New drilling system combines a high-power motor with high-speed rotary closed-loop steering technology to expand the drilling envelope.

Since their introduction in the late 1990s, rotary closed-loop steering (RCLS) systems have changed the way operators plan and execute their drilling programs. A new version, AutoTrak X-treme from Baker Hughes Inteq, combines RCLS technology with a positive displacement motor to extend the capabilities of rotary steerable drilling.
The new system fully integrates a high-performance drilling motor into an RCLS system. By integrating these components, it was anticipated that rotary steerable drilling systems could be employed in tougher drilling environments where previously they had drilled too slowly to be economically viable. In addition, the new system would allow significant extensions to the lateral reach of wells and reduce the risks associated with rotary steerable drilling in smaller hole diameters. This emerging technology has been operating on a limited basis to evaluate performance and confirm the anticipated benefits. The results, in terms of both drilling performance improvement and field drainage efficiency, are very promising.
System description
The high-speed RCLS is capable of operating at rotary speeds of up to 400 rpm. In comparison, conventional rotary steerable technology operates at speeds ranging from 200 to 250 rpm. Taking advantage of the basic system's high-speed specifications and the system's modular bottomhole assembly (BHA) design, the company designed a special high-powered, compact drilling motor to integrate within the downhole assembly using proprietary motor technology. The power sections produce 60% to 100% more power than comparably sized conventional mud motors. This enhanced performance is made possible via a proprietary, pre-contoured stator design. Reducing the stator's rubber content enables greater mechanical and hydraulic efficiency, lower friction and internal heat build-up and minimizes the effect of chemicals in the drilling mud.
This drilling motor is positioned immediately above the steering unit to drive the drill bit with high power for increased drilling performance compared with purely surface driven rotary steerable drilling. The motor is hard wired along its length to provide continuous communication between the rotary steering system below the motor and the measurement while drilling (MWD) systems positioned above it. Since the system has a high total rotary speed capability (surface + motor speed), the entire drill string is still able to be rotated from surface at a speed sufficient for effective hole cleaning, but excessively high surface rotary speeds are no longer required to attain acceptable drilling performance.
Applications
In tough formations, the drilling performance improvement over either standard rotary steerable or steerable motor drilling has been very impressive. Rate of penetration (ROP) is frequently double or more than that of offset wells, saving significant rig time and allowing early production. For example, in the Central North Sea (CNS), most production is from reservoirs located below or in the lower Cretaceous. The chalk in this area is notoriously tough to drill and use of rotary steerable systems here has shown to be very slow compared to conventional motor drilling. For this reason, the CNS Cretaceous was identified as an ideal area to evaluate the performance of the system. Since the new system has been operated there, many wells have been independently benchmarked either "Best in Class" or "Top Quartile" drilling performance. This is a tremendous achievement considering the tough nature of the formations.
The reach envelope of extended reach drilling (ERD) wells is frequently bounded by the torque capability of the rig. As the well becomes longer, friction increases and eventually reaches the limits of the rig capacity to deliver drilling torque. At this point, no further progress is possible. The onset of this limit is often earlier that anticipated. As drillstring rotary friction increases, with a corresponding drop in drillstring rotary speed, torsional oscillations or "stick-slip" vibration becomes more severe to a point where no useful energy is reaching the cutting structure of the drill bit. The new system has been used in ERD wells offshore Denmark to evaluate the effectiveness in extending the reach of wells past limits established by the drilling rig's torque output specifications. Using the new system, horizontal ERD wells here have been drilled significantly past prior reach limits. One well was drilled with a 6,700 ft (2,043 m) additional extension past what was previously possible and stopped only because the extremity of the reservoir was reached. If the reservoir had continued to be viable, drilling could have continued. The key benefits of this capability are enhanced production and more efficient field drainage.
Performance of the system has been evaluated in complex multilateral wells offshore India. The reservoir in this area is a tight limestone that frequently causes high drilling torque and vibration. Rotary steerable systems have been used for some time to drill these wells, but torque and vibration restricts lateral reach and causes drilling equipment damage. Introduction of the new system to this field gave an immediate boost to drilling efficiency and achievable horizontal displacement of the drains. On the first well, ROP increased by 48%, the average run length increased by a factor of three and the longest ever lateral drain in the area was achieved.
To date, most rotary steerable drilling work has been performed in 121/4-in. and 81/2-in. hole sizes, but it is now becoming more prevalent in 6-in. hole as 43/4-in. rotary steerables are developed. However, there are risks associated with rotary steering in this smaller hole size.
First, smaller diameter pipe used in 6-in. sections is weaker than the pipe used in upper hole sections. The need to drill with high surface rotary speed to attain reasonable ROP increases the risk of catastrophic pipe failure in the event of a BHA stall. With the new integrated system, there is no need to rotate the drill string any faster than the speed required to achieve efficient hole cleaning. In addition, because drill bit torque is delivered by the motor and effectively decoupled from the drill string above, torque spikes caused by stalling of the drill bit are not immediately transferred directly to the drill string. Therefore, the drill string is protected from these events.
Second, many 6-in. hole sections are re-entry wells that may be damaged by standard rotary steerable drilling techniques. The parent or donor well being exited may be old, with existing casing already worn from original drilling activities or corroded from production. Every effort should therefore be made to preserve the integrity of the existing well while drilling the re-entry. The need for continuously high speed rotation of the drill string when using standard rotary steerable drilling techniques risks additional wear to the existing casing or tubing, compromising its integrity. Since the new system does not require high drillpipe rotary speed to drill at high ROP, wear damage to the well is minimized. This application is being evaluated offshore Norway, where extension to the life of a mature field requires an extensive through tubing rotary drilling program.
Summary
The integration of a rotary steerable system with a high-powered drilling motor is an emerging technology with great potential. It has been developed to overcome many of the concerns and challenges identified through more than 10 years of rotary steerable drilling experience. Using the new system, it was anticipated that drilling efficiencies would increase dramatically in harsher environments and, as a result, the benefits of rotary steerable drilling would extend to areas which previously struggled to justify its use. This has been evaluated and the results are very promising. The system was also designed to allow extensions to complex ERD wells and again, very promising results have been observed. In addition, it was planned to apply the system in smaller hole sizes to reduce the specific risks associated with rotary drilling in slim holes. Again, this has been evaluated and the results are also very promising.