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Innovative Solution for Drilling Pre-Khuff Formations in Saudi Arabia Utilizing Turbodrill and Impregnated Bits

Authors: Gabriel D. Carrillo, Usman Farid, Michael Albrecht, Perry Cook, Nouman Feroze and Kenneth Nevlud

ABSTRACT

The abrasive pre-Khuff sandstones, combined with high downhole temperatures and the propensity for bottom-hole assembly (BHA) and bit sticking, present a uniquely hostile drilling environment. These limit run durations and the ability to optimize the rate of penetration (ROP) with the use of conventional rotary assemblies, positive displacement mud motors (PDMs)/conventional turbodrills and/or rotary steerable systems. Catastrophic damage or loss of drill strings, poor hole quality and logging problems are common, even with the advancements in polycrystalline diamond compact (PDC) bit technologies. Of the systems listed above, historically turbodrilling systems have best addressed the high temperatures and the abrasive nature of the pre-Khuff formations and held the potential for drilling economics optimization, but they have been unsuccessful in addressing bit sticking challenges. The development of best practices produced only marginal results because they require surface intervention, and therefore do not fully address sticking problems (in particular, bit sticking). The engineering challenge was to develop a downhole device that automatically engages and imparts sufficient drill string torque to maintain bit rotation. The turbodrill device disengages when conditions return to normal and returns the operation to high productivity drilling without surface intervention. The turbodrill device, coupled with advancements in BHA design, stabilizer and jar placement along with formation characterization and drill bit technology, is the solution to bit sticking incidents. Presented with this advancement in turbodrilling technology, the Operator/Service Company team has completed trials using this technology and presents data that supports the use and benefits of anti-sticking technologies. With this success, the team has regained focus on drilling optimization and reset the goal for single run-casing point to casing point.

INTRODUCTION

Saudi Aramco continually seeks ways to improve drilling efficiencies through the difficult pre-Khuff formations. This article is about a fortuitous trial of last resort that has delivered favorable results.

In Saudi Arabia, pre-Khuff formations (Unayzah, Jauf, Tawil, Sharawra, Qusaiba, Sarah, Qasim and Saq) are encountered at depths between 13,000 ft and 17,000 ft. These strata primarily comprise sandstones interbedded with shales, limestone, dolomite, anhydrite and siltstones. Sandstones with some siltstone predominate in the top Unayzah strata downward through the Qusaiba. At the bottom, the Sarah can also include some interbedded shale. The Unayzah and Jauf are generally characterized by hard, abrasive sandstones interbedded with shale and siltstone. Unconfined compressive strengths can reach 40,000 psi with internal angles of friction ranging from 25° to 75°. Hardness, abrasiveness, toughness, irregularity in size and orientation of rock constituents, and problems with sticking all contribute to a significant challenge and expense associated with drilling in the pre-Khuff formations1-3. High wear rates on bits and bottom-hole assembly (BHA) components present a significant potential for premature equipment failures, limit bottom rotating time, and necessitate long and frequent trips. The Upper pre-Khuff (Unayzah to Jauf) strata are the most difficult of the pre-Khuff formations to drill. These strata typically consist of very irregular, fine to coarse grained pebbles and conglomerates that can vary significantly in their dimensions and hardnesses (both in adjacent wells and at different depths in a particular well), and the Jauf formation may also contain pyrite. Pre-Khuff reservoirs are an important source of present-day Saudi Arabian nonassociated gas production. Saudi Arabia is progressively moving toward a significant increase in gas production. The expected growth is primarily for domestic fuel and petrochemical feedstock, although Saudi Arabia is also a large exporter of natural gas liquids (NGLs). To meet these goals, significant portions of Saudi Aramco drilling assets have been directed toward pre-Khuff onshore and offshore gas exploration and development during the past several years. Saudi Aramco aggressively seeks drilling solutions that mitigate inefficiencies. This article describes drilling the preKhuff sections in two offshore trial wells, which will be called Trial Well "1" and Trial Well "2" in this article. A total of 11 turbodrill with diamond impregnated bit runs, during 661 rotating hours, had four principal goals:

