Finding optimal balance in rotary steerable systems
In push mode, the steering unit pushes the bit sideways in the desired direction, whereas in point mode, a full-gauge near-bit stabilizer is used as the fulcrum, allowing the bit to tilt in the required direction. There are advantages to both systems, depending upon the application. Typically the system runs in push-the-bit mode if excessive hole wash-out is expected or if higher degree doglegs are required. The former relates to the fact that the pad extension is considerably larger than that of conventional push tools and thus can maintain borehole contact in enlarged holes.
Tuning drill bit design
There are four fundamental characteristics of the fixed-cutter bit: durability, steerability, stability, and aggressivity. Drill bit designs need to be tuned in terms of bit profile, cutting structure, and gauge design to optimize performance for a given BHA, formation, interval, mud type, and directional requirement. One key for a rotary steerable assembly is the operating mode of the specific tool itself; an in-depth knowledge of the tool and field performance enables both bit cutting structure and gauge geometry to be matched with a specific RSS drive mechanism.
To determine optimal gauge geometry for the tool, a series of systematic tests was conducted with emphasis on the affect of bit gauge design on stability and steerability. The same primary cutting structure was used so that any variance in recorded stability and steerability would be directly related to the difference in gauge geometry used during testing. This testing was done in both push and point modes, not only identifying the optimal gauge geometry for each mode, but also allowing a true and accurate comparison between push and point mode using the same steering unit and bit design. The following four gauge geometries were tested:
- 1. Active gauge: This arrangement increases the lateral cutting ability of the bit design using exposed, full round cutters on the leading edge of the gauge pad
- 2. Dual action gauge (DAG): This gauge geometry incorporates a full diameter gauge pad with a PDC cutter selectively placed in a recess at the back angle of the gauge pad
- 3. SteeringWheel gauge: This gauge design features a continuous ring at the gauge diameter rather than discrete pads as in conventional bits. SteeringWheel bits were designed to improve directional response by improving lateral stability of the bit to reduce the propensity of the bit to whirl. By minimizing lateral vibration, the ring bit reduces torque fluctuations that can occur due to bit whirl. The majority of ring gauge designs used commercially feature partial ring geometry, which was applied within this test program
- 4. SmoothSteer gauge: The gauge pad of the drill bit is tapered at an angle that matches the tilt imposed by the RSS. The specific geometry of the proprietary tapered gauge avoids the frictional resistance produced in directional drilling, thus lowering torque and instability.
Systematic testing
Previous Page
|
Next Page
Page 2 of 4
Volume 68 Issue 9
September 2008
Add RSS Feed