7/21/2019 PDC Bit Technology Improvements
2/452 D R I L L I N G C O N T R A C T O R March/April 2005
Stephane Menand, Laurent Gerbaud,
Paris School of Mines
Alfazazi Dourfaye, Varel International
PDC (POLYCRYSTALLINE DiamondCompact) bits were introduced in the
oiland gas industry in the mid 1970s. Dur-ing the past 30 years,
numerous techno-
logical improvements brought to the PDCcutters and bits have
enabled them totake an important and growing share ofthe drilling
bit market (50% of the totalfootage drilled in 2003 was with PDC
bitscompared to 26% in 2000). The total rev-enue of PDC bits sales
in 2003 wasaround $600 million.
Improvements in PDC bit hydraulics,PDC cutter (tougher and more
abrasionresistant cutter) and PDC bit dynamicstability have
resulted in continuouslyand significantly increasing the
averagerate of penetration (ROP) and bit life of
PDC bits, and extend their application inharder and more
abrasive formations.
Historically, the use of PDC bits has beenrestricted to soft to
medium and non-
abrasive formations.
Today, thanks to the numerous innova-tions and technological
breakthroughs,PDC bits drill faster, better and deeper,extending
their application in harder andmore abrasive formations, but
basicsremain.
M A T E R I A L
PDC bits, as opposed to roller cone, haveno moving parts. The
body of PDC bitscan be manufactured from two differentmaterials,
steel bodied and matrix bod-ied bits. The steel bodied bit,
machinedand manufactured with steel stock, isbetter able to
withstand impact load thanmatrix bodied bits. Steel bodied bits
aregenerally preferred for soft and non-abrasive formations and
large hole size.
The main disadvantage of steel is that itis less erosion
resistant than matrix and,consequently, more susceptible to wearby
abrasive fluids.
To reduce the bit body erosion, bits are
hardfaced with a coating material thatis more erosion resistant,
and sometimesreceives an anti-balling treatment forvery sticky rock
formations such asshales.
Matrix bits are manufactured with tung-sten carbide, which is
more erosionresistant than steel. They are preferredin high
solid-content drilling mud.
The PDC cutters are composed of a thin(up to 3.5 mm) layer of
polycrystallinediamond bonded to a cemented tungstencarbide
substrate. These PDC cutters
are generally cylindrical (diameter gen-erally from 8 mm up to
24 mm) but maybecome available in other forms (oval ortriangle) and
are generally chamfered toincrease the cutters impact
resistance.
Many improvements have been made inthe quality and variety of
the cutters andin new manufacturing techniques to pre-vent cutter
wear and breakage. Theimprovements concern a better impactand
abrasion resistant diamond materi-al.
The interface geometry between the dia-mond layer and the
tungsten carbidesubstrate are also improved. Due to thethermal
limitations of the PDC (above700C the diamond layer disintegrates
asa consequence of cobalt expanding),much work has been done to
produce aThermally Stable Polycrystalline (TSP)cutter, stable up to
1,150C.
Cutters are attached to the bit bodyusing an alloy that must
have the lowestpossible melting point, good flow proper-ties,
excellent wettability and shear
The different components of matrix PDC bit. As opposed to roller
cone bits, PDC bits have no moving
parts. PDC bits can be manufactured from two different
materials, steel bodied and matrix bodied bits.
Many improvements have been made in the
quality and variety of the cutters and in new
manufacturing techniques to prevent cutter
wear and breakage. The improvements concern
a better impact and abrasion resistant diamond
material. The interface geometry between the
diamond layer and the tungsten carbide sub-
strate are also improved.
PDC bit technology improvementsincrease efficiency, bit life
BACK TO BASICS
DRILLINGCONTRACTOR
7/21/2019 PDC Bit Technology Improvements
4/4
D R I L L A B I L I T Y
PDC bit drillability is certainly the mostimportant factor
affecting global drillingcosts. The PDC cutter characteristics,back
rake angle, cutter layout, cuttercount and cutter size are the
mainparameters that control the drillability ofthe bit.
The back rake angle is defined as the
angle the cutter face makes with respectto the rock. The back
rake controls howaggressively cutters engage the rock
for-mation.
Generally, as the back rake is decreased,the cutting efficiency
increases (highROP) but the cutter becomes more vul-nerable to
impact breakage. A large backrake angle will result in lower ROP
butwill enable to give a longer PDC bit life.
