development of a directional drilling system for operation
TRANSCRIPT
GRC Transactions, Vol. 40, 2016
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Development of a Directional Drilling System for Operation at 300°C for Geothermal Applications
Kamalesh Chatterjee, John Macpherson, Aaron Dick, Harald Grimmer, Sundaie Klotzer, Jon Schroder, Dave Epplin, Carsten Hohl, and Sobieslaw Gacek
KeywordsDirectional drilling system, Measurement-While-Drilling (MWD), Enhanced Geothermal System (EGS), mud motor, high temperature
ABSTRACT
Many countries around the world, including the USA, have untapped geothermal energy potential. Enhanced Geo-thermal Systems (EGS) technology is needed to economically utilize this resource. If hot rock is sufficiently fractured with continuous channels interconnecting large volumes of rock with a very large surface area it is possible to economi-cally extract geothermal energy from deep in the ground. The Department of Energy (DOE) spearheads research and innovation in tools and technologies required for successful and economic development of EGS reservoirs. Temperature in some EGS reservoirs can exceed 300°C. To effectively utilize EGS resources, an array of injector and production wells must be accurately placed in the formation fracture network. This requires a high temperature directional drilling system. Most commercial services for directional drilling systems are rated for 175°C while geothermal wells require operation at much higher temperatures. Two US Department of Energy Geothermal Technologies Program (GTP) projects have been initiated to develop a 300°C capable directional drilling system, the first consisting of a drill bit, directional motor, and drilling fluid, and the second adding navigation and telemetry systems.
The development of the 300°C directional drilling system will be discussed focusing on the drill bit and directional motor. Two complete tools were assembled and tested at Baker Hughes Experimental rig (BETA) at Oklahoma in April 2014. This high temperature directional drilling technology will be equally useful to the oil and gas industry where the temperature occasionally rises to above 260°C.
1. Introduction
The United States has vast untapped geothermal energy potential. Using Enhanced Geothermal Systems (EGS) technology, geothermal wells can supply the energy consumption for the USA for 2,000 years [MIT study 2006]. Drilling geothermal wells for EGS require precise placement of injection and production wells [GTP 2008]. Any short circuit (or direct pathway) in the water path from the injection to production well causes insufficient heat exchange. On the other hand, when heat exchange impedance is too high (for example, if the wells are too far apart lacking any connecting frac-tures) the flow becomes poor and heat exchange remains insufficient. Precise well positioning achieves maximum heat exchange and could render EGS economical. The precise placement can only be done with a directional drilling system and measurement-while-drilling (MWD) system. There is no directional drilling system in the industry capable of sur-viving temperatures encountered in deep EGS wells, where formation temperatures can be in excess of 300°C. Current commercial offerings for directional drilling systems are rated to 175°C.
Two US Department of Energy (DOE) Geothermal Technologies Program (GTP) projects were awarded to Baker Hughes to develop a 300°C (572°F) drilling system. The first project (DE-EE0002782) covers the development of drill bit,
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steerable motor, and drilling fluid all rated for 300°C and capable of operating at least 50 hours at depths up to 10,000 m [Dick A.J, 2012]. The system is built for an 8.5-in. (diameter) hole. The second DOE project (DE-EE0005505) covers the power source, telemetry, navigation sensors and directional electronics for a 300°C measurement while drilling (MWD) system. We have completed the conceptual system design phase of the EE0005505 and started detailed design. The two systems together provide a complete directional measurement while drilling solution.
In the past few years [Dick A.J, 2013, Chatterjee K, 2014] we have designed and developed a directional drilling system consisting of drill bit, steerable motor, and drilling fluid all rated for 300°C. We have conducted drilling simulator tests of the drill bit and drilling fluid under controlled conditions. We also have conducted flow loop tests to evaluate the motor performance in controlled conditions. We also have tested many critical components and subsystems in ovens at 300°C temperatures and above. After the successful assembly of the complete drilling system the tool was used to drill in granite with a full scale rig at the experimental test facility in BETA, Tulsa, OK. Adjustable Kick-Off rings (AKO) were used to build angle to test directional drilling capability. The remainder of the paper describes the directional drilling system focusing on the design challenges of the drill bit and the steerable mud motor.
Both hybrid PDC-rolling cone bits and Tricone rolling cone bits were selected for this application. The main design challenge at 300°C operation is the removal of all elastomeric components in the system. The metal mechanical face seals, high temperature bearing grease and metal bellows grease pressure compensator, all capable of sustaining a 300°C are described in section 2.
A directional motor design based on the positive displacement motor (PDM) was selected. A PDM is driven by mud flow through the power section of the tool, and its simple mechanical design enables excellent reliability. A typical industry standard mud motor uses elastomer to reduce friction and prevents fluid from leaking. At 300°C the challenge is finding an alternative to this elastomer. The developed mud motor features an industry-first, metal-coated metal-to-metal (M2M) contact between the rotor and the stator. Section 3 elaborates further on motor development.