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· Decreasing trip frequency. · Decreasing bit and BHA component failures that lead to fishing. · Increasing productive, on-bottom bit life. · Increasing the rate of penetration (ROP) through use of high efficiency downhole drives and analytically engineered BHAs. Historically, vertical turbodrill runs through the pre-Khuff formation have been unsuccessful because of their inability to transmit rotary torque to the bit when sticking events occur4. Still, a turbodrill paired with a specially designed diamond impregnated drill bit promises a favorable response to all of the above goals. Although the bit sticking problems clearly remained in everyone's mind, interest in the tool remained as a result of its potential for durability and productivity in the event the sticking problems could be prevented5. This article begins with an offshore well, Trial Well 1, at a point where pre-Khuff problems had become acute. The well has had a troubled history. At a depth of 16,617 ft in the Unayzah formation, a twist-off occurred, necessitating a sidetrack. The sidetrack was started at the top of the Unayzah at a depth of 16,200 ft with a positive displacement mud motor (PDM) and polycrystalline diamond compact (PDC) bit. Saudi Aramco's pre-Khuff drilling is typically achieved with PDC bits drilled with rotary power. Because of the sidetrack, however, PDC with rotary was not possible in Trial Well 1. The assembly could not, of course, negotiate the change of direction at the sidetrack. The utilization of a down drive system increases the rotational speed and torque delivered to the drill bit, while isolating these increased forces from the drillstring. The drillstring rotary speed and torque required to drill effectively is reduced. This reduces the stress and tortuosity induced on BHA components and improves drillstring reliability. At Unayzah depths in Trial Well 1, temperatures of 340 °F were encountered. These temperatures cast doubt on reliability expectations for key PDM rubber components. Turbodrills easily withstand much higher temperatures, and in spite of past bit-sticking problems, there was little to lose: It was an opportunity for the evaluation of innovative turbodrill technology.

wear. This makes long drilling runs a typical and expected result. The vendor has spent years matching diamond impregnated bit performance and run life to the turbodrill. Diamond impregnated bit cutting structures with increased blade heights enable the incorporation of larger volumes of diamond material, thus longer bit life, into turbodrill bit cutting structures6. Bit aggressivity can be increased with use of larger diamonds in the impregnation mix (and vice versa) and/or modified by changing the number of blades. Cutting structure geometries also enhance nose and shoulder durability (and bit run life) without interfering with gauge protection. Thermally stable polycrystalline (TSP) inserts are positioned on the gauge and shoulders to ensure the bit has the capability to drill a gauge hole in hard and abrasive rock, and increase durability and wear resistance in the shoulders. Incorporated PDC cutters in the cone area improve ROP. While a matched bit and turbodrill promised long run life, bit sticking presented what seemed an insurmountable problem. If jarring was required to free a stuck bit attached to a turbodrill, there was a high probability of damage to the turbodrill requiring a tool replacement trip.

DEVELOPMENT OF MECHANICAL RECOVERY FROM STICKING

While every effort was made to optimize performance and durability potentials for all parts of the drilling system, preKhuff turbodrilling could be successful only if bit sticking events were avoided. Some months prior to the Well 1 problems, the vendor ran a series of development trials on a new optional tool feature that allows drillstring torque to be directly applied to a drill bit mounted on a turbodrill. The purpose of this device is to prevent the bit sticking, and it had shown solid potential during the initial prototype tests. This tool feature was disclosed and Saudi Aramco agreed to a trial. The device works as follows: If, while drilling ahead in rotary, the turbodrill drive shaft (attached to the bit) begins to slow/stall, a locking clutch (LC1) mechanism automatically (with no intervention from the surface) engages and transmits string torque via the drive shaft to the bit to prevent the stall. As most sticking originates during times in which a turbodrill is stalled, the clutch will prevent most incidents of bit sticking. Subsequently, as sticking forces are overcome, the need for string torque decreases, and the LC1 automatically disengages (again without surface intervention) and returns the system to high productivity turbodrilling. In turbodrilling conditions in which the potential for bit sticking is a significant operational risk, the LC1 showed promise as a very useful innovation, and a turbodrilling trial commenced using the innovative feature. At the time of the Saudi Aramco trial, experience with the LC1 was limited to about 1,800 hours of field testing. Based on performance data derived from this testing, Saudi Aramco and the vendor elected to mitigate the risk of a catastrophic bit sticking event by limiting the operational on-bottom run time to

