State-of-the-art rotary steerable systems have minimal interaction with the bore- hole. They use tool internal rather than external means to reference their position and attitude within the borehole and thereby preserve bore- hole quality. Indeed, the most advanced systems simply exert consistent side force similar to traditional
steerable motors in the rotary drilling mode as much as possible. Drillers clearly saw the potential for a continuously rotating directional- drilling tool to eliminate slide drilling, improve hole cleaning, increase penetration rate and reduce the risk of differential sticking, among a host of other benefi ts.2 Today, drillers realize these and many other benefits with a new generation of rotary steer- able systems that garner fewer headlines but offer substantial improvements over previous technology. For example, it is now possible to use a bicenter bit to enlarge boreholes while drilling with a rotary steerable system, saving rig time consumed by separate drilling and reaming operations.3
stabilizers that rotate with
the drillstring or orient the bit in the desired direction while continuously rotating at the same number of rotations per minute as the drillstring. This latter approach uniquely provides dogleg severity of over 8° /30 m [8° /100 ft] in the softest of formations in which constant borehole interaction is impossible.4
Advanced rotary steerable systems must con- tend with more than complicated trajectories. These systems are now built to perform in the toughest environments, taking advantage of enclosed components that protect the tool in wellbore temperatures to 302° F [150° C], highly abrasive formations, all types of fl uids and high-
1. For more on Wytch Farm drilling: Modi S, Mason CJ, Tooms PJ and Conran G: “ Meeting the 10 km Drilling Challenge,” paper SPE 38583, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, October 5– 8, 1997.
Allen F, Tooms P, Conran G, Lesso B and Van de Slijke P:
“ Extended-Reach Drilling: Breaking the 10-km Barrier,” Oil eld Rev iew 9, no. 4 (Winter 1997): 32– 47.
2. Slide drilling refers to drilling with a mud motor rotating the bit downhole without rotating the drillstring from the surface. This directional-drilling operation is conducted when the bottomhole assembly has been tted with a bent sub or a bent housing mud motor, or both. Without turning the drillstring, the bit is rotated with a mud motor, and drills in the direction it points. The wellbore trajectory can be controlled by limiting the amount of hole drilled in the sliding versus the rotating mode.
3. A bicenter bit is an integral bit and eccentric reamer used to simultaneously drill and underream, or enlarge, the hole. For more on drill bits: Besson A, Burr B, Dillard S, Drake E, Ivie B, Ivie C, Smith R and Watson G:
“ On the Cutting Edge,” Oil eld Rev iew 12, no. 3 (Autumn 2000): 36– 57.
4. The build angle is also referred to as dogleg severity. A dogleg is an abrupt turn, bend or change of direction in a wellbore, and can be quantied in degrees or degrees per unit of distance.
shock environments. These advances in hardware are coupled with durable, proven electronics that improve reliability and reduce risk. Oilfield Review presented early achieve- ments in specialized rotary steerable drilling operations several years ago.5
In this article, we
briefl y review directional-drilling technology and then focus on recent successes in Brazil and Norway involving novel applications of the most advanced rotary steerable systems.
Drilling in New Directions
Directional drilling began in the early 1970s as a technique to reach otherwise inaccessible reserves, particularly those drilled from offshore well templates (previous page). The earliest directional-drilling technology involved devices such as whipstocks that defl ected the drill bit, a method that offered limited control and that all too frequently resulted in missed targets.6
The
introduction of the positive-displacement motor offered steering capability and with it, direc- tional control, but the motor lacked the efficiency drillers sought. Eventually, steerable motors allowed rotation and sliding of the drill- string from surface, which further improved directional control. Nevertheless, this technology remained inefficient and risky because the extreme torque and drag limited drilling capabil- ity in sliding and rotating modes, leaving some targets inaccessible. In addition, the wellbore tortuosity the steerable motors produced in the sliding mode was unacceptable, in part because tortuosity makes future sliding more diffi cult and because tortuosity can impede critical operations for formation evaluation and running casing; log quality also suffers in rough holes.
5. Downton G, Hendricks A, Klausen TS and Pa tis D:
“ New Directions in Rotary Steerable Drilling,” Oilel d Rev iew 12, no. 1 (Spring 2000): 18– 29.
6. A whipstock is an inclined wedge placed in a wellbore to force the drill bit to start drilling in a direction away from the wellbore axis. The whipstock must have hard steel surfaces so that the bit will preferentially drill through either casing or rock rather than the whipstock itself. Whipstocks may be oriented in a particular direction if needed, or placed into a wellbore blind, with no regard to the direction they face. Most whipstocks are set on the bottom of the hole or on top of a high-strength cement plug, but some are set in the open hole. Use of a whip- stock presents some risk of mechanical sticking in a casing shoe or milled window.
7. Packing off refers to plugging of the wellbore around a drillstring. This can happen for a variety of reasons, the most common being that either the drilling uid is not properly transporting cuttings and cavings out of the annulus, or portions of the wellbore wall collapse around the drillstring. When the well packs off, there is a sudden reduction or loss of the ability to circulate, and high pump pressures follow. If prompt remedial action is not suc- cessful, an expensive episode of stuck pipe can result.
The introduction of rotary steerable technol- ogy eliminated several disadvantages of previous directional-drilling methods. Because a rotary steerable system drills directionally with contin- uous rotation from the surface, there is no need to slide the tool, unlike drilling with a steerable motor. Continuous rotation transfers weight to the bit more effi ciently, which increases the rate of penetration (ROP). Rotation also improves hole cleaning by agitating drilling fl uid and cut- tings, allowing them to flow out of the hole rather than accumulating as a cuttings bed. Advanced rotary steerable systems are designed to improve fluid circulation and cuttings removal. Effi cient cuttings removal reduces the chance for the bottomhole assembly (BHA) to become stuck or pack off.7
Continuous rotation and better hole cleaning reduce the chance of mechanical and differen- tial sticking of the drillstring. No stationary components contact the casing or borehole. In addition, rotary steerable technology improves directional control in three dimensions. The net result is a smoother, cleaner and longer well- bore, drilled faster with fewer stuck-pipe and hole-cleaning problems (below). The higher
Hole “ spiraling”
> Comparison of hole quality using a mud motor and the PowerDrive Xceed rotary steerable system. The mud motor tends to produce an uneven, overgauge or spiral borehole (top). The PowerDrive Xceed borehole (bottom) is smooth and in gauge.
Spring 2004
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