2. Drill Bit
High temperatures such as 300°C pose several unique challenges for traditional rock bits. There are various types of rock bits available, each with unique application specific advantages. In order to make appropriate selection decisions in bit style, past EGS wells were reviewed. These installations typically drill deep into the basement where hard igneous formation is present, along with high confining pressure which further increases rock strength. Applications in Southern Australia and Vietnam granites, and Iceland basalt formations were studied. Additional information on formations from the Newberry crater site 55-29 well was considered as this represented the closest lithology for target EGS well sites. Cores of volcanic tuff were sourced from an area close to the Newberry Crater site, and performance tests were conducted on all currently available bit styles. Bit performance parameters, rate of penetration (ROP) and torque are plotted in Figure 1 from drilling simulator tests on various bit styles. These results are further elaborated in Dick. A.J, 2012. Based on the results of these tests, and considering the projected weight and torque capability of the metal to metal motor assembly, a tungsten carbide insert (TCI) roller cone bit and hybrid PDC-roller cone bit were selected. In some of the shallower sections of the Newberry crater site, the granites, dolomites, and other harder formations are less prevalent and are softer formation like the volca-nic tuff are more prevalent. In these applications, the hybrid bit can take advantage of the easier drilling and achieve higher rates of penetration. For harder rocks including granite, the tungsten carbide insert roller cone bit would be required.
After the selection of bit styles the next task was to develop the specialized subcomponents capable of operating at 300°C. A review of bit design and materials of construction identified the components most sensitive to the extreme operating temperature requirement. Both bit types utilize a lubricant, elastomeric pressure compen-sation system, and elastomeric seal components. Though there have been strides made to improve the temperature capability of these, there is still nothing commercially available to withstand continuous operation at 300°C.
Figure 1. Bit performance from bottomhole simulator testing. All tests were per-formed at 41.4 MPa (6,000psi) confining pressure with 9.5 ppg water-based mud [Dick. A.J, 2012].
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Based on prior experience in energized metal face seal technology, an all metal sculptured seal that is self-energizing was selected. Typically these seals use elastomer components to energize a metal rotary seal. Eliminating the elastomers in this system required a singular seal design comprised of four key features. The material had to be suitable to operate at 300°C, static seal on one end, dynamic seal on the other, and could energize the dynamic sealing face to be capable of sealing a differential pressure. Several designs were investigated and analyzed using finite element analysis. After several itera-tions, a design was finalized for independent seal tests. Tests were completed at elevated temperatures. It showed capability to effectively seal a low pressure differential during 50 hour endurance testing in and out of a mud bath, and without yielding the material being deflected. The seal was then packaged into the full bit design to be utilized in the TCI bits. An example of this seal is shown in Figure 2.
A hybrid bit typically uses smaller bearing sizes due to the smaller cone size. Developing a new sculptured metal seal to fit is considerably more complicated. To develop a solution for the hybrid, high temperature elastomer compounds were used in the energizing seals in a metal face seal design. These specialized compounds are capable of intermittent but not continuous exposure to 300°C as reported in Shakhovsko. D. 2009. With new material properties, the geometries were slightly adjusted to obtain the nominal seal preload. This would be adequate because the hybrid bits are expected to be used in shallower sections where the tem-peratures are expected to be lower.
In order to equalize the differential pressure from the lubricant reservoir to the downhole environment, a pressure compensation system is used. This reduces the pressure differential across the rotary seal as the downhole pressure in-creases and decreases by allowing the diaphragm to be exposed to the same pressure. Typically an elastomeric diaphragm is used to accomplish this; however, they are not capable of withstanding 300°C. To accomplish the system temperature requirements, an all-metal bellows with welded flanges was developed. The Inconel material utilized was capable of operating at the required temperatures, while allowing weld attachment to the bit body and excellent fatigue strength. The bellows were successfully fatigue tested to almost 3 million cycles without failure. Two sizes were designed to fit both bit types. A recess was created to fit the bellows in its extended, relaxed state. The outside of the bellows is towards the grease reservoir, and the inside is exposed to the downhole environ-ment through a hole in the weld flange. A schematic cross section of the metal bellow is shown in Figure 3.
Conventional greases used in drilling bits were not able to maintain properties at high temperatures. Using a standard bit-grease as a baseline, it became evident that the thickener would break down early at these elevated temperatures. Beyond a certain temperature, the coefficient of friction increases rapidly. Several new formulas were developed and put through seizure and wear tests at temperature. One particular formula stood out; yielding up to 10-times lower friction and significantly smaller wear scar di-ameters. During aging tests, it was able to maintain critical properties after 50 hours of exposure to 300°C in contrast to the baseline which was severely degraded, loosing lubricity as shown in Figure 4.