TURBODRILL AND MATCHED BIT CHARACTERISTICS

Turbodrills running diamond impregnated drill bits are normally an excellent combination in hard, abrasive materials. Diamonds easily cut through all hard rock. Although diamond exposure in a diamond impregnated drill bit is quite small, the high rotating speeds of the turbodrill (1,000 rpm to 1,200 rpm or more) produce attractive ROPs because the number of rotations is so high. Turbodrills are similar to aircraft engines. Power is produced by rotating vanes that are subjected to very little

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80 hours. This time period was sufficient to provide both engineering application and fiscal justification for the trial, which would determine future use of the turbodrilling assembly equipped with the locking clutch device. For now, summarizing ahead, it can be mentioned that the turbodrills equipped with the LC1 produced no drilling disruptions of any kind, including bit sticking or locking clutch equipment failure.

OFFSET WELL

Well 1 could not be used for offset comparison to the Trial Well 1 sidetrack because it had not reached sufficient depth. Saudi Aramco, therefore, selected another offshore pre-Khuff well for comparative evaluation to the Trial Well 1 sidetrack. In this article, the offset is referred to as Offset Well. In the Offset Well, the top of the Unayzah begins at 16,416 ft, and TD 17,970 ft. The Offset Well was drilled with 17 rotary runs using PDC bits.

TRIAL PROGRAM

Key Saudi Aramco goals for a turbodrilling trial were: Operational cost reduction, mitigation in the required number of trips, avoidance of downhole component failures, increased on-bottom drilling time, and an evaluation of turbodrilling performance in hard, high temperature preKhuff rock. On historical turbodrill runs, Saudi Aramco has had a number of bit sticking problems and almost quit using turbodrills across the pre-Khuff. The intervening years brought stabilization engineering and development of matched diamond impregnated bits designed for drilling efficiency and for extended run durations. Turbodrilled hole quality and straightness are greatly influenced by BHA design. To ensure optimization, the vendor's engineers in Houston used proprietary BHA simulation software to model critical rpm and stabilizer placement. Rig hydraulic equipment and drilling fluid characteristics are another key factor in turbodrill optimization. Available pump pressures and flows are optimized to turbodrill requirements with proprietary software used by on-rig turbodrill supervisors prior to runs and regularly during drilling. Turbodrilling crews are, obviously, the controlling influence on tool performance. The turbodrilling supervisor is responsible for training the rig crew on drilling safety and procedures, and ensures there is no deviation from procedure during operations. This supervisor also works closely with the Mud Engineer to be aware of changes to the mud and reoptimize the tool when changes occur. The crews help prevent accidents of all types, including bit sticking, and ensure better optimized drilling performance.