The side rake generally affects the clean-ing of the cutters, as
it helps to direct the
cutting toward the periphery of the bit.PDC cutter count and
size are selectedfor a specific formation under specificoperating
conditions.
The general rule is that small cutters andhigh cutter count are
chosen for hardand abrasive rock formation, whereaslarge cutters
and a reduced cutter countare preferred for soft to medium
forma-tion. The cutter count determines thenumber of blades
required.
S T A B I L I T Y
PDC bit stability is extremely importantfor the global drilling
performance. A sta-ble bit increases rate of penetration andbit
life, improves hole quality andreduces the damage caused to
downholeequipment.
The three main vibration modes areaxial, resulting in bit
bouncing; torsional,resulting in stick-slip; and lateral,
result-ing in whirl motions.
Considerable research in PDC bit dynam-ic led to highly balanced
PDC bits (mini-
mization of imbalance force), in particu-lar as a result of the
developments of spi-ralled blades.
Other techniques are anti-whirl bits, low-friction gauge pad,
and full gauge contactdesign to make the bits more stable.
A widely spread innovation consists inplacing some impact
arrestors (smallround inserts) behind the PDC cutters,which provide
a better stabilization toaxial and lateral modes of vibration.
S T E E R A B I L I T Y
The steerability of a bit corresponds tothe ability of the bit
to initiate a devia-tion. For example, high steerability for abit
implies a strong propensity for devia-tion, enabling a maximum
dogleg poten-tial.
Generally speaking, and all things beingequal, the short-gauge
design is more
steerable than long-gauge design, butmay lead to poor borehole
quality.
To enhance toolface control during thesliding phase of a mud
motor, some PDCbits have been designed to control later-al and
axial aggressivity. This enablesthe directional driller to control
a PDCbit as well as a roller cone bit.
D U R A B I L I T Y
Advancements in PDC cutter technologyhave increased the
development and per-formance of PDC bits. Cutters have main-ly been
evaluated in terms of their resist-ances to impact and abrasion
becausethe primary reasons of bit failure areabrasive damage and
impact loadingdamage.
Additionally, other characteristics suchas interface strength,
thermal stabilityand fatigue are also analyzed. Maximiz-ing these
properties improves cutterdurability that subsequently enhancesPDC
bit performance and drilling effi-ciency.
H Y D R A U L I CThe size of nozzles (made of tungstencarbide)
that are interchangeabledepends on many factors, the main fac-tors
being the size of the bit and the rec-ommended hydraulic program.
The bithydraulic is fundamental for two mainpurposes.
First, the drilling mud cleans the cuttingsfrom the bit and
prevents bit balling. Sec-ondly, the mud cools the cutters to
main-tain the temperature below the critical700c.
The conventional nozzles are circularand create a symmetric
pressure distri-bution at the rock interface.
Some improvements have been the devel-opment of nozzles with
non-circular jets(fluted jets) with specialized interiorshapes.
This enables a more efficient cleaningand cutter removal
(increased turbu-lence under the bit) resulting in a higher
ROP. Computational fluid dynamics(CFD) programs enable modelling
thefluid flow around bits inside a borehole,to investigate quickly
many bit designsand optimize fluid flow.
P D C B I T S E L E C T I O N
Today, a PDC bit is designed for a specif-ic application,
depending mainly uponthe rock formation to be drilled. It is
therefore important to study the type ofrock encountered during
drilling, usingdata and logs from offset wells.
The mechanical and physical character-istics of the formation
(compressivestrength, abrasiveness, elasticity, sticki-ness, pore
pressure) govern the choice ofPDC bit.
Advanced design software can estimaterock strength from well
logs and evaluatePDC bit performance to help in drillingbit
selection. At the same time, drilling
parameters or hydraulic aspects shouldalso be studied to adjust
the bit design.
PDC bits are also chosen for the type ofapplication: directional
drilling, slimhole, horizontal, motor drilling, turbo-drilling,
reaming drilling (bi-center bits)etc.
Today, most bit manufacturers have theirown line of PDC bits for
rotary steerablesystems (RSS), their own specializedPDC bits for
drilling salt or shales, or forany particular application.
The objective is always the same: to drillas fast as possible in
a smooth way, andterminate the run with minimum wear toreduce
overall drilling costs.
54 D R I L L I N G C O N T R A C T O R March/April 2005
Computational fluid dynamics (CFD) programs
enable modelling the fluid flow around bits
inside a borehole to investigate bit designs and
optimize fluid flow.
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