Joe Green, Schlumberger

ADVANCES IN polycrystalline dia-mond compact (PDC) cutter technologyhave resulted in the development of newcutters with greatly improved abrasionproperties.

Fixed cutter bits fitted with these newcutters have demonstrated substantialincreases in wear life and drilling ratesof penetration (ROP) in field tests con-ducted around the world.

While the faster drilling rates saved rigtime, less bit wear translated intogreater total footage drilled with fewerbit changes and lower wear rates toBHA tools, resulting in reduceddrilling costs for operators.

The newly developed Schlum-berger TReX* cutters aredesigned with an ultra wearresistant layer of diamond at thecutting edge. In the laboratory,the new cutters consistentlymeasure 400% more abrasionresistance than standard PDCcutters, without jeopardizingtheir impact properties.

This translates into a 45%increase in cutter wear lifeproperties. These laboratoryresults have been confirmed innumerous field tests conductedworldwide in which the same bittypes were compared to those inclose offset wells, with the cut-ters being the main variable.

These field trials achieved greater than50% improvements in ROPs and sectionlength drilled.

N E W C U T T E R T E C H N O L O G Y

The development of TReX cutter tech-nology signifies a step change in thelong-standing efforts by the PDC synthe-sis industry to develop a material havingincreased abrasion resistance, or wearlife, without sacrificing impact resist-ance, or toughness.

These properties are influenced prima-rily by the size distribution of the syn-thetic diamond microparticles, or dia-mond grit, which are used in pressingthe PDC wafer. The relative particlesizes will define the properties of the

PDC in responding to bottomholedrilling actions, with larger particle-sized material being more resistant toimpact forces and smaller sizes more soto abrasive wear.

Historical development efforts havefound that altering these particle sizedistributions has served to improve oneproperty, only at the expense of theother.

Depending on the diamond grit size mix,cutters either have been more wearresistant or more impact resistant.

The inverse relationship between tough-ness and wear life is shown in Figure 1.

TReX technology has now succeeded inimproving the historic compromisebetween a PDC cutter's properties ofresistance to wear and toughness. How-ever, the technology progressed throughseveral developmental stages in order toovercome the previous PDC cutter limi-tations.

These stages are illustrated in theaforementioned graph, with technicalimprovements advancing toward the topright corner (TRC) of the wear life vs.toughness plot (Figure 1).

Successful PDC bit performancedepends upon maintaining a sharp dia-mond edge on each individual cutterthroughout the bit run. Although dia-mond is the hardest known material, it

will deteriorate from the impact shocksand abrasive wear that occur during thedrilling process.

Even small impact shocks can createmicro-fractures that lead to chipping ofthe sharp cutter edges. Friction andcontinuous contact with the drilled rockgenerates heat, resulting in thermaldegradation of the diamond and blunt-ing of the cutting edge.

Much effort has been made by the dia-mond synthesis industry to improvePDC properties over the past threedecades, with some significant mile-stones achieved. The introduction of

multi-modal (MM) tech-niques marked one of theearlier stages of PDC cutterdevelopment. MM technologyenabled the diamond parti-cle sizes to be more evenlydistributed than was previ-ously possible.

This resulted in increaseddiamond density, which inturn increased cutter abra-sion resistance withoutdecreasing impact resist-ance, thus taking a steptoward the TRC of the graph(Figure 1).

Implementing non-planarinterfaces (NPI) comprisedthe next stage of PDC devel-opment. NPI advancesfocused on de-localizinginterfacial stresses and

reducing peak stresses, thereby sub-stantially increasing cutter toughness.These significant cutter improvementsalso are illustrated on the graph with amarked TRC movement.

TReX technology builds on the combina-tion of these and other enhancements toprovide a PDC cutter that has the basicproperty of remaining sharp as it wears,therefore causing the carrier bit to drillfarther and faster. The surface of thecutter is a thin layer of diamond that is400% more abrasion resistant than thelower diamond layer.

Analysis of used TReX bits reveals thatthe cutters develop interesting wearcharacteristics, in that sharp lips andflinted edges maintain the bit's capabili-

January/February 2002 D R I L L I N G C O N T R A C T O R 55

New PDC bit cutters improve wear life and ROP

2.0

1.5

1.0

0.5

1.00.0 0.5 1.0 1.5

Relative wear life

Relative toughness

MMNPI TReX

TRC

PDC technical improvements advance toward the top right corner (TRC) ofthe wear life vs. toughness plot. TReX technology results in cutters withlonger wear life, without sacrificing their toughness.

ty to drill at an optimum performancelevel. These self-sharpening featuresare clearly evident, and they are themain contributors to the outstandingperformance of bits that are fitted withthe new cutters.

