Finding optimal balance in rotary steerable systems
Stability, steerability, borehole quality challenges
Steve Jones, Junichi Sugiura - PathFinder
Steve Barton - ReedHycalog
The selection of optimum gauge geometry for a rotary steerable system (RSS) varies considerably and depends on the RSS driving mechanism, application, and rock type. Since each RSS is different, the optimal drill-bit design must be customized for each system. Systematic testing is required to evaluate the dogleg and drillabililty of rotary steerable systems.
Directional drilling revolution
The first commercial RSS revolutionized directional drilling in the 1990s. The technology has made improvements in reliability and is now a standard drilling tool, with both push-the-bit and point-the-bit RSS applied in directional and vertical wells worldwide. Their use is not limited to high-cost offshore markets but is becoming more common in lower-cost land markets.
The advancement of RSS drilling goes hand in hand with the use of polycrystalline diamond compact (PDC) bits. Continual development of advanced modeling software and cutters with significantly increased abrasion resistance have led to PDC designs that can drill faster, further, and with a high degree of stability.
In directional applications, drill bit stability, and steerability are critical to performance. Without a smooth torque response or a laterally stable design, neither the desired rate of penetration (ROP) nor durability to drill the required interval is achieved. It is also important to match the side-cutting capability of the cutting structure and gauge geometry to the operating mechanism of the specific tool and the directional objectives. These factors affect borehole quality, which has a direct impact on the directional control of the RSS. Maintaining wellbore quality, particularly borehole gauge, is crucial for obtaining predictable directional response in both push- and point-the-bit systems. Also, with any RSS, directional control and borehole quality is linked directly to how quickly and precisely the internal control system operates.
Rotary steerable systems
The principle and drive mechanism of the commercial RSS have diversified over the years, with evolution in both push and point tools. The steering unit of the tool has evolved uniquely to be run in both point-the-bit and push-the-bit configurations. It consists of three hydraulically actuated pads for directional control, which also act as an anti-rotation device, to hold the steering unit stationary while drilling ahead. All three pads make full contact with the formation, and steering is accomplished by offsetting the steering unit in the desired direction. The RSS controls the dogleg severity (DLS) by varying the amount of offset that the steering unit creates from the center-line of the hole and the pads are constantly adjusted to achieve the target settings. This continuous adjustment to maintain orientation of the steering unit allows the system to drill a constant curve.
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A typical 12 ¼-in. hole size PathMaker RSS in a point-the-bit configuration.
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Volume 68 Issue 9
September 2008
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