Results of Extensive RSS Testing With PDC Bits

Abstract

Technology Update Rotary steerable systems (RSSs) have made remarkable improvements in reliability since their introduction in the 1990s and have become a standard drilling tool. Today, point-the-bit and push-the-bit RSSs are used on both directional and vertical wells worldwide. No longer limited to high-cost offshore markets, their use is becoming more common in lower-cost land markets. In the following analysis, the results of an extensive series of test wells are described. The wells were drilled in a controlled, noncommercial environment, allowing single-step changes in the drill bit features and rotary steerable (RS) configurations. The testing was unique in that the specific RSS worked in shop-configurable point-the-bit and push-the-bit modes. Between the two distinct RSS operation modes, consistency in stiffness, weight, force-applying capability, and control systems led to a direct comparison of bit performance. The advancement of RS technology goes hand in hand with the use of polycrystalline-diamond-compact (PDC) bits. Continual development of advanced modeling software and cutters with significantly increased abrasion resistance have led to PDC designs that can drill faster, further, and with a high degree of stability. These benefits, combined with the appropriate RSS, can also deliver superior directional control, improved borehole quality, higher-quality logging, and easier casing runs. The resultant improvement in economics is particularly attractive in high-cost offshore wells. The bit-design optimization is key to the success of RS drilling. In directional applications, stability and steerability of the drill bit are critical to performance. Without a smooth torque response or a laterally stable design, drilling will not achieve the desired rate of penetration (ROP) or the durability necessary to drill the required interval. It is also important to match the side-cutting capability of the cutting structure and the gauge geometry to the operating mechanism of the specific tool and the directional objectives. These factors will affect the borehole quality, which has a direct impact on the directional control of the RSS. Maintaining wellbore quality, particularly borehole gauge, is crucial to both push- and point-the-bit systems for obtaining predictable directional response. Also, with any RSS, the directional control and borehole quality is directly linked as to how fast and precise the internal control system can operate. Rotary Steerable Bits There are four fundamental characteristics of the fixed-cutter bit: durability, steerability, stability, and aggressivity. Drill-bit designs need to be tuned with respect to bit profile, cutting structure, and gauge design to optimize performance for a given bottomhole assembly (BHA), formation, interval, mud type, and directional requirement. As with most RSSs, 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 play a direct part in this, particularly the cutting structure. Optimal RSS performance is obtained with drill bits that are laterally stable and drill with smooth response in terms of torque and rotation speed. Erratic torque and rotation-speed fluctuations will lead to problems with tool operation, inconsistent steering response, and ultimately to bit and tool failure.