DOWNHOLE TOOLS

26 D R I L L I N G C O N T R A C T O R March/April 2006

Improved design, matrix materials reducecosts while drilling interbedded formations

A Bell and F Lugo, ReedHycalog; AJ Currie,Petroleum Development Oman LLC

EVEN WITH THE significant develop-ments in polycrystalline diamond com-pact (PDC) cutter technology over thepast decade, bits comprising diamond-grit impregnated working surfaces arestill preferred for drilling hard and abra-sive formations. Such “impreg” bits mayalso be required to operate on interbed-ded formations, including those contain-ing softer and stickier lithologies such asshale. Functionality must be consideredon both micro- and macroscopic scales.

Microscopically, impregnated bits oper-ate by providing a balanced wear of thediamonds and surrounding matrix, con-tinuously regenerating the surfacethrough the exposure of new diamonds.Matrix wear rates should not be so highas to limit the useful working life of thediamond. Neither should they be so slowas to limit the protrusion of the diamondand restrict the passage of detritus fromthe face of the bit. Macroscopically, theinclusion of geometries that promoteshearing of softer interbedded forma-tions, whilst contributing to the cleaningand cooling of the diamond metal matrixcomposite (DMMC) are considered bene-ficial to improved penetration rates.

A development project was identified;the objective was to increase impreg-nated drilling efficiency through hardand interbedded formations and totransfer the knowledge and improve-ment model to other sizes and applica-tions. Petroleum Development Oman(PDO) was willing to develop andimprove impreg performance. The SaihRawl and Saih Nihayda fields in Omanwere proposed locations, containinginterbedded formations. Geologicaldata were available for the intendedapplication. The Oman application alsoprovided the timely return of dullimpregnated bits for evaluation.

A project team was created. Materialsengineers were responsible for improv-ing the existing manufacturing process-es and characterising the wear of the

DMMC. The geometric design was cre-ated and adapted by the eesign engi-neer, while the applications engineersprovided detailed run and formationinformation.

Developments and improvements wereachieved iteratively. From the initialdesign, subsequent modifications weremade continuously.

To fully evaluate the wear modes of thedull impregnated bits, a quantitativeanalytical technique was developed.The technique has since proven to be

valuable in recommending improve-ment strategies for Oman and otherapplications. Central to the techniqueis detailed microscopic examination ofthe diamonds and the surroundingmetal matrix composite (MMC). Out-comes include the applicability of thesize, grade, concentration and reten-tion of the diamonds and relative wearrate of the MMC.

O M A N A P P L I C A T I O N

The Saih Rawl and Saih Nihayda fieldsare located in northern Oman in theGhaba Salt Basin. Deep liquefied naturalgas (LNG) wells target the two mainreservoirs of the Barik and Amin sand-stones, both of which are hard, abrasivePalaeozoic sandstone sequences of lateCambrian age. These reservoirs are eachcapped by sequences of sand and shale.The Ghudun sandstones, Barakat &Mabrouk shale units overlay the Bariksandstone, which then leads into theheavily interbedded Al Bashair carbon-ate and shale sequence followed by theMiqrat shales and siltstones overlayingthe Amin. LNG wells on Saih Nihaydafield drill deeper into the Amin as it is aprimary LNG reservoir.

The lower formations of the Saih Rawland Saih Nihayda fields consist of pre-dominantly dense clay, with silt, sandand pebble horizons interbedded. TheBarik sequence is dominated by fine-grained sandstones interbbeded withthin red and green shales. The Al-Bashair section contains an interbbededsequence of fine sandstones, siltstones,shales and limestones. The Miqrat for-mation is a sequence of red brown shalesand siltstones thinly interbbeded withfine sandstones. The final Aminsequence of sandstones with variousdeveloped siltstones and conglomeratesin the lowermost part. Typically the final100 m of the Amin interval contains thehighest confined compressive strength ofthe section.

To develop an experimental range of dia-mond impregnated drill bits, significant

Figure 1: Field experience of historic bit designshad shown that an interrupted blade profile asshown here produced increased penetrationrates when operating through interbeddedshale containing formations.

This article contains additional content not found within the pages of the printed versionof Drilling Contractor magazine—exclusive to the online version of the magazine.

changes to the processing, materials anddesign of legacy product were required.These improvements led to unprecedent-ed levels of casting integrity and provid-ed a capability for rapid execution ofmodifications to the design and materi-als based on dull evaluations.

