Dragon Oil Plc used a new powered rotary steerable system (RSS) from Schlumberger to drill an almost 2,625-ft (800 m), 81/2-in. section in less than 3 days through the difficult Cheleken reservoir series in the LAM field, in the Caspian Sea offshore Turkmenistan. Normally taking 7 to 10 days to drill a similar section in a formation that had previously exhibited high stick-slip tendencies, using the PowerDrive vorteX system saved essentially 1 week of rig time, which translated into cost savings of US $875,000 for the operator.

The challenge

Dragon Oil, the sole owner of a Russian era offshore production sharing agreement (PSA), undertook an infill drilling project in the LAM field, Caspian Sea, that initially involved expanding the LAM 21 platform and drilling four production wells from it. The directional drilling, measurement-while-drilling (MWD) and logging-while-drilling (LWD) services were to be provided by Schlumberger, which has been drilling in Turkmenistan for Dragon Oil since November 2003. The wells generally have an S-shaped profile with a maximum 30? angle and a long, 81/2-in. diameter vertical section through the Cheleken reservoir series. The Cheleken reservoir is difficult to drill through using a conventional bottomhole assembly (BHA) because conventional assemblies do not rotate while sliding, thus increasing the chances of differential sticking. High stick-slip tendencies exhibited in previous wells also made drilling difficult.

Because of the differential sticking risks and necessary high mud overbalance, sliding through the Cheleken series was not an accepted practice. Furthermore, the build tendency of the formation required that a reduced weight-on-bit (WOB) be used, which subsequently decreased the rate of penetration (ROP). While drilling the first well of the new infill project, LAM 21/107, a formation-induced build tendency of up to 1?/30 m was encountered in the Cheleken series. The only practical means of controlling it was by decreasing the WOB, and therefore, the ROP. As a result of the problems encountered in this first well, Dragon Oil asked Schlumberger for a recommended strategy to improve the drilling of the project's next well, LAM 21/108.

The plan

Schlumberger proposed the use of its PowerDrive vorteX RSS. While the tool system had been used in different areas around the world since its introduction in February 2004, this would mark its first use in the Caspian Sea. The new RSS would fulfill a number of criteria leading to a viable solution to the challenges faced by Dragon Oil in the area: First, the system would help meet the operator's high bit revolutions per minute (rpm) requirements in its drilling operations. Second, it would allow the use of a motor to help alleviate possible casing wear in the upper sections of the well bore. Finally, the vorteX system would provide precise verticality without the need for sliding, thus helping to eliminate the differential sticking tendencies encountered in the past.

In fact, the new system enabled RSS drilling in conditions that were less than optimal for an RSS alone. By combining the PowerDrive motor and vorteX technologies, the challenges of the drilling environment would be met and done so at higher ROPs. The vorteX system has a high-torque power section that converts mud hydraulic power to mechanical energy. When this added energy is combined with surface torque generated by the rig's top drive, overall torque capacity is significantly increased, as is bit rpm. The additional torque and higher rpm enables faster, more cost-effective hard rock drilling. For the drilling of the LAM 21/108 well, an A700GT 7:8 power section was proposed to drive the system, using a maximum flow of 500 gpm. Mud weight would be maintained at or below 14 lb/gal.

The use of a polycrystalline diamond compact (PDC) bit also was proposed, specifically, the Smith MRS74PX bit with 13 mm and 16 mm cutters. By operating at increased rpm, the powered RSS optimizes PDC bit performance, further improving drilling efficiency and penetration rates. The combination of increased torque capacity and bit rotation translates to faster drilling rates and longer runs. As a result, significant rig time and drilling cost savings are generated with its use. For the LAM 21/108 well, 7 days of rig time were saved, which translated into cost savings of $875,000 for Dragon Oil.

The vorteX system has a feature inherent with every PowerDrive RSS: all external parts rotate, which serves to further improve ROP. Full rotation also helps clean and condition the hole which, in turn, reduces drilling torque, drag problems and the risks of differential or mechanical sticking. This was particularly important while drilling the Cheleken reservoir series because of the inherent differential sticking risks that occurred while sliding. Because the RSS system always rotates, the possibility of differential sticking is reduced. Additionally, directional control is maintained without having to reduce WOB and ROP.

The execution

A rotary drill out assembly was run to drill out the 95/8 in. casing shoe because the cement plug had not bumped. After confirming a good casing seat existed, the vorteX system was picked up and run in the hole. Hole inclination was dropped in a controlled manner (less than 2?/30 m dog leg severity) from 9? back to vertical and then the reservoir section was drilled with a maximum hole inclination of 0.81? throughout 1,968 ft (600 m) of vertical hole, with 90% of the section drilled at less than 0.5? inclination. The bit came out graded 1-1-WT-A-XIN-NO-TD and the vorteX power system had less than 1.0 mm of bearing play. To summarize, 2,539 ft. (774 m) of 81/2-in. hole were drilled in less than 3 days (60 hours), which would have taken 9 to 10 days with a standard motor assembly. The RSS enabled problem-free drilling and saved Dragon Oil the costs of essentially one week of rig time.

Moreover, vorteX usage greatly reduced the stick-slip occurances. This was made possible by the system's power section, discussed previously. The integrated power section not only rotates the bit rapidly, but also optionally slows drill string rotation. Consequently, stick-slip and other damaging vibration modes common to conventional RSS drilling are reduced. Rather, all available energy is used to drill the hole efficiently and maximize ROP.

The future

Other existing technology currently available is not capable of drilling an 81/2-in. hole section in a similar manner as was accomplished using the vorteX system. Dragon Oil has now requested that Schlumberger make available the PowerDrive vorteX 900 for the 121/4-in. hole sections in its future wells. According to the company, the PowerDrive vorteX RSS overcame difficult challenges to increase drilling performance and penetration rates, reducing overall well costs for Dragon Oil by saving days of rig time. With its unique energy combination, the powered RSS promises to perform high ROP drilling in hard-rock and other challenging environments like the Cheleken reservoir series in the LAM field, offshore Turkmenistan.

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