Pre-Khuff

ABSTRACT

The abrasive pre-Khuff sandstones, combined with highdownhole temperatures and the propensity for bottom-holeassembly (BHA) and bit sticking, present a uniquely hostiledrilling environment. These limit run durations and the abilityto optimize the rate of penetration (ROP) with the use ofconventional rotary assemblies, positive displacement mudmotors (PDMs)/conventional turbodrills and/or rotarysteerable systems. Catastrophic damage or loss of drill strings,poor hole quality and logging problems are common, evenwith the advancements in polycrystalline diamond compact(PDC) bit technologies.

Of the systems listed above, historically turbodrillingsystems have best addressed the high temperatures and theabrasive nature of the pre-Khuff formations and held thepotential for drilling economics optimization, but they havebeen unsuccessful in addressing bit sticking challenges. Thedevelopment of best practices produced only marginal resultsbecause they require surface intervention, and therefore do notfully address sticking problems (in particular, bit sticking).

The engineering challenge was to develop a downholedevice that automatically engages and imparts sufficient drillstring torque to maintain bit rotation. The turbodrill devicedisengages when conditions return to normal and returns theoperation to high productivity drilling without surfaceintervention. The turbodrill device, coupled withadvancements in BHA design, stabilizer and jar placementalong with formation characterization and drill bit technology,is the solution to bit sticking incidents.

Presented with this advancement in turbodrillingtechnology, the Operator/Service Company team hascompleted trials using this technology and presents data thatsupports the use and benefits of anti-sticking technologies.With this success, the team has regained focus on drillingoptimization and reset the goal for single run-casing point tocasing point.

INTRODUCTION

Saudi Aramco continually seeks ways to improve drillingefficiencies through the difficult pre-Khuff formations. Thisarticle is about a fortuitous trial of last resort that hasdelivered favorable results.

In Saudi Arabia, pre-Khuff formations (Unayzah, Jauf,Tawil, Sharawra, Qusaiba, Sarah, Qasim and Saq) areencountered at depths between 13,000 ft and 17,000 ft.These strata primarily comprise sandstones interbedded withshales, limestone, dolomite, anhydrite and siltstones.Sandstones with some siltstone predominate in the topUnayzah strata downward through the Qusaiba. At thebottom, 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 withinternal angles of friction ranging from 25 to 75.Hardness, abrasiveness, toughness, irregularity in size andorientation of rock constituents, and problems with stickingall contribute to a significant challenge and expenseassociated with drilling in the pre-Khuff formations1-3. Highwear rates on bits and bottom-hole assembly (BHA)components present a significant potential for prematureequipment failures, limit bottom rotating time, andnecessitate long and frequent trips.

The Upper pre-Khuff (Unayzah to Jauf) strata are the mostdifficult of the pre-Khuff formations to drill. These stratatypically consist of very irregular, fine to coarse grainedpebbles and conglomerates that can vary significantly in theirdimensions and hardnesses (both in adjacent wells and atdifferent depths in a particular well), and the Jauf formationmay also contain pyrite.

Pre-Khuff reservoirs are an important source of present-daySaudi Arabian nonassociated gas production. Saudi Arabia isprogressively moving toward a significant increase in gasproduction. The expected growth is primarily for domesticfuel and petrochemical feedstock, although Saudi Arabia isalso a large exporter of natural gas liquids (NGLs). To meetthese goals, significant portions of Saudi Aramco drillingassets have been directed toward pre-Khuff onshore andoffshore gas exploration and development during the pastseveral years.

Saudi Aramco aggressively seeks drilling solutions thatmitigate inefficiencies. This article describes drilling the pre-Khuff sections in two offshore trial wells, which will be calledTrial Well 1 and Trial Well 2 in this article. A total of 11turbodrill with diamond impregnated bit runs, during 661rotating hours, had four principal goals:

SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2009 37

Innovative Solution for Drilling Pre-KhuffFormations in Saudi Arabia UtilizingTurbodrill and Impregnated Bits

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

38 FALL 2009 SAUDI ARAMCO JOURNAL OF TECHNOLOGY

wear. This makes long drilling runs a typical and expectedresult. The vendor has spent years matching diamondimpregnated bit performance and run life to the turbodrill.Diamond impregnated bit cutting structures with increasedblade heights enable the incorporation of larger volumes ofdiamond material, thus longer bit life, into turbodrill bitcutting structures6. Bit aggressivity can be increased with useof larger diamonds in the impregnation mix (and vice versa)and/or modified by changing the number of blades. Cuttingstructure geometries also enhance nose and shoulderdurability (and bit run life) without interfering with gaugeprotection. Thermally stable polycrystalline (TSP) inserts arepositioned on the gauge and shoulders to ensure the bit hasthe 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 insurmountableproblem. If jarring was required to free a stuck bit attached toa turbodrill, there was a high probability of damage to theturbodrill requiring a tool replacement trip.

