Significant technical and economic benefits define a uniquely designed slimhole point-the-bit rotary steerable system combined with a new generation of hostile environment measurement-while-drilling (MWD) and loggingwhile- drilling (LWD) tools. This highly efficient suite of drilling tools includes the Revolution* rotary steerable system and Hostile Environment Logging (HEL*) MWD system - provided by Precision Energy Services. Together, they have been successfully applied to several complex geometry wells offshore Europe, North Africa and the Middle East, with recent deployment 100 km offshore Libya in deep Mediterranean waters.
This technology duo has enabled operators to drill wells faster with efficiency gains of more than 100 per cent - and in some cases as much as 200 per cent - resulting in significant expense reduction. Operators are also accessing depths previously beyond the technical reach of conventional tools, allowing economic development of reservoir sections previously untapped. When faced with the objective of drilling its Al Jurf oilfield offshore Libya, Total used the slimhole rotary steerable system in tandem with the hostile environment MWD system to meet challenges associated with the reservoir's extended reach and complex geometry. The successful outcome exceeded expectations and raised the bar for future drilling in the region.
Wide variety
The Revolution point-the-bit rotary steerable system is a simple, compact design comprised of two principle components - a hydraulically actuated mechanical unit and a self-powered electronic control unit. The control unit is mounted inside a rotating non-magnetic drill collar and contains navigation sensors, batteries, microprocessor control and communication and memory functionality. The control system is designed to operate in a wide variety of downhole environments and can operate at temperatures up to 1650C.
The mechanical unit features a central rotating drive shaft that allows drilling fluid to pass through its center. Mounted on the rotating shaft is a non-rotating sleeve with an outer diameter close to full gauge and in contact with the wellbore wall. The central shaft is deflected in the required direction within the non-rotating sleeve by a hydraulic actuation system, featuring multiple arrays of hydraulic pistons mounted in a non-rotating manifold. All moving parts of the actuation system operate in a clean hydraulic bath to ensure efficiency of operation and longevity. Also, the hydraulic actuation system is pressure balanced to minimize wear on sealing systems that might otherwise prevent communication between the clean hydraulic oil bath and drilling fluid as a result of excessive wear.
The entire mechanical unit is built from nonmagnetic, high-strength metals. The required deflection of the center shaft is achieved by commanding a solenoid and rotary valve arrangement to direct energized ydraulic fluid from the self contained mechanical pump to specific arrays of sleeve positioning hydraulic pistons.
By operating the solenoid valve in a synchronous manner in relation to shaft rotation, selected banks of actuator pistons are energized to deflect the drive shaft in a particular direction. The control and operation of the solenoid valve is determined by a computer located in the control unit. The computer uses sensor and accelerometer information to compute and feedback tool-face and deflection information.
The deflection and tool-face demand information is either preset at surface or acquired by downward telemetry transmission. When the drive shaft is rotating and the solenoid stop valve is operated, fluid is fed through the distribution valve to the piston banks.
The Hostile Environment Logging (HEL) MWD system was developed by Precision Energy Services as part of an initiative to apply a broad suite of leading-edge capabilities to the development of breakthrough technology. With the industry's highest bottomhole pressure rating of 30,000 psi and the ability to operate in temperatures up to 1,800 C and survive up to 2,000 C, the HEL MWD system is designed to meet the demands of drilling in the most extreme conditions. In fact, it has successfully operated at bottomhole circulating temperatures up to 1,840 C.
Optimizing efficiency
The system features six key design components, including critical tools and sensors. Among them are a Pressure Modulated Telemetry System (PMT), an Environmental Severity Measurement Sensor (ESM), Dual Battery Module assembly (DBM), a High Temperature Azimuthal Gamma Ray tool (HAGR), Bore/Annular Pressure system (BAP) and Integrated Directional Sonde assembly (IDS).
When drilling deep and hostile environments, each of the system's components work together to reduce downtime typically associated with time-consuming multiple singleshot surveys required for traditional MWD systems with significantly limited capability. The reliability offered by the HEL MWD system under crippling high temperatures and high pressures combined with the system's higher flow rate capability, which improves hole cleaning, are additional factors in optimizing operational efficiency.
Long before the system's successful deployment, Precision Energy Services designed a test bench, enabling thorough testing of the rotary steerable system and HEL MWD at various stages during the development cycle. The design of the test bench stimulated downhole drillstring rotational dynamics and slippage of the rotary steerable's outer sleeve. The test bench also featured two high-powered servo motors, one to rotate the drive shaft and the other to control rotation of the outer sleeve.
Computer-controlled servo motors performed complex functions to simulate drill-string transient behavior or play back recorded data. The test bench accelerated the drive shaft of the rotary steerable system at up to 400 rpm within half a shaft rotation.
High accuracy displacement transducers that monitored sleeve position relative to the drive shaft enabled measurement of shaft deflection. Also, force was applied to the bias unit outer sleeve to verify load capacity of the internal hydraulic system. The test bench proved critical in development and testing of the rotary steerable control system.