New manufacturing processes devel-oped during the project providedrobust, reliable and high integrity prod-

uct. Infiltration and solidification cycleswere optimized through the use of finiteelement models and validated throughsectioning and examination. Improve-ments in processing provided a moreregular diamond distribution and muchreduced levels of contaminationthroughout the MMC compared withhistoric techniques. In addition to thebenefit of improved structural integrity,improved diamond to MMC interfaceswere achieved. The adoption of newmoulding methods enabled geometricand materials changes to be madequickly.

As a result of research into the wearbehaviour of diamond metal matrixcomposites, a technique was applied tocharacterise dull bit surfaces. In devel-oping the method, legacy diamondimpregnated bits were initially exam-ined, resulting in material improvementopportunities that were immediatelytransferred to the experimental range.Using a light microscope, the conditionof the diamonds and surrounding MMCwere assessed quantitatively. Evalua-tion results revealed the wear behav-iour during the final stage of the bitrun. The assessment was cognizant ofits performance through its last litholo-gy. In particular the predicted rockstrengths, abrasivity and final penetra-tion rate formed a basis for recommen-dations. Understanding the wear mech-anism(s) was key to improving perform-

ance; examination at different radiallocations over the surface of the bitrevealed regional differences in wearbehaviour of the DMMC. Outcomes fromthe technique included an assessmentof the appropriateness of the diamondgrit size, grade and concentration.Cases were also identified when thewear rate of the surrounding materialwas out of balance compared with dia-mond breakdown and loss.

The proposed lithologies were consid-ered in selecting the geometry for theexperimental range of impregnatedbits. Field experience of our historic bitdesigns had shown that an interruptedblade profile as shown in Figure 1 pro-

DOWNHOLE TOOLS

March/April 2006 D R I L L I N G C O N T R A C T O R 27

Figure 2: A computer-generated model revealingfluid dynamics of the experimental design.

E X C L U S I V E W E B C O N T E N TFigure A: Protrusive elements providing shear-ing and cleaning capabilities in experimentalbit. Increased protrusive heights compared withthe historical series were introduced to offer anuniform and improved duration of shearingcapability.

EXCLUSIVE WEB CONTENT Figures B (1 and 2): Spiral location of protrusive shearing elements for the experimental series. The protrusive elements werelocated in a spiral configuration to further improve the shearing potential and incorporating drag bit cutter location philosophies.

duced increased penetration rateswhen operating through interbeddedshale containing formations. The initialadvantages of this profile were limitedby the height of the interruption, sincethey wore completely and no furthershearing potential was available.

Adapted from initial concepts, the newdesign adopted an interrupted profilefor operating through interbedded for-mations. Increased protrusive heightscompared to the historical series wereintroduced to offer improved durationof shearing capability. To further

improve the shearing potential andincorporating drag bit cutter locationphilosophies, the protrusive elementswere located in a spiral configuration.Effective cleaning of the detritus andcooling of the diamonds were importantconsiderations. Ports were positionedto provide effective and uniformhydraulics on all blades. During thedesign validation process, computation-al fluid dynamics models were used toevaluate hydraulic energies and flowpaths. Figure 2 shows a typical result.Improved cross blade lubrication waspromoted by waterways locatedbetween the protrusive elements.

The results of comparative field andlaboratory data when comparing a newgeneration of polycrystalline diamondcompact (PDC) demonstrated clearsuperiority in terms of ROP and inter-val drilled to conventional cutters.Their inclusion has extended the appli-cation of drag bits to fields that werenot historically PDC drillable. The newgeneration of PDCs were included with-in the cone of the experimental bitoffering a shearing action, withimproved durability and penetrationrates compared with conventional cut-ters. To avoid coring but not detractfrom the free cutting characteristics ofthe DMMC, subsurface thermally stableproduct (TSP) were included in thecone to act as bearing surfaces duringthe final stage of the bit’s life.

P E R F O R M A N C E C H R O N O L O G Y

The following reveals the developmentand improvement process for the experi-mental range of diamond-impregnatedbits. Run data were selected when pos-sible, to provide comparisons betweensuccessive iterations. Within the devel-opment program, significant geometricchanges were identified alphabetically,while alterations to the materials withinsimilar geometries were distinguishednumerically.

