After considerable experience, ADCO reports excellent results using rotary steerable systems to drill onshore Abu Dhabi wells.

The Abu Dhabi Company for Onshore Oil Operations (ADCO) prides itself on performance, and in the utilization of technology to improve it. The company produces oil from five fields located onshore and in the shallow coastal waters of the Persian Gulf. The majority of their wells have a horizontal lateral drainhole through the carbonate reservoir. Accordingly, ADCO has closely followed the evolution of steerable drilling assemblies, and are alert to any new designs or techniques that will improve hole quality or drilling efficiency.

Aggressively following a Well Delivery Limit approach, ADCO tested rotary steerable systems when they became available and quickly recognized the advantages in their specific situation. Efficiencies over the sliding mode of directional drilling were considerable and included greater extended reach, ability to drill more complex trajectories, better hole cleaning and higher rates of penetration (ROP). Drilling efficiency was enhanced because changes in toolface could be made on the fly without tripping the pipe. Whereas deviation changes could be made easily enough using conventional motors and bent subs, when a combination of deviation and azimuthal changes were required rotary steerable systems proved to be clearly superior.

Previously, ADCO experienced problems with borehole quality attributed to conventional directional drilling. This affected the company's ability to get good logging while drilling (LWD) data, specifically the LWD density log. Density tools hung up on borehole ledges causing poor log quality right in the middle of critical zones. Since the advent of rotary steerable systems, the problem has disappeared because the continuously rotating tool string does not hang up.

Borehole quality is a priority for ADCO. Besides improving LWD quality, smoother boreholes enable better casing, cementing and completion installation. The company has used rotary steerable technology almost from its inception. At the time, hole direction was achieved using "push-the-bit" technology. Simply put, this means that servo-actuated pads on the circumference of the bottomhole assembly (BHA) near the bit sequentially pushed against the borehole wall during drilling to nudge the bit in the opposite direction. By changing the orientation of the pushing action, the borehole could be easily steered in any direction as well as up or down.

A second generation rotary steerable technology called "point-the-bit" shows greater promise for ADCO. It this scheme, the bit is attached to a long drive shaft encased in a housing. An eccentric cam is used to bend the shaft on a bearing that acts as a fulcrum thus the bit can be pointed in any direction. One of the key advantages of point-the-bit technology is that it enables the use of extended-gauge bits that have a significant impact on hole quality, in particular by reducing ledges and hole spiraling. Most of the time, point-the-bit holes are gun-barrel smooth.

GeoPilot system tested

The result of a collaboration between Japan National Oil Corp. (JNOC) and Halliburton, the Sperry-Sun GeoPilot tool employs point-the-bit technology. As described, the bit is attached to a long drive shaft which is flexed to orient the bit in the desired direction. The tool's outer housing is allowed to slowly rotate to provide a reference orientation and aid in hole cleaning and friction abatement. Seals protect the moving parts from mud or cuttings and act as barriers to contain the internal lubricating oil that keeps everything cool and turning smoothly. The housing contains an inclinometer package that continually advises which was is "up," but the key component is the compound bias unit located halfway along the shaft. Here, two counter-rotating cams deflect the shaft to point the bit. When the cams are oriented 180° relative to each other, they cancel and bit orientation is coaxial with the BHA. However, the closer the cams are oriented toward each other, the greater is the deflection of the shaft, and consequently the angle of the bit.

Once it is tilted to the desired angle, orientation of the bit is achieved by rotating the assembly. A closed loop mud pulse system is used to send steering commands to the bias unit and send confirmation pulses back to surface that the unit has executed the commands.

Because the fulcrum bearing is located within 2 ft (.61 m) of the bit, a high degree of bit tilt can be achieved with relatively little shaft deflection. And because the bit itself is pointed, as opposed to being pushed to the side by pads, long gauge bits can be used that improve hole quality. No side-cutting action is required, and bit performance is on par with straight hole drilling.

Two examples make the point

Although ADCO has used point-the-bit steering on several wells, two examples illustrate the advantages. Wells in the Rumaitha Field have previously been very difficult to drill using conventional directional drilling techniques in sliding mode. Problems included low ROP, continual hanging-up and poor hole quality. The RA-17 well was drilled with a conventional steerable motor to 6,945 ft (2,118 m) and a deviation of 7.2?. The remainder of the well was drilled using the GeoPilot rotary steerable system with a full triple combo LWD string. Initially, hole angle was built from 7.2? to 18.4? at a rate of 3.3?/100-ft. Then 1,000 ft (305 m) of tangent section was drilled before hole angle was increased to 85? with an average build rate of 2.86?/100-ft. At the same time, the hole azimuth was turned from 3? to 32? and the section bottomed at 10,503 ft (3,203 m). A total of 114 hours were required to drill the point-the-bit second including 10 reaming/circulating hours. Two problematic formations were drilled with relative ease, the Nahr Umr shale and the cherty Shilaf formation. Previously, it had been difficult to maintain bit orientation in these formations let alone steer, but with GeoPilot they were able to drill through both with no loss of ROP and no need to pull the bit.

The RA-22 well involved drilling an 81/2-in. hole using an extended gauge bit. A triple combo LWD tool with roller reamer to reduce torque was run. The GeoPilot was run to the shoetrack at 10,403 ft (3,172 m) and drilled out the float collar and cement. Then the horizontal section was drilled at 90? inclination as the hole azimuth was geosteered using real-time LWD data from 41? to 100? with maximum turn rate of 5?/100-ft. In spite of encountering hard stylolites, the hole was drilled in a single run of 63.5 drilling hours. Final measured depth was 14,757 ft (4,500 m).

With more than 20 successful runs behind them, ADCO concludes that rotary steerable systems deliver overall improvements in drilling rate. In particular, the point-the-bit technology gives additional benefits including reliable directional response in problem formations, excellent directional control and cost effectiveness. With the GeoPilot system it was noted that it is possible to kick off from vertical without using a whipstock, and to drill reliably using extended gauge bits. They also experienced improved tripping when an undergauge roller reamer was installed in the BHA above the tools.

Comments