Drilling Engineering
Drilling Engineering - PE 311
Rock Failure Mechanisms
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Drilling Engineering
Rock Failure Mechanisms
Bits are designed to induce rock failure. Because rock failure can occur in different
ways, depending on the formation and on downhole conditions, there are a large
number of design variations among rolling cutter and fixed cutter bits. To evaluate
these design variations and select a bit, we first need a basic understanding of
how rocks fail and how formation conditions affect drilling performance.
The purpose of the design is to take advantage of the formation most efficient
failure mechanisms.
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Drilling Engineering
Rock Failure Mechanisms
The Stress/Strain Relationship
Stress is the internal force applied to a unit area of material. An analysis of the
stresses acting on a particular object can become quite involved. For the purpose
of this discussion, however, we can define three basic components of stress:
• Compressive stress: a pushing or squeezing force;
• Tensile stress: a pulling or elongating force;
• Shear stress: a slicing or cleaving force.
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Drilling Engineering
Rock Failure Mechanisms
The Stress/Strain Relationship
Strain is the deformation experienced by a material in response to an applied
stress. This deformation may take one of two forms:
•
Elastic: If the applied stress is below the elastic limit of the material, the
material returns to its original shape and size once the stress is removed.
•
Plastic: if the applied stress exceeds the material's elastic limit, the material
experiences permanent deformation

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L
L0

L  L0
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L0
L0
Drilling Engineering
Rock Failure Mechanisms
The Stress/Strain Relationship
If rupture takes place before significant plastic deformation occurs, the material is
described as brittle. If the material ruptures only after experiencing significant
plastic deformation, it is considered ductile.
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Drilling Engineering
Rock Failure Mechanisms
The Stress/Strain Relationship
If formation is under brittle failure, bits which have crushing and chipping actions
are preferred. On the other hand, if formation is under ductile failure, bits which
have gouging and scrapping actions are chosen.
All rocks exhibit brittle stage under atmosphere pressure. This is applicable for
under balanced drilling conditions
Under high pressure, the failure mechanism transfers from brittle to ductile. This
condition is for overbalanced drilling.
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Drilling Engineering
Rock Failure Mechanisms
The Stress/Strain Relationship
At atmospheric pressure, sedimentary rocks are normally brittle. They become
ductile, however, under high confining stress if there is no communication between
the internal rock pore pressure and the surrounding pressure medium.
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Drilling Engineering
Rock Failure Mechanisms
The Stress/Strain Relationship
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Drilling Engineering
Rock Failure Mechanisms
Underbalanced Condition
If the pressure exerted by the fluid column is less than the pore pressure of the
formation, the differential pressure is less than zero, and the well is being drilled in
an underbalanced condition. This condition most often occurs when drilling with
air, fresh water or muds weighing less than 8.6 lb/gal.
In underbalanced drilling, the rock exhibits brittle behavior — it has a relatively low
failure strength and fractures very easily. Because the rock surface is in tension, it
virtually explodes under the compressive loads of the bit. There is no downward
pressure to promote chip hold-down, and so there is very little regrinding of
already-drilled cuttings. This helps attain very high rates of penetration.
Prepared by: Tan Nguyen
Drilling Engineering
Rock Failure Mechanisms
Balanced Condition
When the pressure of the fluid column is equal to the pore pressure, the hole is in
a balanced condition. This condition generally occurs when drilling with brine water
or mud weighing 8.6 lb/gal.
Under balanced conditions, the rock is still in the brittle state and fractures
relatively easily. The bottom of the hole is in pressure equilibrium, so there is
minimal stress concentration present to either enhance or slow penetration rates.
Penetration
rates
are
generally
slower
than
those
experienced
in
an
underbalanced drilling, because there is some chip hold-down resulting from
cohesive forces between the rock cuttings, along with interference due to fluid
viscosity.
