Even as rotary steerable assemblies prove their worth around the world, exceptional drilling challenges call for special solutions.

Rotary steerable assemblies are replacing conventional motors and bent subs because they offer huge advantages in terms of higher penetration rates (ROP) and better hole quality. The more the drill string rotates, the less friction there is to impede drilling progress and transport of cuttings back to surface. Steerability is facilitated by a rotating assembly as is the ability to drill a smooth trajectory.

However, some formations challenge even the latest drilling technology. Extremely hard, abrasive formations, and extremely soft, unconsolidated formations as well as high pressure/high temperature/high shock environments call for special solutions. The PowerDrive Xceed system from Schlumberger has been designed and built as a niche tool to address these special drilling challenges. Mike Williams, marketing manager for Schlumberger Drilling & Measurements, said, "There should be no limit to anything the driller wants to do, and the Xceed system is designed to give drillers this capability."

Among the capabilities of the system is the ability to drill out shoes, perform openhole sidetracks without a whipstock, ream and back-ream and steer while drilling with bi-center bits. It can tackle tough formations and drill doglegs to accommodate difficult well designs while maintaining good hole quality. According to the company it enables lower drilling costs because of its high ROP, efficient hole cleaning, excellent wear resistance and reliability. Williams said, "The PowerDrive Xceed system accomplishes all of this mainly because of one unique feature - everything rotates."

Everything rotates

The tool is internally stabilized, meaning that it does not depend on contact with the borehole wall to maintain a reference in geospace. With its smooth external profile, the fully rotating assembly has minimal friction, and that delivers maximum power to the bit while facilitating the removal and transport of cuttings. Efficient hole cleaning not only prevents pack-offs but keeps the bit from wasting effort re-grinding already drilled cuttings.

Because the steering reference does not depend on borehole contact, the system can steer through washouts, extremely heterogeneous strata and overgauge holes, all with high build rates and doglegs of as much as 8?/100-ft. Its ability to open the hole while rotary steering is advantageous in deepwater or extended reach applications.

The system uses a mud turbine to generate power to operate the steering assembly as well as the control system electronics and sensor package. All attitude sensors are internal, and are designed to withstand very high shock loading in excess of 1,000 hr without problems. The steering assembly continuously orients a tilted bit shaft to control the drilling direction and dogleg severity (DLS).

Early global experience

The Xceed system has been used in difficult drilling operations around the world. Recently, drilling thin sands in Qatar's Nahr Umr reservoir for Maersk Oil, the system provided geosteering control that allowed it to stay in the target sand section for 99% of the 6,581-ft (2,006-m), 81/2-in. hole, while making a 90? turn at an extended step-out. More than 90% of the drain hole was placed within the optimum zone of the pay sand.

In West Africa, drilling the prolific Girassol formation for Total, the system was used to achieve a large number of 4.5?/30-m doglegs in unconsolidated formations. In this application the advantage of continuous rotation benefited critical azimuthal density/neutron (ADN) logging while drilling (LWD) measurements. The ADN measurement does not work while sliding, and real-time ADN measurements were required to correlate with seismic to hit formation sweet spots. In this application the drillers benefited from the tool's ability to receive and implement directional commands via downlink while drilling. This allowed them to respond quickly to changes in formation dip observed in real-time.

One Middle East operator used the Xceed system to successfully drill a heavily-faulted sandstone reservoir. A 52% (US $1.5 million) reduction in total well costs, resulting primarily from 91% increase in drilling efficiency per bit run, was realized, despite loose sands, abrasive conditions, and the need to maintain trajectory control while drilling multiple, faulted sand layers. Nearly 90% of the wells required openhole sidetracks to overcome obstacles in the laterally-variable sandstone reservoir. According to the company, rotation of the entire drill string and reduced wellbore tortuosity added up to 68% reduction in trip time as a result of better hole quality. Among the records set were the longest well ever drilled in the field, a 13,689-ft (4,175-m) sandstone section drilled in a single run, two successful openhole sidetracks and run lengths exceeding 191 circulating hours.

The Xceed system is part of Schlumberger's PowerDrive Xtra family of rotary steerable systems. Presently it is available in 61/2-in. size to drill holes ranging from 83/8-in. to 97/8-in. using concentric bits. It can also drive bi-center bits.

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