Fully rotating PowerDrive X5 RSS is available in 4.75-, 6.75-, 8.25-, 9.0-, and 11.0-in. diameters to drill boreholes ranging from 5 ¼ in. to 26 in.

For many years the conventional wisdom held that rotary steerable systems (RSS) were so expensive to run that they should be reserved for deep, high-risk wells in tough areas like offshore or Alaska. But the tide is turning, as more operators find that the improvements realized in drilling efficiency and wellbore quality when RSS are used easily result in bottom-line savings. Higher penetration rates, better hole cleaning, fewer bit trips, and improved steerability add up. And with straighter boreholes, casing and completion jobs are less problematic. The ultimate result is that operators are able to get production into the pipeline faster, with fewer problems, and that is pure profit.

Several factors contribute to the success of RSS. In the first place, since the drill pipe is always rotating, axial friction is greatly reduced, allowing maximum power to translate to the bit. Pipe rotation aids in hole cleaning. In addition, it enables selection of more aggressive bits than are appropriate for motors; in fact, PowerDrive Xceed RSS is the only rotary steerable system that can steer with a bi-center bit.

Continuous drill pipe rotation makes the bit work more efficiently. Many operators find that they can drill borehole sections with a single bit trip, called shoe-to-shoe drilling. One operator cut US $2 million off a $12 million AFE because of the ability to drill both the 121?4-in. and 81?2-in. sections of a well through a highly abrasive formation with one bit each, compared to 10 bits per section required on two offset wells drilled conventionally.

Power steering

Drilling efficiency is only one of the factors to be gained by using RSS in well construction. Another is precise well placement. Whether the objective is to drill a gun-barrel straight borehole or steer the bit through a complex reservoir structure to optimize placement of the production interval in the highest quality reservoir sections, RSS can perform as required. Successful directional drilling results from the integration of three important factors: using a high-performance drilling technology, acquiring sufficient geological data while drilling so proactive steering directions can be given to the bottomhole assembly (BHA), and having real-time decision-making capability so effective course corrections can be decided and implemented. The importance of this third factor can be illustrated in the following case study from a well recently constructed in the Permian Basin of West Texas.

Drilled in a hot new play in the Delaware Basin called the 3rd Bone Springs Sand, the well was planned to traverse 2,200 ft (671 m) of a thin sand reservoir situated between two other sand bodies. After its construction, the well would be hydraulically fractured to connect with the upper and lower sand bodies. The target sand had been described by data from the best of 50 to 100 offset wells in the area, and everyone believed that offset control was excellent. The plan called for a soft landing about 500 ft (152 m) above the target sand, at which point the 475 PowerDrive X5 RSS with PeriScope bed boundary mapper and sonic logging-while-drilling (LWD) BHA was picked up.

The new BHA was intended to land the well in the target and continue to drill the reservoir sand for about 2,200 ft. The PeriScope tool has the ability to see formation as much as 21 ft (6.4 m) away from the tool radially in all directions. As the bit ate up the last 500 ft of the landing section, the geologists noticed that the target sand was coming in 14 ft (4.3 m) shallower in true vertical depth (TVD). They started steering the well bore immediately to intersect the sand. The closer the tools got to the target, the better they were able to see that the sand, instead of sloping away slightly as suggested from regional mapping, was in fact dipping gently upward. The operator was contacted and, in conference with the geologist and the drilling team, elected to believe the information from the downhole tool, even though it contradicted everything they knew up to that point. What was universally believed to be a uniformly flat formation was shown to have both upward and downward undulations and dip magnitudes varying from 1.2° to 2.4° in opposite directions.

The LWD tool was able to provide the real-time formation data so the directional driller could steer the RSS by issuing commands via downlink telemetry, keeping the bit in the thin sand body for 85% of the horizontal section. A total lateral length of 2,224 ft (678 m) was drilled in a single run. Because of the precise positioning information the driller was receiving, it was possible to maintain less than 1° of dogleg severity through the target zone.

The PowerDrive X5 RSS has three servo-controlled pads spaced at 120° intervals around the tool near the bit. The driller programs the tool to drill a certain azimuth and inclination, then uses commands from the surface to steer the wellbore into the highest quality portions of the target reservoir. The commands cause the pads to push in sequence against the borehole wall opposite the direction intended as the tool rotates, thus providing a constant force that steers the bit in the desired direction. Because steering is effected by a continuous directional force, changes in borehole trajectory are smooth.

Many benefits realized

Besides the obvious benefit of identifying the up-sloping target and staying within it, other important benefits were realized. Because of the higher drilling efficiency delivered by the RSS, the well was completed in only 9.5 days, 10.5 days less than a conventional mud motor could drill, and saved $450,000 in rig time. The sonic LWD measurements were combined with those of a subsequent wireline sonic log to permit a stress analysis of the reservoir and its surroundings to design the hydraulic fracture job. The multizone stimulation was carried out without incident thanks to the smooth borehole profile and low dogleg severity. These benefits reflect the excellent collaboration between the operator and the many service company technical teams that cooperated to construct and complete this well.

Straightening out the curves

Not every well in West Texas is horizontal. The benefits of RSS also accrue in vertical wells. The Permian Basin may be flat at the surface, but the subsurface strata include steeply dipping beds off the slopes of the Diablo and Central Basin Platforms (see Figure 1) and in the overthrust area of the Southern Marathon Shelf as well as numerous shallow salt and anhydrite sections that cause drill bits to “walk” off course.

Crooked wells cost money. If the borehole represents the vertical section of a well that will transition to a horizontal drainhole, having unplanned doglegs in the surface pipe can cause casing and drillpipe wear damage when the steerable assembly is introduced to drill the target zone. If the well is planned as a vertical completion, the doglegs cause undue tubing and sucker rod wear. Some wells can get so far out of directional control that they cross a lease line, and that causes big problems.

The solution is to drill the vertical sections with a PowerV bias unit. The system achieves very high penetration rates because it integrates the RSS with a powerful positive displacement mud motor. This allows the driller to get the high rpm needed to achieve high penetration rates with reduced weight-on-bit (WOB). Excess WOB is one of the causes of bit walk with conventional systems. But with this system continually measuring and controlling the verticality of the well bore, drillers can apply as much WOB as they like for maximum drilling efficiency. When introduced into a deviated well, it immediately steers the borehole back to vertical and keeps it there. Not only is the well drilled straight down, but typical results show a 57% reduction in drilling days compared to motors in a similar situation.

A typical example

Recently a well drilled in Ector County, Texas, targeted a 10,000 ft (3,049 m) deep formation. Hitting a tough spot, the bit started to walk off vertical. Efforts to straighten it were unsuccessful, and it reached more than 8° of inclination. The bias unit was deployed at about 8,384 ft (2,557 m). Within 466 ft (142 m) the borehole was brought automatically back to vertical, where it continued at less than 0.1° deviation all the way to bottom (Figure 4). Up until the point the RSS was introduced, average daily penetration rate was 271 ft (82.6 m). As soon as the RSS was deployed, the average daily rate rose to 465 ft (142 m).