However, it is worth noting that analysisof worn TReX cutters requires care toensure that the flinted edge is not mis-taken for chipping and spalling, whichare customary blunt wear features ofnon TReX PDC cutters.

Consequently, the new cutter technologyhas enabled wear characteristics to begreatly advanced without negativelyaffecting impact resistance. The newPDC material keeps cutters sharperwhile maintaining their toughness.

Because their design incorporates a lipthat might be relatively fragile, TReXbits are preferentially applied in abra-sive rather than high-impact applica-tions.

Nonetheless, the new cutters have beensuccessfully applied in a broad range ofdrilling environments, with impressiveresults in numerous field applicationsconducted worldwide. In this article, twoareas will be discussed in particular: theNorth Sea and Australia's Cooper Basin.

N E W C U T T E R S I N T H E N S E A

The new cutters were initially tested inthree North Sea fields for both UK and

Norwegian operators. The results ofthese field trials are discussed in the fol-lowing paragraphs.

An 8-1/2 in. bit equipped with TReX cut-ters was used to drill a high-angle,reservoir tangent section in thedemanding Britannia formation for aUK North Sea operator.

One of the tougher sections of the for-mation was a hard, abrasive sandstonecontaining interbedded mudstones,which had compressive strengths ofmore than 30 Kpsi.

When drilling through this section, athree-times higher ROP was achievedover a 25% longer reservoir intervalthan was accomplished in close offset

wells drilled with bitsfitted with standard cut-ters.

In another UK NorthSea field, a 12 1/4-in.DSX 97 bit fitted withthe new cutters wasused to drill the Floun-der and Herring forma-tions. The drilled sec-tion, which included ademanding marl/lime-stone sequence, waspenetrated at almosttwice the ROP--89%higher--than a similarsection drilled with astandard cutter bit.

The TReX bit showed nowear and was pulled forTD whereas the bit withstandard cutters hadIADC grade 1:2:PR.

For a Norwegian opera-tor's North Sea well, the new cuttersalso produced impressive results whendrilling the Fangst and Baat Group(Jurassic) sandstone.

An 8 1/2-in. DSX94 bit fitted with TReXcutters achieved increases in drilledfootage and ROP of 55% and 25%,respectively, compared to the same bittype fitted with standard cutters thatwas run in a close offset well.

Both bits were pulled for TD with anequal IADC wear grading. Several other8 1/2-in. DSX94 bits fitted with the newcutters that were run in this field hadcomparable results.

However, it is striking that in these and

the majority of other test runs, consis-tent ROP improvements are recorded,thus confirming that TReX cuttersremain sharp as they wear.

A U S T R A L I A N A P P L I C A T O N

In Australia's Cooper Basin, the newcutters increased footage drilled by38%, thus allowing an operator to pene-trate in one trip an entire section thatincluded the challenging Hutton forma-tion.

The hard and abrasive Hutton formationpresent a significant challenge due toconfined compressive strengths rangingfrom 30-50 Kpsi, with occasional thinbeds exceeding 50 Kpsi.

The section prognosis comprises 3,500-4,500 ft of soft claystone, followed by300-400 ft of Hutton siliceous sandstone.The objective is to have a bit fast enoughto maintain high ROPs in the soft sec-tion, but tough enough to survive theHutton sandstone.

A TReX cutter-equipped 6 3/4-in. DSX68bit drilled a total interval of 4,297 ft, 305ft of which included the difficult Huttonformation.

While ROP was sacrificed in extendingthe run, the bit averaged 16.2 ft/hrthrough this hard abrasive sandstone.In an offset Hutton well, the same bit fit-ted with standard cutters drilled just 85ft of the sandstone.

In the previous benchmark for the field,190 ft of the Hutton was drilled at 20ft/hr using a different bit equipped withstandard cutters.

In additional Cooper Basin bit runs, thenew cutter bits have consistently drilledmore of this hard rock than bits havingstandard PDC cutters.

In summary, TReX cutters representone of the most significant advance-ments in PDC bit technology in recentyears. In numerous field trials conduct-ed worldwide, increased penetrationrates and extended bit life have resultedin lower well costs for operators.

While these benefits will continue, it isanticipated that the TReX characteristicof "wearing sharp" will provide otherimportant economic benefits, includingmore reliable steeribility and a higherquality well bore.

*Mark of Schlumberger n

56 D R I L L I N G C O N T R A C T O R January/February 2002

Bit cutters used to drill the North Sea Flounder and Herring forma-tions, including a limestone sequence, show the significant differ-ence in wear exhibited by the TReX (left) and standard cutters.

52 D R I L L I N G C O N T R A C T O R July/August 2000