Understanding Torque: The Key to Slide-Drilling Directional Wells

Abstract

Abstract An in-depth study of surface torque and its effect on drillstring and bit movement has led to the development of automated technology for optimizing directional drilling with a downhole motor/measurement while drilling (MWD) system. By assimilating surface torque with downhole bit and drillpipe behavior, the technology allows drillers to maximize drilling efficiency and improve wellbore quality (due to less trajectory tortuosity) during the sliding part of the drilling process. This paper describes this proprietary1,2,3 surface system and how developers used torque control to optimize slide drilling without introducing new equipment downhole. The new technology integrates surface and MWD data to provide the following benefits in the sliding mode:Improved ROP and horizontal reach capabilityImproved tool-face correction while drillingImproved well trajectoryImproved motor life (less stalling)Quick and accurate tool-face orientationNo lost-in-hole exposureTime savings from switching from rotating to sliding without coming off bottom; faster tool-face orientation; overall performance optimization (as listed above) Introduction In the oil industry today, two primary methods are used to directionally drill a well: rotary steering and drilling with a motor/MWD system. Rotary steering systems are configured so that the entire drillstring rotates continuously with steering capabilities. A motor/MWD system is designed with a downhole motor and a bent housing. The system rotates the entire drillstring to drill a tangent section, and turns only the bit to produce a curve or bend. Drilling with only the mud motor is commonly called slide drilling because the drillpipe slides along the wellbore without rotating; the bent housing (and thus, the tool face) is oriented for trajectory control (Fig. 1). The value added is different for each system and depends on the total drilling costs involved, the availability of this technology, and the logistics of a particular job. With the motor/MWD system, sliding drilling efficiency is largely determined by the driller's ability to transfer the proper amount of weight to the bit without stalling the motor, and to reduce longitudinal drag sufficiently to achieve and maintain a desired tool-face angle. Several techniques are available for reducing longitudinal drag. They include addition of lubricity agents, rollers, downhole vibrators within the bottomhole assembly (BHA), and the use of a "rocking" procedure that consists of turning the pipe to the right and then to the left by an amount that avoids interference with the tool face. The effectiveness of each technique varies with well conditions, however, each technique is limited in some respect(s). Limitations can include cost, rig downtime, permanent installation requirements, and increased risk for downhole failure (or loss) due to introduced equipment, introduced vibrations, and fishing restrictions (especially if downhole vibrators or other obstructive devices are introduced above the MWD equipment). The authors surmised that an understanding of how torque impacts the drillpipe and bit movement allows significant improvements in the accuracy with which slide drilling could be controlled. By defining the relationships between torque, drag, and downhole pipe movement, the Noble team established the foundation for a control system for automating the slide drilling process, helping ensure optimum performance on every job. Directional Drilling Challenges As weight is increasingly applied to the bit, torque at the bit increases. Bit torque is proportional to the tangential force vector to bit rotation to the right. Thus, the downhole motor with its bent housing just above the bit experiences an equal force in the opposite direction (left) called reactive torque. Because of the relationship between bit weight, bit torque and tool-face orientation, the tool face responds to changes in bit weight.