Well 1-Saih Rawl. A1 Version

The first experimental run of the A1version was undertaken in the SaihRawl field, in well 1 through the lowerboundary of the Mabrouk and upperBarik formations. The performance ofthe initial A1 design was considered tohave delivered a satisfactory rate ofpenetration, with no evidence of plug-ging or balling. The run length was amodest 150 m and the rate of DMMCloss was thought to be high withregions of preferential wear observed,

DOWNHOLE TOOLS

EXCLUSIVE WEB CONTENT Figure C: During the design validation process, computational fluid dynam-ics models were used to evaluate hydraulic energies and flow paths. Shown here is a typical result.

EXCLUSIVE WEB CONTENT Figure D: In order to avoid coring, but so as to not detract from the freecutting characteristics of the DMMC, sub-surface thermally stable product (TSP) were included in thecone to act as bearing surfaces during the final stage of the bit’s life.

28 D R I L L I N G C O N T R A C T O R March/April 2006

together with an undergauge condition.Microscopic evaluation revealed that alarge proportion of the diamonds hadbeen lost prematurely. To provide areduced DMMC wear rate and producemore preferential wear, the diamondsize and concentration were adapted.Methods were proposed to provideimproved retention and reduce theamount of missing diamonds. The bitand turbine sleeve gauge contact areaswere increased to further improvedurability.

To examine the relative performance offuture iterations to the A1, the drillingcosts per meter were indexed from theA1 run.

Well 2-Saih Nihayda A2 Version

The A2 version incorporated improve-ments made as a result of examination ofthe A1. It was run in well 2 of the SaihNihayda field. Compared with the A1,penetration rates were higher, particu-larly through the Miqrat shale section;dropping significantly on entering theharder Amin consolidated sandstone.

The onset of a ring out was observed inthe cone, but the overall the run wastwice the interval drilled of the A1 andhalf the drilling cost. Changes to the dia-mond selection and incorporation ofretention methods had reduced the pro-portion of prematurely lost diamonds by50%. Proposed changes for the futureiterations, included modification of mate-rials toward the cone of the bit to reducethe propensity for coring. The mainobjective was to be able to drill atincreased ROP and longer intervalsthrough the Amin section. Changes to the

diamond size range used were proposedso as to better overcome the fracturestrength of sandstone.

Well 3-Saih Nihayda A3, A4 and A5Versions

The A3, A4 and A5 versions werereturned and evaluated collectivelyafter being run in the same well. TheA3 was designed primarily for theMiqrat, whilst drilling as far as possi-ble into the Amin formation. Like theA2, it recorded higher penetrationrates in the Miqrat before slowing inthe Amin and provided 65% of thedrilling costs of the A1. Crushed dia-monds were observed on the surface ofthe A3, indicating that they were tooweak for the hardest final formation.

Although the A4 indicated increaseddurability than comparable competitivebits run in the Amin formation of well2, further improvements were required.The A4 contained an experimentalmethod to provide still further improve-ment in diamond retention. Since sig-nificant diamond crushing was evident,the adequacy of the alternative dia-mond retention method within the A4could not be verified.

The A5 was proposed to drill the Amin,where the diamond size and gradewould be better suited. Erosive wear ofthe metal matrix predominated in theA5 before the diamonds could becomeactive with the formation.

Well 4-Saih Nihayda. A6 Version

Incorporating changes made as aresult of evaluating previous itera-tions, the A6 adopted improved gaugeprotection and material selection to

provide increased durability throughthe hardest sandstone section. Run inwell 4, it achieved TD completing thedemanding Amin section and part ofthe shale containing Miqrat formation.Its durability and pentration rate wasimproved compared to A4 and A5 runsin well 3 and comparable competitiveruns through well 1. Allowing for amore arduous formation, drilling costswere 64% of the of the A1 version.

Well 5-Saih Nihayda. B2 Version

In order to overcome coring towardsthe cone encountered on the A ver-sions, an extra row of new generationPDCs were included within the B ver-sions. Material changes were made tothe outside diameter and gauge toimprove gauge durability. Like the A6,the B2 was able to complete the Aminsection but at a higher ROP when runin well 5.

Well 6-Saih Nihayda. C1 and C3 Ver-sions

The C versions included increasedblade height, providing increased dia-mond composite volume and the poten-tial for improved run duration. Bladeends were tapered and the gaugeextended to provide increased contactarea and volume for improved durabil-ity. The C1 runs through the Al Bashirand Barik sandstone and shale forma-tions produced high penetration ratesin well 6. Surface examinationrevealed an optimised diamond size

DOWNHOLE TOOLS

March/April 2006 D R I L L I N G C O N T R A C T O R 29

E X C L U S I V E W E B C O N T E N TFigure E: High proportion of prematurely lostdiamonds on the face of the A1 experimentaldiamond impregnated bit. In the Saih Rawl field,microscopic evaluation revealed that a largeproportion of the diamonds had been lost pre-maturely.