DEVELOPMENT OF MECHANICAL RECOVERY FROMSTICKING

While every effort was made to optimize performance anddurability potentials for all parts of the drilling system, pre-Khuff turbodrilling could be successful only if bit sticking eventswere avoided. Some months prior to the Well 1 problems, thevendor ran a series of development trials on a new optional toolfeature that allows drillstring torque to be directly applied to adrill bit mounted on a turbodrill. The purpose of this device isto prevent the bit sticking, and it had shown solid potentialduring the initial prototype tests. This tool feature was disclosedand Saudi Aramco agreed to a trial.

The device works as follows: If, while drilling ahead inrotary, the turbodrill drive shaft (attached to the bit) begins toslow/stall, a locking clutch (LC1) mechanism automatically(with no intervention from the surface) engages and transmitsstring torque via the drive shaft to the bit to prevent the stall.As most sticking originates during times in which a turbodrillis stalled, the clutch will prevent most incidents of bit sticking.Subsequently, as sticking forces are overcome, the need forstring torque decreases, and the LC1 automatically disengages(again without surface intervention) and returns the system tohigh productivity turbodrilling. In turbodrilling conditions inwhich the potential for bit sticking is a significant operationalrisk, the LC1 showed promise as a very useful innovation, anda turbodrilling trial commenced using the innovative feature.

At the time of the Saudi Aramco trial, experience with theLC1 was limited to about 1,800 hours of field testing. Based onperformance data derived from this testing, Saudi Aramco andthe vendor elected to mitigate the risk of a catastrophic bitsticking event by limiting the operational on-bottom run time to

Decreasing trip frequency.

Decreasing bit and BHA component failures that lead tofishing.

Increasing productive, on-bottom bit life.

Increasing the rate of penetration (ROP) through use ofhigh efficiency downhole drives and analyticallyengineered BHAs.

Historically, vertical turbodrill runs through the pre-Khuffformation have been unsuccessful because of their inability totransmit rotary torque to the bit when sticking events occur4.Still, a turbodrill paired with a specially designed diamondimpregnated drill bit promises a favorable response to all ofthe above goals. Although the bit sticking problems clearlyremained in everyones mind, interest in the tool remained as aresult of its potential for durability and productivity in theevent the sticking problems could be prevented5. This articlebegins with an offshore well, Trial Well 1, at a point wherepre-Khuff problems had become acute. The well has had atroubled history. At a depth of 16,617 ft in the Unayzahformation, a twist-off occurred, necessitating a sidetrack. Thesidetrack was started at the top of the Unayzah at a depth of16,200 ft with a positive displacement mud motor (PDM) andpolycrystalline diamond compact (PDC) bit. Saudi Aramcospre-Khuff drilling is typically achieved with PDC bits drilledwith rotary power. Because of the sidetrack, however, PDCwith rotary was not possible in Trial Well 1. The assemblycould not, of course, negotiate the change of direction at thesidetrack.

The utilization of a down drive system increases therotational speed and torque delivered to the drill bit, whileisolating these increased forces from the drillstring. Thedrillstring rotary speed and torque required to drill effectivelyis reduced. This reduces the stress and tortuosity induced onBHA components and improves drillstring reliability.

At Unayzah depths in Trial Well 1, temperatures of 340 Fwere encountered. These temperatures cast doubt onreliability expectations for key PDM rubber components.Turbodrills easily withstand much higher temperatures, andin spite of past bit-sticking problems, there was little to lose:It was an opportunity for the evaluation of innovativeturbodrill technology.

TURBODRILL AND MATCHED BIT CHARACTERISTICS

Turbodrills running diamond impregnated drill bits arenormally an excellent combination in hard, abrasive materials.Diamonds easily cut through all hard rock. Although diamondexposure in a diamond impregnated drill bit is quite small, thehigh rotating speeds of the turbodrill (1,000 rpm to 1,200rpm or more) produce attractive ROPs because the number ofrotations is so high.