TRIAL WELL 1 SIDETRACK

The Trial Well 1 sidetrack begins at a depth of 16,200 ft and was competed at 16,394 ft. This portion of the well is not included in summaries that are shown below. High bottom-hole temperatures (approximately 350 °F) are a problem at pre-Khuff depths. Because of temperature, PDMs have unsatisfactory operational life and are not suitable for drilling in these sections. Moreover, the utilization of a downhole drive system, which does not allow drillstring torque to be transmitted to the bit, disqualifies PDMs and conventional turbodrills alike from drilling potential bit sticking sequences. The wish to avoid the reoccurrence of catastrophic experiences with rotary drilling on the original well, combined with the incompatibility of a PDM, resulted in the turbodrill being the sole remaining alternative. Thus, a 43/4" turbodrill equipped with a locking clutch, and a matched diamond impregnated bit was selected, subject to the 80 hour drilling limit described above. Trial No. 1 goals included: An evaluation of turbodrilling performance and reliability, operational cost reduction in hard, high temperature pre-Khuff drilling, mitigation in the required number of trips, avoidance of downhole component failures and increased on-bottom drilling time. The first turbodrill trial run began at a depth of 16,394 ft. This tool drilled 377 ft in 73 hours. As indicated by surface torques during the run, bit sticking events did occur and were overcome by engagement of the locking clutch. There were no problems disruptive to drilling attributed to the turbodrill. As a result, Saudi Aramco approved continuation of the trial, on a run-by-run basis, to a TD of 17,900 ft. A PDC bit on rotary drilled 127 ft in a formation that was not conducive to efficient penetration by a diamond impregnated bit.

Trial No. 1 Run Summary

ORIGINAL WELL 1

The original Well 1 is an offshore well. The well targets a pre-Khuff reservoir drilled with PDC bits and rotary. The well has had a troubled history. At a depth of 16,617 ft, well short of the planned total depth (TD), in the Unayzah Formation, a twist-off occurred that necessitated continuing the well via a sidetrack that begins at the top of the Unayzah formation (16,200 ft). This sidetrack was to become turbodrilling Trial Well 1.

A total of 1,379 ft (six runs total, including five turbodrill runs and one PDC bit in rotary run) were drilled at an average ROP of 5.18 ft/hr. All bits were in good condition at the end of their respective runs. The hole condition was good, and there was no spiraling. High temperatures had no adverse effect on the turbodrill durability or performance. No loss of productive time was attributable to bit sticking or problems with either the turbodrills with LC1s or the matched diamond

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impregnated drill bits during any of this group of trial runs. Favorable results were achieved for all trial goals.

in the Trial Well 1 sidetrack, and results for both trials are, accordingly, beneficiaries of added credibility.

TRIAL WELL 2

Based on the favorable results in the Trial Well 1 sidetrack, Saudi Aramco authorized continuation of the turbodrill with LC1 trials in the pre-Khuff sections of Trial Well 2. The 80 hour drilling limit was continued. Primary goals for Trial Well 2 were the same as for the Trial Well 1 sidetrack. Of additional importance, Saudi Aramco wanted to determine whether or not the favorable Trial Well 1 results could be consistently expected, and both the Offset Well and the Trial Well 1 sidetrack served as offsets for Trial Well 2.

Trial No. 2 Run Summary

CONCLUSIONS

1. During this trial, there were no events in which bit sticking or BHA problems resulted in lost on-bottom drilling time. 2. The high temperature pre-Khuff environment had no adverse effect on turbodrill durability or performance. 3. Turbodrills equipped with the LC1 locking clutch and matched diamond impregnated bits drilled 11 runs. In addition, three PDC runs are included for a total of 3,863 ft drilled in comparable pre-Khuff sections in two trial wells, Table 1. 4. During these runs there were no events causing downtime related to either turbodrills or drill bits. 5. Turbodrills equipped with the LC1 were tripped from the well after a maximum of 80 on-bottom hours. Based

Required Trips

A total of 2,357 ft (eight runs total, including six turbodrill runs) were drilled at an average ROP of 4.9 ft/hr. Two PDC bits on rotary, between the first and second turbodrill runs, drilled 171 ft. A 60 ft core run between the fifth and sixth turbodrill runs is not included in feet drilled. As with the first trial, hole condition was good, and no nonproductive time was attributable to either turbodrills with LC1s or the matched diamond impregnated drill bits during the runs. Trial Well 2 results are exceptionally consistent with those produced Runs Days Footage Days/ Ft/Day Ave. 1,000 Ft Ft/Run 17 36.8 1781 20.7 48.4 104.8 6 8 22.5 35.3 15.6 2357 14.9 15.0 67.1 66.7 251.0 294.6

20

17

16

12

8

8

Offset Well Trial Well "1" ST Trial Well "2"

Table 1. Summary data

6

4

0

Offset

Fig. 2. Required trips.