E X C L U S I V E W E B C O N T E N TFigure F: The A2, run in the Saih Nihayda field,shows a 50% reduction in prematurely lost dia-monds compared with the A1 due to changes tothe diamond selection and incorporation ofretention methods.

E X C L U S I V E W E B C O N T E N TFigure G: In an alternative application in theRockies, the first experimental 6 ½-in. A1 pro-duced a satisfactory ROP, but after a modest run(225m) encountered significant blade wear(shown).

and grade for the application. Evi-dence of thermally generated damagewithin the metal matrix composite,combined with a desire to provide stillfurther run lengths resulted in anincrease in the diamond concentrationin the C3.

The C3 run produced high penetrationrates through the Miqrat formation andwas able to complete the notoriously dif-ficult Amin. It was run for a second occa-sion drilling a further 240 m through asand and shale formation in anotherfield. Allowing for more difficult forma-tions, the cost per meter drilled repre-sents a 60% saving compared with thefirst A1 version.

Summary of Run Performances in Oman

Changes made to the materials anddesign based on detailed dull evalua-tion has continuously improved bit per-formance. Increased penetration ratesand interval drilled has been encoun-tered with successive, comparable iter-ations. The difficulty in completing theAmin section is demonstrated by thenumber of competitive runs in well 2and the need for two experimental A4and A5 bits in well 3. Modifications tothe A6 enabled the Amin section to becompleted with increased pentrationrates, duration and reduced costs com-pared with the A4 and A5 versions.Changes to the cone of the B2 providedfurther improvements in ROP com-pared with the A6. Providing increasedavailable volumes of DMMC, a moredurable gauge and materials modifica-

tions to the C3 further increased dura-bility compared to the B2 and reducedcosts by 60% compared to the first A1version.

Alternative Applications and Sizes

The knowledge gained in the Omanapplication and the continuousimprovement model, has been appliedconcurrently to other sizes and appli-cations. Both design and materialshave been selected specifically for eachapplication and improvements madeiteratively as a result detailed dullevaluation.

In the Rockies, the first A1 version of a 6½-in. experimental impreg bit operatedthrough an interbedded section contain-ing sandstones, plastic shales and coal.The first run produced a satisfactoryROP but after a modest run encounteredsignificant blade wear see Figure 12.Detailed post run wear characterisationrevealed that preferential blade wearhad been encountered as a result erosionof the MMC predominating regionally.Materials and geometric modificationswere incorporated into the A2 version topromote even wear and extended runlengths. The A2 run in similar lithologiesto the A1 produced similar penetrationrates, with even and much reduced bladewear over an increased interval.

In Venezuela, the first run of the exper-imental 6½-in. version produced a fieldrecord for durability and penetrationrate. The bit design was selectedspecifically for the application andcompleted 280 m at 2.2 m hr-1, provid-ing the lowest cost impregnated bit inthe section.

C O N C L U S I O N S

• Effective teamwork between operatorand supplier has reduced drilling costs inOman for the experimental series ofimpregnated bits by 60% within a shorttime scale.

• Modifications to the design and materi-als have provided an ongoing continuousperformance improvement cycle.

• Informed recommendations were madeas a result of detailed run informationcombined with detailed macro andmicroscopic dull evaluation.

• Improvement to processing provided ahigh integrity product where design andmaterial modifications could be madequickly.

• Combined materials and geometricchanges provided expected performanceresponse.

• Objective of increasing drilling efficien-cy through interbedded formations hasbeen met.

• Applying design and materials knowl-edge, together with the teamwork/itera-tive improvement model has been suc-cessfully transferred to other sizes andapplications.

References:

Bell. A. Development of a DiscriminatoryWear Test for Diamond Containing MetalMatrices. M.Sc. Thesis. 2004.

Carraway. D. Drag-Type Rotary Drill Bit. USPatent 6,371,226 B1. 2002.

Griffin. N.D. and Hughes. P.R. High VolumeDensity Polycrystalline Diamond with Work-ing Surfaces Depleted of Catalyzing Material.US Patent 6,544,308 B2. 2003.

DOWNHOLE TOOLS

30 D R I L L I N G C O N T R A C T O R March/April 2006

E X C L U S I V E W E B C O N T E N TFigure H: The dull condition of the experimental6 ½-in. A2 in the Rockies after 360m.