Turbodrills are similar to aircraft engines. Power isproduced by rotating vanes that are subjected to very little

80 hours. This time period was sufficient to provide bothengineering application and fiscal justification for the trial,which would determine future use of the turbodrilling assemblyequipped with the locking clutch device.

For now, summarizing ahead, it can be mentioned that theturbodrills equipped with the LC1 produced no drillingdisruptions of any kind, including bit sticking or lockingclutch equipment failure.

TRIAL PROGRAM

Key Saudi Aramco goals for a turbodrilling trial were:Operational cost reduction, mitigation in the requirednumber of trips, avoidance of downhole component failures,increased on-bottom drilling time, and an evaluation ofturbodrilling performance in hard, high temperature pre-Khuff rock.

On historical turbodrill runs, Saudi Aramco has had anumber of bit sticking problems and almost quit usingturbodrills across the pre-Khuff. The intervening yearsbrought stabilization engineering and development of matcheddiamond impregnated bits designed for drilling efficiency andfor extended run durations.

Turbodrilled hole quality and straightness are greatlyinfluenced by BHA design. To ensure optimization, thevendors engineers in Houston used proprietary BHAsimulation software to model critical rpm and stabilizerplacement.

Rig hydraulic equipment and drilling fluid characteristicsare another key factor in turbodrill optimization. Availablepump pressures and flows are optimized to turbodrillrequirements with proprietary software used by on-rigturbodrill supervisors prior to runs and regularly duringdrilling.

Turbodrilling crews are, obviously, the controlling influenceon tool performance. The turbodrilling supervisor isresponsible for training the rig crew on drilling safety andprocedures, and ensures there is no deviation from procedureduring operations. This supervisor also works closely with theMud Engineer to be aware of changes to the mud and re-optimize the tool when changes occur. The crews help preventaccidents of all types, including bit sticking, and ensure betteroptimized drilling performance.

ORIGINAL WELL 1

The original Well 1 is an offshore well. The well targets apre-Khuff reservoir drilled with PDC bits and rotary. Thewell has had a troubled history. At a depth of 16,617 ft, wellshort of the planned total depth (TD), in the UnayzahFormation, a twist-off occurred that necessitated continuingthe well via a sidetrack that begins at the top of the Unayzahformation (16,200 ft). This sidetrack was to becometurbodrilling Trial Well 1.

OFFSET WELL

Well 1 could not be used for offset comparison to the TrialWell 1 sidetrack because it had not reached sufficient depth.Saudi Aramco, therefore, selected another offshore pre-Khuffwell for comparative evaluation to the Trial Well 1 sidetrack.In this article, the offset is referred to as Offset Well. In theOffset Well, the top of the Unayzah begins at 16,416 ft, andTD 17,970 ft.

The Offset Well was drilled with 17 rotary runs usingPDC bits.

TRIAL WELL 1 SIDETRACK

The Trial Well 1 sidetrack begins at a depth of 16,200 ft andwas competed at 16,394 ft. This portion of the well is notincluded in summaries that are shown below.

High bottom-hole temperatures (approximately 350 F) are aproblem at pre-Khuff depths. Because of temperature, PDMshave unsatisfactory operational life and are not suitable fordrilling in these sections. Moreover, the utilization of adownhole drive system, which does not allow drillstring torqueto be transmitted to the bit, disqualifies PDMs and con -ventional turbodrills alike from drilling potential bit stickingsequences. The wish to avoid the reoccurrence of catastrophicexperiences with rotary drilling on the original well, combinedwith the incompatibility of a PDM, resulted in the turbodrillbeing the sole remaining alternative. Thus, a 434 turbodrillequipped with a locking clutch, and a matched diamondimpregnated bit was selected, subject to the 80 hour drillinglimit described above.

Trial No. 1 goals included: An evaluation of turbodrillingperformance and reliability, operational cost reduction inhard, high temperature pre-Khuff drilling, mitigation in therequired number of trips, avoidance of downhole componentfailures 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 surfacetorques during the run, bit sticking events did occur and wereovercome by engagement of the locking clutch. There were noproblems disruptive to drilling attributed to the turbodrill. Asa result, Saudi Aramco approved continuation of the trial, ona run-by-run basis, to a TD of 17,900 ft. A PDC bit on rotarydrilled 127 ft in a formation that was not conducive toefficient penetration by a diamond impregnated bit.