After solving the technical chal-lenges at the component level we proceeded with the design and manu-facturing of 5 hybrid bits and 4 roller cone bits. The bits were built to support internal testing on a downhole pressure simulator and at the BETA test facility in Oklahoma. The remaining bits will be used for field tests. Currently, two roller cone bits have been successfully tested, one at the BETA test facility in Oklahoma, and the other on the downhole pressure simulator drilling rig. The same bit from the simulator drilling rig was also used for a second test at BETA.
In the simulator, the bit was run on Sierra white granite at 6000psi confining pressure with water, high-temperature drilling fluid, drilling fluid with lube additive, and a standard 9.5ppg drilling mud. All subcomponent systems performed
Figure 2. Metal sculptured self-ener-gizing seal.
Figure 3. Metal bellows in the pressure compensation system.
Figure 4. Contrast of standard high temperature grease after exposure to 300°C.
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as expected. This simulator bit, along with a separate roller cone bit, were also run at the BETA test facility in combination with the drilling fluid and all metal motor developed for this project. Due to limited drilling time available, one bit was able to drill ahead for 16 hours and the other bit for 12 hours. Although these tests were not at 300°C, they were the first functional tests of the entire system. All of the specialized subcomponents functioned without failure, and with seals effective at the end of the runs. The 16 hour tested bit has since been disassembled and an extensive forensic evaluation performed to assess the integrity of the new design features, materials and lubricant. No contamination was found in the grease, rotary seals had no signs of seizure and were maintaining sealing efficiency. The metal bellows compensators were not damaged either as shown in Figure 5.
3. Drilling Motor
In addition to the 300°C power section the complete drilling motor consists of several subcomponents. These typically include top stabilizer, top subassembly, power section, Adjustable Kick-Off (AKO) subassembly, bearing assembly, upper bearing housing stabilizer, and the drive subassembly with the bit box connection. These subsystems are shown in Figure 6.
Commonly used drilling motors for the power section in deep drilling operations are positive displacement motors (PDM). The motor employs a reverse application of the pump principle first established by René Moineau, in which circulating drilling fluid is used to drive the drill bit independent of drill string rotation. The Moineau principle holds that a helical rotor with one or more lobes will rotate when placed ec-centrically inside a stator having one more lobe than the rotor. The rotor and the stator form a series of sealed cavities so that when drilling fluid is pumped into the tool, the rotor will be driven in an eccentric, rotary motion relative to the stator, allowing the fluid to pass while transmitting rotational power to the drive train and bit. The characteristics of a PDM are a function of the design of the stator/rotor geometry. The available torque and rotational speed depend on the pitch angle and the number of lobes in the stator and rotor.
At Baker Hughes two different stator types are widely used, which differ in the use of the elastomeric sealing material. The conventional Ultra stators and the pre-contoured or even-wall X-treme stators are depicted in Figure 7. Pre-contoured stators have the advantages of higher torque and power output, better overall efficiency and higher temperature rating up to 190°C (374°F). Though this temperature rating is quite high, it is still well below 300°C (570°F). The temperature limit is primarily imposed by the elastomeric sealing material. There is no elastomer material available that withstands such high temperatures paired with the high dynamic loads seen in a Moineau power section.
In order to make the motor operate at 300°C metal to metal seal instead of an elastomeric seal was used. Power sections with elastomer are usually assembled with a compression fit between rotor and stator to ensure good sealing, which leads to a good (volumetric) efficiency. An all-metal power section on the other hand doesn’t have the high flexibility of the elastomeric material and therefore can’t be assembled with a compression fit. To ensure that the power section can be assembled a (slight) flush fit is necessary. To ensure good sealing and no fluid loss the gap between rotor and stator needs to be minimal. The rotor and stator need to be machined with the highest possible accuracy to keep the flush fit tolerance to the lowest achievable. We have optimized our proprietary pre-contour manufacturing process for the stator and enhanced a Weingärtner rotor mill for milling the rotor. As a result the flush fit of the 6 ¾” all-metal motor prototype is well below one millimeter.
With such narrow clearances, the big challenge in assembly of this metal to metal motor is to find the correct machine and processes that could place the rotor into the stator without damaging the rotor and stator coatings. Figure 8 shows the assembly process and the torque machine used to assemble the rotor and stator.
When the motor is running both the stator and rotor components are exposed to high sliding velocities and high con-tact pressures. High sand content in the drilling fluid makes the rotor and stator of the all-metal power section susceptible to wear by erosion and abrasion. To mitigate this, both the stator and the rotor are coated with a wear resistant coating. In
Figure 5. Metal bellows compensa-tor after simulator and 12 hour BETA run still containing majority of grease volume, undamaged.