In addition to the system test bench, a fullsize drilling simulation test rig was designed and constructed to allow dynamic testing of the rotary steerable tool. The test rig was designed to allow drilling of multiple 80-ft-long, principally horizontal holes thorough a block of concrete. The composition of the concrete was varied, within the length of the block, to provide different compressive strengths and simulate downhole formation changes.
The test rig consisted of a traveling power swivel to provide rotation and hydraulic rams to rush the BHA into the concrete and provide 'weight-on-bit.' A conventional mud pump supplied fluid. The test rig drilled multiple holes into a large block of concrete, typically 80 ft long by 5 ft wide by 8 ft high. With regards to drilling 6-in. holes, about 12 holes were drilled in each concrete block.
Quality
Each hole was accurately measured for size, using caliper and plug gauge devices. They were surveyed using a bespoke wireline survey tool and measured using laser survey instruments. Due to easy access to each end of the hole, video cameras were used to accurately record the quality of the wellbore.
Since the initial testing, about 100 holes have been drilled on Precision's test rig, and tests have continued to evaluate and analyze directional performance of the rotary steerable system and optimize its bottomhole assembly (BHA). The test rig has also been used for endurance testing of the rotary steerable system under pseudo-downhole conditions.
The test rig has proven to be a valuable component in verifying and refining computerbased models used to predict directional behavior and explore operational issues. It has enabled scientific evaluation of the effect of each variable through comparative testing. Key findings from the various concrete drilling tests revealed that (1) the borehole produced by a drill bit is always larger in diameter than the drill bit; however, BHA optimization can minimize this effect, (2) the mechanical advantage of point-the-bit configuration can be optimized to improve tolerance of the steering system to over-gauge the hole, (3) push-the-bit steering techniques tend to produce an imperfect borehole with significant spiraling and ledges produced while point-the-bit steering eliminates these features, producing a very smooth, accurate, high-quality well bore and (4) directional performance of the rotary steerable system has proven to be predictable and controllable.
Testing of the HEL MWD system was among the industry's most stringent, including flow loop testing for erosion and LCM tolerance determination, high-pressure testing at elevated temperatures and aggressive vibration qualification. Vibration testing included randomon-random standards during multiple hightemperature cycles.
The rotary steerable system was used with the HEL MWD system from the BD-1 platform in Libya's Al Jurf field for Total in direct response to difficulties the operator had previously experienced in the well using conventional steerable motor tools.
When using the steerable motor assembly, problems of high string torque and complete tool loss in the lateral section of certain zones occurred after a measured depth of 12,000 ft, resulting in extremely difficult drilling conditions. The operator specifically faced challenges drilling a 6 -in. build-up section and lateral section from the platform. The directional plan called for a build-up section beneath the 7 5/8-in. casing shoe followed by a long lateral section. Inclination at the casing shoe was 53.4. The vertical section was drilled at an average build-up rate of 2.3/100 ft and a turn rate of 0.75/100 ft. The assembly initially built from 53.4 to horizontal while turning the azimuth through 16 to the left.
The build and lateral section of 6,283 ft was drilled in 213 hours at an average penetration rate of 31.2 ft / hr during the build-up section and 29.5 ft/hr during the entire 6 -in. hole section, enabling the operator to quickly meet its objectives and log the well using pipeconveyed logs, and confirm that a good quality wellbore had been drilled.
Before implementation of the point-the-bit rotary steerable system and hostile environment MWD system, the longest lateral section drilled on previous wells in the field was 3,992 ft with a measured depth of 13,152 ft and an inability to slide after 11,873 ft.
With implementation of the new tools, full directional control was maintained throughout the build-up and lateral section of the well with a starting depth of 9,286 ft and TD being reached at 15,569 feet. This was the longest well drilled in the field to date, with a TD 2,417 feet deeper than any previous well.
Conclusion
The directional drilling performance of the rotary steerable system combined with MWD tools in both laboratory tests and deployment in the field has proven the technology's superiority vs. standard systems. Most notably, rotary steerable systems that point the bit - bend the assembly so that the bit is pointed toward the intended direction while drilling instead of pushing the bit -- exert lateral side force on the bit as it drills ahead, resulting in a much higher quality wellbore.
For Total, the impact was significantly more efficient use of rig time and reduction in total drilling costs compared with previous wells.
*This article was written by Simon Peach, Rotary Steerable Product Manager for Precision Energy Services, and is derived from a paper planned for presentation at the 26th International Petroleum and Gas Conference in Tihany, Hugary, September 22-24, 2005. The author acknowledges the combined efforts of the staff at Smart Stabilizer Systems in Tewksbery (UK) and Advantage Engineering in Houston.
*Trademark of Precision Drilling Corporation or an affiliate.
© Oil Review Middle East 2005