Trial No. 1 Run Summary

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



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

CONCLUSIONS

1. During this trial, there were no events in which bit stickingor BHA problems resulted in lost on-bottom drilling time.

2. The high temperature pre-Khuff environment had noadverse effect on turbodrill durability or performance.

3. Turbodrills equipped with the LC1 locking clutch and matcheddiamond impregnated bits drilled 11 runs. In addition, threePDC runs are included for a total of 3,863 ft drilled incomparable pre-Khuff sections in two trial wells, Table 1.

4. During these runs there were no events causing downtimerelated to either turbodrills or drill bits.

5. Turbodrills equipped with the LC1 were tripped fromthe well after a maximum of 80 on-bottom hours. Based

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

TRIAL WELL 2

Based on the favorable results in the Trial Well 1 sidetrack,Saudi Aramco authorized continuation of the turbodrill withLC1 trials in the pre-Khuff sections of Trial Well 2. The 80hour drilling limit was continued.

Primary goals for Trial Well 2 were the same as for theTrial Well 1 sidetrack. Of additional importance, SaudiAramco wanted to determine whether or not the favorableTrial Well 1 results could be consistently expected, and boththe Offset Well and the Trial Well 1 sidetrack served asoffsets for Trial Well 2.

Trial No. 2 Run Summary

A total of 2,357 ft (eight runs total, including six turbodrillruns) were drilled at an average ROP of 4.9 ft/hr. Two PDCbits on rotary, between the first and second turbodrill runs,drilled 171 ft. A 60 ft core run between the fifth and sixthturbodrill runs is not included in feet drilled. As with the firsttrial, hole condition was good, and no nonproductive timewas attributable to either turbodrills with LC1s or thematched diamond impregnated drill bits during the runs. TrialWell 2 results are exceptionally consistent with those produced

Table 1. Summary data

Runs Days Footage Days/ Ft/Day Ave.1,000 Ft Ft/Run

Offset Well 17 36.8 1781 20.7 48.4 104.8Trial Well 1 ST 6 22.5 15.6 14.9 67.1 251.0Trial Well 2 8 35.3 2357 15.0 66.7 294.6

Fig. 1. Typical condition of post-run drill bits.

Fig. 2. Required trips.

0

4

8

12

16

20

Req

uir

ed T

rip

s

Offset Trial 1 Trial 2

17

6

8

Fig. 3. Days per 1,000 ft drilled.

0

5

10

15

20

25

Day

s /

1,00

0 Fe

et


20.7

14.9 15.0

on positive trial results, the limit on drilling time iscorrectly established.

6. All diamond impregnated bits were tripped from the wellin excellent condition after a maximum of 80 hours. Onebit was reused with good results. All bits possessedadditional life/value and their respective run lives couldhave been extended, Fig. 1.

7. The turbodrill and impregnated bit combination reducedthe tripping required by the offset well by more than 50%.This is a positive result for the trial goal pertaining totripping, Fig. 2.

8. Days per 1,000 ft drilled are 28% lower for the turbodrilland 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 drilledby PDC bits in the offset well. This contributes to a positiveresult for the trial goal of increasing time on-bottom, Fig. 4.

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 com parisonbetween offset and Trial Wells with and withoutcoring runs.

12. It can be concluded from the study of reduction in daysper 1,000 ft drilled, and for the required number of runsthat turbodrilling with the innovative locking clutch andimpregnated bits produced important fiscal benefits.

In conclusion, the turbodrill locking device innovationhad a very positive impact on the vertical pre-Khuffdrilling program through two well trials. Saudi Aramco, inconjunction with Smith International, continues to seekand expand the applications for the concept whilesignificantly increasing value through optimization of theturbodrilling system.

ACKNOWLEDGMENTS

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


Fig. 4. Average ft drilled per run.

0

50

100

150

200

250

300

350

Ave

. Fee

t /

Ru

n


104.8

251.0

294.6

Fig. 5. Average ft per day.

0

10

20

30

40

50

60

70

80

Ave

. Fee

t /

Day


48.4

67.1 66.7

Fig. 6. Comparative depth vs. days.

Fig. 7. Comparative depth vs. days (including coring).