Figure 6. Typical drilling motor components.
Figure 7. Type of stator contour used in positive displacement motors.
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addition, the drilling mud needs to have lubricant added to it. An extensive coating evaluation program was performed to locate a coating technology that could successfully meet the 50 hour tool lifetime requirement and the operating parameters required by the drill bit.
We have built two complete drilling systems consisting of a 6 ¾” all-metal motor with an adjustable kick-off for directional drilling in a 300°C environment. The motor was first tested for 50 hours in a dynamometer endurance test. The test was conducted with water and about 4% lubricant. The motor successfully passed the test, the wear rate was low. Performance of the motor was promising; the reduction in motor speed over time was very small. Test results are summarized in Figure 9. Although the overall efficiency is lower than that of elastomer type motors, the efficiency is higher than expected. The overall efficiency is approximately 30%, which is just under that of conventional motors (40 – 60%).
In addition to the motor power section there have been several up-grades to make the tool operate at 300°C. The flow diverter seals have been upgraded to a 300°C rating. Fatigue is expected to be more detrimental at such high temperatures, hence the flex shaft design is modified to enhance fatigue resistance. And finally, near the drill bit subsystem high capacity diamond thrust bearings were used to withstand the high weight on bit encountered in hard rock drilling.
3.
Successful Testing of the Directional Drilling System
The first test of the complete drilling system was at BETA, south of Tulsa, Oklahoma in December 2013. The system drilled 197 ft for 16 hours in granite at the target weight-on-bit, with the mud providing lubrication of the motor and bit. Repeat testing occurred in April 2014; 180 feet were drilled through granite and steering with a BUR of 6°/100ft with higher rate of penetration than the initial test. Figure 10 shows the picture at the test site as the tool is lowered into the borehole during the first test. The ROP varied depending on weight-on-bit and bit rotation rate (motor speed plus drillstring rotation rate) but ranged between 15 to 25ft/hour for most of the run. Weight-on-bit was gradually increased to 40klbs.
Complete testing of the directional drilling system at the BETA test site was conducted with a Wyoming bentonite/polyanionic cellulose polymer (GEL/PAC) drilling fluid conditioned with the lubricant. The test rig also
Figure 8. Assembly of the precision flush fit metal to metal motor requires skilled professionals.
Figure 9. Metal to metal motor performance in flow loop test. Torque increases with pressure as expected and the slight reduction in speed with pressure is also to be expected.
Figure 10. The drilling tool ready at the BETA test facility.
Figure 11. Granite sample at 5202 ft depth (left) and at 5270 ft (right).
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allowed for the evaluation of a solids control equipment package while drilling granite with the directional drilling system. At the experimental test run mud samples were collected daily. Three cuttings samples were collected and submitted for X-Ray diffraction analysis to determine mineralogical composition. The formation of granite was visibly different as the drilling progressed. Figure 11 illustrates the differences in appearance of the circulated granite at two different depths. At the shallow depth, the granite has the reddish-brown color while at the deep depth the granite contains darker segments.
Based on the successful test at the experimental rig at BETA the system is ready for field trial. The ability of the system to build an angle and perform directional drilling is adequate. We are confident of the high temperature perfor-mance because many critical components and subsystems have been tested in oven at 300°C temperatures and above. A test bench evaluation of one of the motors run at BETA was conducted. In addition to 9 hours downhole, this motor has been tested at an additional 50 hours in a surface flow loop to evaluate wear on the system. It survived that test and will now be run at an additional 50 hours with a bent sub to increase the side load within the motor.
4. Conclusion
The US Department of Energy Geothermal Technologies Program awarded a project to Baker Hughes to develop directional drilling system rated for operation at 300°C and suitable for EGS and conventional geothermal wells. The challenges were to develop drill bit, drilling motor and drilling fluid, all capable of operating at 300°C. The complete drill-ing system has been designed, developed and tested at a test well at BETA. The test run showed excellent performance; customer field testing is scheduled for Q3 2016.
The US Department of Energy also awarded a second project for development of measurement while drilling sys-tem consisting of telemetry, directional electronics and power source for operation at 300°C. The detailed design of the complementary measurement-while-drilling system is complete. These two systems together enable accurate directional wellbore placement in deep EGS wells by providing a complete measurement while drilling solution. This has the potential of reduce EGS well installation costs to bring EGS energy production closer to commercialization.
Acknowledgements
This material is based upon work supported by the Department of Energy Geothermal Technologies Program under award numbers DE-EE0002782 and DE-EE0005505. The authors thank AltaRock, Geodynamics and the Geothermal Resource Group for providing well data and advice on EGS and conventional geothermal well requirements.
Disclaimer
This paper was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe upon privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
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