Steerable Motor

Key Takeaways

• Bent housing angle selection is the primary design variable that controls the build rate capability (dogleg severity) of the steerable motor in the sliding mode: a larger bent angle (2.5 to 3 degrees) produces a higher build rate (typically 8 to 15 degrees per 30 meters in the sliding mode) but also creates more torque and drag on the drillstring during rotation (because the offset of the stabilizers and bit from the wellbore axis is greater), limiting the rotating weight on bit and the rate of penetration in the rotating mode; a smaller bent angle (0.5 to 1.0 degree) produces a lower build rate (2 to 5 degrees per 30 meters in slide mode) with less rotary torque and drag, giving better rotating performance but less directional authority in formations that require frequent trajectory corrections; the selection of bent housing angle is made at the time of BHA design and cannot be changed without tripping out, so the appropriate angle must be chosen to match the build rate required by the well plan (accounting for formation tendencies and the maximum dogleg severity allowed by the casing program and completion hardware); adjustable bent housings (ABHs, where the bent angle can be changed from surface by rotating the drillstring through a specific torque sequence that moves a mechanical cam) provide the flexibility to change the build rate without tripping out and are increasingly used in complex well profiles where different sections require different build rates.
• Toolface control in the sliding mode determines the direction in which the bent housing orients the bit, and hence the azimuth and inclination change that the curved drilling produces: in sliding mode (no surface rotation), the MWD tool continuously measures the toolface angle (the angular orientation of the bent housing high side relative to a gravity reference in the vertical plane or a magnetic reference in the horizontal plane) and transmits this value to the surface via mud pulse telemetry; the driller adjusts the surface drillstring orientation (by rotating the top drive or kelly in small increments) to set the toolface to the desired orientation before applying weight on bit to resume sliding; gravity toolface (GTF, referenced to the high side of the wellbore when the wellbore inclination exceeds approximately 5 degrees) is used for inclination control, while magnetic toolface (MTF, referenced to magnetic north, used in near-vertical wells where gravity reference is unreliable) is used for azimuth control; maintaining consistent toolface during sliding requires the driller to periodically correct toolface creep (caused by reactive torque from the mud motor rotation at the bit, which tends to rotate the drillstring and shift the toolface), either manually by applying small surface torque corrections or automatically using an automated toolface control system at the top drive; toolface control quality is the primary determinant of the achieved dogleg severity relative to the theoretical maximum for the BHA configuration, with poor toolface consistency resulting in actual doglegs 20 to 50 percent lower than the theoretical maximum.
• Steerable motor performance comparison to rotary steerable systems (RSS) involves trade-offs between cost, capabilities, and operating conditions: steerable motors cost $2,000 to $8,000 per day to rent and require no surface equipment beyond the standard MWD tool, making them significantly less expensive than RSS tools ($5,000 to $20,000 per day plus surface equipment); however, steerable motors alternate between sliding (directional control, lower ROP due to no surface rotation and higher friction) and rotating (higher ROP, less directional control, average dogleg diluted to near-zero in long rotating sections), creating an uneven wellbore profile (a "staircase" pattern of short curved sections and long straight sections) that increases friction in extended-reach wells, can cause casing running difficulties in wells with high total dogleg severity, and limits the achievable bottomhole location accuracy in complex trajectories; RSS tools (push-the-bit and point-the-bit designs) drill continuously in rotation with continuous directional control, producing smoother, lower-torque wellbore profiles that are essential for extended-reach wells beyond 5 to 8 km horizontal reach, for wells requiring high landing accuracy in thin pay zones, and for high-temperature or hard formation drilling where mud motor stator elastomers degrade rapidly.
• Stator elastomer selection for the mud motor in a steerable assembly is critical for performance and reliability in high-temperature (above 120 degrees Celsius) and oil-based mud environments: the Moineau mechanism requires a flexible stator (typically made of nitrile rubber (NBR) in standard service, hydrogenated nitrile (HNBR) for temperatures up to 150 degrees Celsius, and fluoroelastomer (Viton or FKM) for temperatures to 200 degrees Celsius) that forms a fluid-tight progressive cavity against the harder steel rotor; in oil-based mud (OBM), the stator elastomer is exposed to oil continuously, and elastomers swell or soften in OBM at a rate that depends on the base oil chemistry (diesel causes more elastomer swelling than internal olefin or ester base oils), the elastomer compound, and the temperature; stator swelling reduces the internal clearance between rotor and stator (causing the motor to over-torque and potentially stall at lower WOB than designed), while stator softening causes progressive wear and internal bypass leakage (reducing motor output torque and speed below specification); motor manufacturers provide temperature and mud compatibility ratings for each stator elastomer grade, and the mud engineer and drilling engineer must jointly specify the stator grade appropriate for the combination of formation temperature, OBM type, and expected motor run duration; a stator failure during a sliding drilling run requires an unplanned trip out, which in deep wells costs $200,000 to $500,000 in rig time and drilling fluid costs.
• Steerable motor applications in horizontal and multilateral drilling demonstrate the technology's continued relevance despite the wider adoption of RSS in high-value wells: in coalbed methane, coal seam gas, and tight sandstone formations requiring multiple horizontal laterals from a single vertical wellbore (multilateral well architectures), steerable motors with medium-radius build rates (5 to 10 degrees per 30 meters) are used to kick off from the main bore and build inclination to horizontal across the transition from vertical to horizontal in 300 to 600 meters of drilling, with the lower motor cost per lateral being the primary economic advantage over RSS in wells with 4 to 8 laterals; in plug-and-perf horizontal completions in North American shale plays (Permian Basin, Eagle Ford, Bakken, Montney), steerable motors with high build rates (8 to 15 degrees per 30 meters) are used in the landing section of the wellbore (the 300 to 600 meter section where the well turns from vertical to horizontal and lands in the target formation), with RSS used for the long lateral if the formation requires precise trajectory control or if the extended reach exceeds 2,000 meters of horizontal drilling; the hybrid RSS-motor approach (motor in the kick-off section, RSS in the lateral) is common in unconventional plays where the motor's high build rate capability is valued in the landing section but the RSS's superior ROP and trajectory control is preferred in the long horizontal lateral.