Trial 1

Trial 2

25

20.7

20

Days / 1,000 Feet

14.9

15

15.0

10

5

0

Offset

Fig. 1. Typical condition of post-run drill bits. Fig. 3. Days per 1,000 ft drilled.

Trial 1

Trial 2

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350

294.6

300

251.0

250

Ave. Feet / Run

200

150

104.8

100

50 Fig. 6. Comparative depth vs. days. 0

Offset

Fig. 4. Average ft drilled per run.

Trial 1

Trial 2

80 70 60

67.1

66.7

Ave. Feet / Day

48.4

50 40 30 20 10 0 Fig. 7. Comparative depth vs. days (including coring).

Offset

Fig. 5. Average ft per day.

Trial 1

Trial 2

10. Feet drilled per day are 38% higher for the turbodrill, and impregnated bit combination than for the offset well. Again, this result is a positive trial result, Fig. 5. 11. Figures 6 and 7 show drilling performance comparison between offset and Trial Wells with and without coring runs. 12. It can be concluded from the study of reduction in days per 1,000 ft drilled, and for the required number of runs that turbodrilling with the innovative locking clutch and impregnated bits produced important fiscal benefits. In conclusion, the turbodrill locking device innovation had a very positive impact on the vertical pre-Khuff drilling program through two well trials. Saudi Aramco, in conjunction with Smith International, continues to seek and expand the applications for the concept while significantly increasing value through optimization of the turbodrilling system.

on positive trial results, the limit on drilling time is correctly established. 6. All diamond impregnated bits were tripped from the well in excellent condition after a maximum of 80 hours. One bit was reused with good results. All bits possessed additional life/value and their respective run lives could have been extended, Fig. 1. 7. The turbodrill and impregnated bit combination reduced the tripping required by the offset well by more than 50%. This is a positive result for the trial goal pertaining to tripping, Fig. 2. 8. Days per 1,000 ft drilled are 28% lower for the turbodrill and impregnated bit combination than for the offset well, Fig. 3. This result contributes positively to trial goals. 9. The impregnated drill bits more than doubled the feet drilled by PDC bits in the offset well. This contributes to a positive result for the trial goal of increasing time on-bottom, Fig. 4.

ACKNOWLEDGMENTS

The authors wish to thank Saudi Aramco management for their permission to publish this article. Special thanks to W.H. Wamsley, for his valuable assistance and support in the preparation of this article.

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REFERENCES

1. Simpson, M.A., Zhou, S. and Nordquist, D.G.: "Drilling Challenges and Recent Advances of Pre-Khuff Wells, Saudi Arabia," presented at the SPE Technical Symposium of the Saudi Arabia Section, Dhahran, Saudi Arabia, June 7-9, 2003. 2. Simpson, M.A., Zhou, S., Treece, M. and Rondon, C.: "Optimal Horizontal Drilling of Hard and Abrasive Unayzah Sandstones," SPE/IADC paper 85331, 2003. 3. Simpson, M.A., Zhou, S., Treece, M., et al.: "Breakthrough Horizontal Drilling Performance in Pre-Khuff Strata with Steerable Turbines," SPE paper 90376, 2004. 4. Internal Report, "Definition of Bit Sticking Problem with Turbine in Pre-Khuff Sands," Drilling Technology Unit, Drilling and Workover Engineering Department, Saudi Aramco, circa 1998. 5. Nordquist, D. and Zhou, S.: "An Interim Report on Bit Sticking Problem and Recommended Solutions," Internal Report, Saudi Aramco, October 2001. 6. Simpson, M.A., Roed, A.H., Al-Shammari, H.A. and Hoekstra, D.: "Rotary Application of Low Matrix Strength Impregnated and TSP Cutter Bits to Unayzah Sandstone Drilling," SPE/IADC paper 77223, 2002.

