Rotary-Steerable Tool Brings Cost-Effective Performance to High-Volume Drilling
Authors: Carpenter Chris 
Published: January 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 166717, ’Fully Mechanical 3D Rotary-Steerable Tool Brings RSS Performance to the High-Volume Drilling Market at a Fraction of the Cost of Conventional Rotary- Steerable Tools,’ by K. Hershberger and D. Herrington, NOV, prepared for the 2013 SPE/IADC Middle East Drilling Technology Conference and Exhibition, Dubai, 7-9 October. The paper has not been peer reviewed.
A 100% fully mechanical rotary-steerable tool has been developed specifically for the high-volume drilling market, providing full 3D directional control while under full rotation of the drillstring from surface. This durable push-the-bit rotary-steerable system (RSS) has eliminated the costly and sensitive electronics common to most RSS tools and has eliminated a number of vibration-related failure modes, making it suited for applications where shock and vibration are prevalent and for high-temperature applications.
Introduction
With the introduction of the downhole drilling motor in the 1980s, horizontal drilling became much more feasible. Horizontal wells are now the most common well type drilled in the US. Successful horizontal wells rely on accurate placement of the wellbore inside the desired reservoir. To achieve this, a directional driller is provided with drilling targets on a well plan, which are typically selected on the basis of offset-well information, seismic data, and geological models.
In an effort to optimize production while minimizing the number of rig moves and wells drilled, horizontal wells have stepped out farther and farther. As the horizontal displacement of a wellbore increases, torque and drag become major obstacles that must be overcome to enable efficient drilling. RSS tools provide a means to extend the wellbore beyond a steerable motor’s capabilities, but they come at a price that is often not economically feasible for the high-volume market. This forces operators to drill complex wells with either standard steerable motors or to pay for expensive RSS tools that challenge the economic viability of the well.
Drilling With Motors. Drilling with steerable motors has been the backbone of directional drilling since the 1980s. However, steering the wellbore with a motor requires slide drilling, which slows down drilling, reduces hole cleaning, and potentially creates other problematic conditions. Rate of penetration (ROP) is reduced when sliding because of friction between the borehole wall and the bottomhole assembly (BHA) as well as hangup of drillstring components. Without drillstring rotation, the drilling mud is not agitated, allowing the cuttings to collect on the low side of the wellbore and potentially pack off the BHA.
The steerable motor contains a bent-housing section. When sliding, the bent housing has to be maintained in the desired direction, or tool-face setting. This can become difficult because of the reactive torque produced by the motor itself when the bit is on bottom. The reactive torque turns the drillstring in a counterclockwise direction, working against the directional driller and making it difficult to maintain control.
Reactive torque is a function of the formation being drilled, the bit type, and the motor output. It can be mitigated by use of lower-torque- rated motors, light-set polycrystalline- diamond-compact (PDC) bits, or roller-cone drill bits; however, all of these solutions result in lower ROPs. Although PDC-bit designs have advanced and many bits incorporate depth-of-cut-control technology to reduce reactive torque while sliding, the basic inefficiencies still remain.
Rotating steerable motors also can result in inefficient drilling. When rotating a motor, there is no directional control, and the BHA will wander on the basis of formation push, formation dip, bit type, stabilizer placement, and drilling parameters, resulting in the wellbore deviating from the target line. Once the wellbore is off plan, a slide has to be performed to reorient the wellbore to the target.
In addition, steerable motors can cause hole spiraling and an overgauge hole. This results from rotating a motor with a bend in it, and the larger the bend setting on the motor, the more eccentric the rotational pattern will be. This creates a spiraling hole gauge larger than the actual bit diameter, which can create poor-quality imaging logs because of the uneven wellbore walls.