Abstract

The combination of the large number of rotary-steerable systems (RSS) currently available and the variety of well trajectories in which they are applied requires a customized range of drill bits to optimize system performance. A clear understanding of the specific tool operation, combined with in-depth modeling and physical testing, is the ideal approach for determining the demands placed on the drill bit and thus deriving its key characteristics.

This paper describes a series of tests that were performed at a purpose-built drilling facility to examine the variation in directional response with different bit features. Steerability is one of the key criteria involved in RSS bit design, but stability is of equal importance, particularly in ensuring good-quality, gauge borehole for maximum steering potential. To monitor the testing, a proprietary downhole dynamics recorder was used. The recorder consists of a small, sophisticated electronic measuring device that can be either housed in a sub or positioned in a specially modified housing within an existing drillstring component. The recorder was used to gather actual downhole data at a high frequency sample rate to determine both the lateral and torsional stability of the drill bit and RSS assembly with different wellbore trajectories and parameters. 

Results identified that specific gauge features are crucial to the directional response of the RSS tool. In addition, the dynamics recorder allowed quantitative assessment of stability versus steerability of the critical drill bit features tested. Lessons learned from this laboratory testing have been taken to the drilling environment with excellent field results, demonstrating that matching the bit to a specific RSS tool as part of the system delivers proven drilling success.

Introduction

An 8 1/4-in. collar size RSS has been developed (the RS 825), which uses point-the-bit drilling technology incorporating a near-bit stabilizer to orient the drill bit axis with that of the targeted hole trajectory. A non-rotating outer sleeve with anti-rotation devices incorporates a center drive shaft to transmit torque through to the bit, with the sleeve and shaft being decoupled by bearings. Relative rotation between the center shaft and the non-rotating outer sleeve drives a hydraulic pump, which generates the motive force required to eccentrically offset the drive shaft within the sleeve. When changes in wellbore direction are required, hydraulic pistons are activated to deflect the shaft from the stabilizer sleeve centerline, which in turn deflects the bit to point in the required direction. The onboard navigation control electronics of the tool are contained in a lithium-battery-powered insert and enable monitoring and control of bit face, deflection, and relative rotation as well as near-bit inclination, azimuth, and gamma ray measurement capabilities. Uplink telemetry is accomplished with mud pulse (Fig. 1).¹

To evaluate the system with various bits, a testing program was initiated at a purpose-built drilling test rig at a Weatherford research and development (R&D) facility in Houston, Texas (Fig. 2).  The drilling simulation rig consists of a hydraulic motor, which powers a set of hydraulic rams, the whole assembly being mounted within a framework that supports a horizontally mounted bottomhole assembly (BHA). The framework is mounted on rails that allow it to move laterally to and from a large concrete block in which it can be used to drill horizontal boreholes (Fig. 3). The hydraulic motor turns the BHA and transmits torque to the bit while the hydraulic rams apply a lateral force on the BHA, thereby simulating weight on the bit (WOB). A conventional mud pump and flow loop are used to supply the drilling fluid. The test rig allows 80-ft horizontal boreholes to be drilled through a large block of concrete 8 ft high by 5 ft wide, the composition of which can be varied through the addition of aggregates to simulate various formation types with compressive strengths ranging from 10,000 to 40,000 psi. Depending on the size of hole drilled, each block can accommodate as many as 12 holes.