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BIOGRAPHIES

Gabriel D. Carrillo began working for Saudi Aramco in 2007. He received his M.S. degree in Petroleum Engineering from Texas A&M University, College Station, TX. Since 1994, he has worked for ExxonMobil, BP America around the world, and several small independent companies in South America, where his jobs included Field Service Technician, Rig Supervisor and Drilling Engineer. Currently Gabriel works in the Exploration Drilling Department where he monitors daily activities and plans upcoming events in a highly offshore profile well. Usman Farid is an Engineering Supervisor in Saudi Aramco's Exploration Drilling Department. He began working with the company in 2002 and over 23 years experience in rig drilling in the field, as well as in the office, up to the Drilling Superintendent level. Usman also worked as a Fishing Engineer and Sr. Drilling Instructor prior to joining the company. His job includes covering all engineering aspects from planning to completion for high profile HPHT offshore gas exploration wells currently being drilled. Usman graduated with a double B.S. degree in Math and Physics and Petroleum Engineering from the University of Engineering & Technology, Lahore, Pakistan. He also has an International Trainer certificate from the Northern Alberta Institute of Technology, Edmonton, Alberta, Canada. Michael Albrecht works for Smith Neyrfor in Saudi Arabia as a Drilling Product Manager. He started his career with Norward Energy in 1995, working in Canada and the USA. In 2000, Mike took an overseas position in Kazakhstan as a Safety Supervisor implementing safe work practices with TCO and Parker Drilling. He joined Smith Neyrfor in 2004 as a Turbodriller working primarily in Canada, and later took a position as a Drilling Product Manager for the Middle East in 2006. Mike has worked in several countries worldwide, including the Middle East, and has 14 years of oil field experience. He is currently based in al-Khobar, Saudi Arabia and has worked closely with the Saudi Aramco Exploration Group and the Joint Venture Operators on overseeing Turbine Operations and Sales.

Perry Cook is the Middle East Area Manager for Smith Drilling & Evaluation, responsible for the turbodrill product line. Perry started his oil field career with Baker Hughes, working for 7 years in the drilling fluids sector in the areas of technical research and field operations. He joined Smith Neyrfor in 2002 and has subsequently held field, technical, coordination and management positions in the UK, the U.S., Kazakhstan and Saudi Arabia. Perry is currently based in Abu Dhabi supporting all aspects of turbodrill business development throughout the Middle East. In 1995, he received his B.S. in Biotechnology from the University of East Anglia, Norwich, UK. Nouman Feroze is a Petroleum Geologist working as a Senior Sales Engineer for Smith Bits, based in AlKhobar, Saudi Arabia, where he serves Saudi Aramco's Exploration Group and Joint Ventures Operators. He started his career with Sperry Sun, Pakistan and worked 2 years as a mud logger, then he joined Smith Bits and worked in various locations in the Middle East. Nouman has a total 11 years of oil field experience. He received his B.S. degree in Geology in 1994, and in 1996 he received his M.S. degree in Petroleum Geology (Gold Medalist), both from the University of Karachi, Karachi, Pakistan. Nouman is an active Society of Petroleum Engineers (SPE) member. Kenneth Nevlud is a Mechanical Engineer with Smith Neyrfor, where he is currently the Manager of the Neyrfor Sustaining Engineering Group. He started his career with Smith International in 2000, spending 2 years with GeoDiamond focusing on bit design before pursuing turbine design with Smith Neyrfor. In 2000, Kenny received two B.S. degrees, one in Mechanical Engineering and the other in Mathematics, from The University of Texas at Austin, Austin, TX.

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