Point-the-Bit Rotary Steerable System: Theory and Field Results

Abstract

Abstract Many existing rotary steerable systems rely on non-rotating, stationary pads or stabilizers to provide changes in borehole direction. An inherent weakness with this technique is that it relies on contact with the borehole wall to achieve predictable and consistent directional control. Hole washouts and borehole rugosity can negatively impact the directional performance of these systems. A new rotary steerable system has been developed that does not depend on wall contact by non-moving parts. This new technology uses what is being termed a "point-the-bit" technique to enable precise, consistent and predictable changes in well trajectory. This paper presents the design objectives and theory behind the "point-the-bit" system and contrasts it with existing rotary steerable systems. A case history from a successful field trial completed in Alaska is provided and includes well objectives and operational results. Introduction Rotary steerable systems are gaining acceptance in the petroleum industry. As more and more systems become available, the range of potential applications continues to expand. Initially rotary steerable systems were utilized primarily in extended reach drilling (ERD) wells, where the ability to slide steerable motors was limited by hole drag. Today, drilling engineers are considering such systems for additional reasons. These include performance drilling, improved hole cleaning and geologic steering. As rotary steerable systems become more widely used, performance demands will increase. The point-the-bit system described here has been designed in anticipation of these expectations. The market needs may be grouped into the following areas:Longer Runs. The ability to replace traditional steerable mud motor systems will be determined by the cost versus benefit for each project. A critical component of this will be improved tool life and reliability. Today, success is often gauged by the completion of 75 hours of operation; this is clearly insufficient. Future systems must be able to compete with, and surpass runs currently completed with traditional steerable systems.Better Control.Predictable and consistent turns and build rates are important in maximizing performance. When system control parameters are heavily dependent on formation characterstics or individual knowledge of system behavior, dependable performance is compromised. Commercial rotary steerable systems have the potential to remove the perceived art associated with directional drilling. Providing a new level of control and steerability to the industry is a second critical objective.Bit Selection Independence. Bit characteristics for a given application are often selected to optimize performance in specific formations or zones. Some rotary steerable systems on the market require bits specifically designed for the system to ensure proper operation. Ideally, bit selection should not be limited by the rotary steerable system. The flexibility of rotary steerable systems to operate with a wide range of bit types is desirable.Furthermore, rotary steerable operations will require the ability to operate with bi-centered bits and near-bit reaming devices which create overguage holes. This introduces new challenges for controlling the system and potentially exposes the tool to additional shock and vibration.Low Risk. The risks associated with running rotary steerable systems fall into two main categories: the likelihood of tool failure in less time than a traditional steerable assembly and the potential for lost-in-hole situations. Both of these risks will need to be demonstrably reduced for the wider adoption of rotary steerable systems.