Abstract

Historic performance in slim hole rotary steerable (RS) applications in the Middle East was held back by severe torsional and lateral vibration issues. A new family of slim hole polycrystalline diamond compact (PDC) bit designs, specifically matched to the rotary steerable systems (RSS) used, has resulted in a dramatic improvement in performance.

The use of mathematically modeled bit performance indices has resulted in a significant improvement in RSS and bit performance while drilling harsh environments in the Middle East where lateral and torsional vibration events are severe. The bit design in question was further optimized by including design characteristics specifically targeted at reducing torsional events. Further, bit hydraulics have been optimized to minimize erosion and maximize borehole quality while matching system operation requirements.

Case studies are presented clearly demonstrating runs with superior performance in the Kingdom of Saudi Arabia (KSA). The field trials clearly demonstrated that significant reduction in torsional vibration and improved side cutting led to superior dogleg achievement. Improvement in lateral stability and resultant bore hole quality led to extended bit and tool life, thus notably improving penetration rate.

The paper concludes that significant performance increases can be achieved by matching the bit design to the RSS, the well geology and the well trajectory. This has resulted in advances in slim hole drilling.

Introduction

Proper bit design is critical for any application, but essential when a bit is engineered for use on an RSS. The fundamental cutting structure characteristics of rate of penetration (ROP), durability, stability and side cutting need to be considered and complemented by bit profile, gauge and hydraulics design to optimize performance for a given bottom hole assembly (BHA), formation, interval, mud type, and directional requirement. One of the key aspects of an RS BHA is the operating mode of the specific tool itself; an in-depth knowledge of the tool and field performance will enable an accurate system matched approach to drill bit selection.

As with most RSS, gauge hole and good borehole quality are very important in enabling the system to deliver consistent directional response. The lateral stability of the drill bit will have a direct part in this, particularly the cutting structure. Optimal RS tool performance is obtained with drill bits that are laterally stable, and drill with smooth torque and RPM response: Erratic torque and RPM fluctuations will lead to problems with tool operation, inconsistent steering response, and ultimately bit and tool failure. Torque control is essential, though many factors affect this, including lithology, trajectory, and mud type.

RS activity has increased rapidly over recent years in several countries within the Middle East. One of these key applications is the use of RSS in slim hole horizontal sections where directional control is beyond the sliding limits of conventional steerable motor assemblies. However, stick-slip which results from the erratic torque and RPM fluctuations described above, has been a major factor identified in these applications, resulting in numerous trips for tool failures related to high vibration. As an important factor in reducing this vibration issue, the fundamental characteristics of the drill bit designs were evaluated and optimized to provide an effective solution. This is demonstrated in the multiple case studies presented within this paper.

Application

Although the specific drill bits developed have now been run successfully in several locations around the world, the initial design intent was to provide an effective solution for torsional vibration issues within the Middle East. Due to the large number of runs within KSA, the following section will concentrate primarily on the application and subsequent challenges within that specific country.

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