Steerable Motor Assembly
Effect of bend housing angle on build rate and bit side load
Effect of hole erosion on build rate
Baker motor stabilization configuration
- The expected build rate will depend upon the motor configuration
- Build rate for a fully partially stabilized motor assembly Can rotate to a bend of 1.5o
Build rate for a slick motor assembly
Can rotate to a bend of 1.6o
The build rate will depend upon
-Motor stabilization
-Hole size
-Motor size
-Distance from bit to bend
-Stabilizer diameter (undergage)
-Etc.
Rotary steerable
Steerable without sliding (100% rotation)
Can change both inclination and direction Steerable motor in the slide and rotate mode
Theoretically in the rotary mode, it will drill straight ahead
Limitations of steerable
motors in the slide mode
- Sometimes difficult to slide due to hole drag and stabilizers hanging up
- Difficulty maintaining orientation especially as the well gets deeper and the drill string gets smaller
- More difficult to maintain orientation with a PDC bit because of higher bit torques versus bit weight
- Poor hole cleaning while in the slide mode. Rotation helps clean the hole
- Lower effective penetration rate. Harder to keep a constant weight on bit in slide mode. Time is spent orienting the motor.
- Higher wellbore tortuosity
- Differential pressure sticking
- Build rate is formation sensitive
Limitations of steerable motors in the rotate mode
- Higher vibrations lead to motor and MWD failure
- Accelerated bit wear
- Poor hole quality for logs (sometimes)
- Poor performance in air
The rotary steerable system address some but not all of the limitations
These rotary steerable concepts were patented in the 1950’s, but the design is being used today
Guidance systems were required to make them work
Rotary steerable systems being designed and used today
Schlumberger rotary steerable system
Gyrodata rotary steerable system
Economics of rotary steerable
Rotary steerable can improve hole cleaning with 100% rotation
Rotary steerable assemblies have the potential to reduce overall dogleg severity
Rotary steerables can still drill directionally when the pipe will not fall into the hole with its own weight (steerable motor cannot slide)
Service companies eventually want to get the rotary steerable to drill the hole without interference from the surface
The directional program is placed in the MWD
The computer computes a position and determines what it needs to do to get to the target and takes the appropriate action
All the drilling contractor does is add drill pipe like drilling a vertical well
It would not be applicable where the directional target changes based on geosteering data
Equation for calculating toolface angle to change direction and azimuth
Need to change inclination from 14o to 25o and change azimuth from 10o to 48o
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