 ft interval.

Introduction

The operator and bit manufacturer have a history of joint optimization of roller cone technology in this 8-1/2" interval, improving cutting structure and metal face sealed bearing performance. The operator has documented these achievements, along with two failed attempts to drill wells with PDC bits. ¹ Despite advances in directional drilling technology and more powerful positive displacement motors (PDM), even as recently as late 2001, IADC code 517 roller cone bits were the only technology that could provide both tool-face control and build up rates required for the well plans. The lithology for this section consists of short section of limestone, followed by long section of soft shale to the landing point in a limestone reservoir. All PDC attempts failed to achieve any significant drilling improvements because of poor tool face control and inadequate build rates through the soft shale. Seeing an opportunity to reduce drilling costs, engineers from the operator, bit supplier and motor supplier developed a new technology for steerable PDC bits through an open and cooperative process.

Well Plan

The 8-1/2" interval starts at approximately 6,600 ft at the bottom of a limestone interval where unconfined compressive strength (UCS) ranges between 9 and 21 ksi. At approximately 7,400 ft, a 3 to 5 ksi UCS shale is encountered which extends to the top of the reservoir at 8,100 ft. Almost half of the directional work takes place in the soft shale before the reservoir is penetrated at around 40 degrees of inclination. The limestone reservoir features alternating soft and hard layers, with the softer rock around 9 ksi UCS and the firmer rock 15 to 26 ksi UCS. Figure 1 shows a plot of the rock type and compressive strength versus depth while Figure 2 shows two typical well plans. Directional work continues until the well is horizontal and measured depths often extend to 9,500'.