REFERENCES

1. Simpson, M.A., Zhou, S. and Nordquist, D.G.: DrillingChallenges and Recent Advances of Pre-Khuff Wells, SaudiArabia, presented at the SPE Technical Symposium of theSaudi 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 AbrasiveUnayzah Sandstones, SPE/IADC paper 85331, 2003.

3. Simpson, M.A., Zhou, S., Treece, M., et al.: BreakthroughHorizontal Drilling Performance in Pre-Khuff Strata withSteerable Turbines, SPE paper 90376, 2004.

4. Internal Report, Definition of Bit Sticking Problem withTurbine in Pre-Khuff Sands, Drilling Technology Unit,Drilling and Workover Engineering Department, SaudiAramco, circa 1998.

5. Nordquist, D. and Zhou, S.: An Interim Report on BitSticking Problem and Recommended Solutions, InternalReport, Saudi Aramco, October 2001.

6. Simpson, M.A., Roed, A.H., Al-Shammari, H.A. andHoekstra, D.: Rotary Application of Low Matrix StrengthImpregnated and TSP Cutter Bits to Unayzah SandstoneDrilling, SPE/IADC paper 77223, 2002.


Perry Cook is the Middle East AreaManager for Smith Drilling &Evaluation, responsible for theturbodrill product line. Perry startedhis oil field career with Baker Hughes,working for 7 years in the drillingfluids sector in the areas of technical

research and field operations. He joined Smith Neyrfor in2002 and has subsequently held field, technical,coordination and management positions in the UK, theU.S., Kazakhstan and Saudi Arabia. Perry is currentlybased in Abu Dhabi supporting all aspects of turbodrillbusiness development throughout the Middle East.

In 1995, he received his B.S. in Biotechnology from theUniversity of East Anglia, Norwich, UK.

Nouman Feroze is a PetroleumGeologist working as a Senior SalesEngineer for Smith Bits, based in Al-Khobar, Saudi Arabia, where he servesSaudi Aramcos Exploration Groupand Joint Ventures Operators. Hestarted his career with Sperry Sun,

Pakistan and worked 2 years as a mud logger, then hejoined Smith Bits and worked in various locations in theMiddle East. Nouman has a total 11 years of oil fieldexperience.

He received his B.S. degree in Geology in 1994, and in1996 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 MechanicalEngineer with Smith Neyrfor, where heis currently the Manager of theNeyrfor Sustaining Engineering Group.He started his career with SmithInternational in 2000, spending 2 yearswith 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.

BIOGRAPHIES

Gabriel D. Carrillo began working forSaudi Aramco in 2007. He received hisM.S. degree in Petroleum Engineeringfrom Texas A&M University, CollegeStation, TX. Since 1994, he hasworked for ExxonMobil, BP Americaaround the world, and several small

independent companies in South America, where his jobsincluded Field Service Technician, Rig Supervisor andDrilling Engineer. Currently Gabriel works in theExploration Drilling Department where he monitors dailyactivities and plans upcoming events in a highly offshoreprofile well.

Usman Farid is an EngineeringSupervisor in Saudi AramcosExploration Drilling Department. Hebegan working with the company in2002 and over 23 years experience inrig drilling in the field, as well as in theoffice, up to the Drilling

Superintendent level. Usman also worked as a FishingEngineer and Sr. Drilling Instructor prior to joining thecompany. His job includes covering all engineering aspectsfrom planning to completion for high profile HPHToffshore gas exploration wells currently being drilled.

Usman graduated with a double B.S. degree in Mathand Physics and Petroleum Engineering from theUniversity of Engineering & Technology, Lahore, Pakistan.He also has an International Trainer certificate from theNorthern Alberta Institute of Technology, Edmonton,Alberta, Canada.

Michael Albrecht works for SmithNeyrfor in Saudi Arabia as a DrillingProduct Manager. He started his careerwith Norward Energy in 1995,working in Canada and the USA. In2000, Mike took an overseas positionin Kazakhstan as a Safety Supervisor

implementing safe work practices with TCO and ParkerDrilling. He joined Smith Neyrfor in 2004 as a Turbodrillerworking primarily in Canada, and later took a position asa Drilling Product Manager for the Middle East in 2006.Mike has worked in several countries worldwide, includingthe Middle East, and has 14 years of oil field experience.

He is currently based in al-Khobar, Saudi Arabia and hasworked closely with the Saudi Aramco Exploration Groupand the Joint Venture Operators on overseeing TurbineOperations and Sales.

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Pre-Khuff