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Drilling Engineering
Rock Failure Mechanisms
Overbalanced Condition
In overbalanced drilling, the pressure of the mud column exceeds the formation
pore pressure. In areas with normal pressure gradients, this condition occurs when
the mud weight exceeds 8.6 Ib/gal. For safety reasons, overbalanced drilling is
normal practice in most areas.
As the differential pressure increases in an overbalanced hole, the rock below the
bit becomes increasingly strong and ductile. The hole bottom is in a state of
compression, thus retarding fracture propagation caused by the bit. These factors,
along with a high degree of chip hold-down, tend to slow penetration rates. If the
differential pressure is too high, the mud can fracture the formation, resulting in
lost circulation and possibly a blowout.
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Drilling Engineering
Rock Failure Mechanisms
Overbalanced Condition
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Drilling Engineering
Rock Failure Mechanisms
Chip Hold Down
Penetration rate is also affected by a pressure-related phenomenon known as chip
hold-down. Chip hold-down occurs when a mud filter cake or fine solids block
fractures produced by the bit. This prevents the liquid phase of the mud from
invading the fractures, and results in a positive pressure differential across the top
surface of the chip. The hold-down force is equal to the area of the chip times the
differential pressure
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Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Introduction
Drag bits are designed to drill primarily by a wedging mechanism.
A vertical force is applied to the tooth as a result of applying drill collar weight to
the bit, and a horizontal force is applied to the tooth as a result of applying the
torque necessary to turn the bit. The result of these two forces defines the plane of
thrust of the tooth or wedge. The cuttings are sheared off in a shear plane at an
initial angle to the plane of thrust that is dependent on the properties of the rock.
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Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Mohr Failure Criterion
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Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Mohr Failure Criterion
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Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Mohr Failure Criterion
f
f: Angle to the
direction of the
compressive load
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Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Mohr Failure Criterion
Summing forces normal to the fracture plane gives
Where
dA3 = dAncosf
and
dA1 = dAnsinf
Making these substitutions in the force balance equation gives
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Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Mohr Failure Criterion
Summing forces parallel to the fracture plane gives
With dA3 = dAncosf
dA1 = dAnsinf
These two equations represent graphically by the Mohr’s circle. Any combination
between t and sn gives a new circle representing a new failure condition.
The rock will fail when the combination between WOB and shear force t gives a
point out side the Mohr’s circle
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Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Mohr’s Circle
Positive
shear
would
cause
clockwise rotation of the element.
s12 is negative and s21 is positive
Compression is positive
Tensile is negative
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a
Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Mohr’s Circle
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Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Mohr Failure Criterion
c = t - sntanq
q is the angle of internal friction
sn is the stress normal to the fracture plane
c is the cohesive resistance
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Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Mohr Failure Criterion
Example: A rock sample under a 2,000 psi confining pressure fails when subjected
to a compressional loading of 10,000 psi along a plane that makes an angle of 270
with the direction of the compressive load. Using the Mohr failure criterion,
determine the angle of internal friction, the shear strength and the cohesive
resistance of the material.
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Drilling Engineering
Rock Failure Mechanisms of Drag Bits
Mohr Failure Criterion
Solution: the angle q and 2f must sum to 900. Thus the angle of internal friction is
given by
q = 90 – 2(27) = 36 0
The shear strength is computed as follows
t = ½(s1 – s3)sin(2f) = ½(10,000 – 2,000)sin(540) = 3,236 psi
The stress normal to the fracture plane is
sn = ½(s1 + s3) – ½(s1 – s3)cos(2f) = 3,649 psi
The cohesive resistance can be computed
c = t - sntanq = 585 psi
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Drilling Engineering
Rock Failure Mechanisms of Rolling Cone Bits
Percussion or crushing action is the predominant mechanism present for the
rolling cutter bits. Since these types of bits are designed for use in hard, brittle
formations in which ROP tend to be low and drilling costs tend to be high, the
percussion mechanism is of considerable economic interest.
The apparatus allowed the borehole pressure, rock pore pressure, and rock
confining pressure to be varied independently. The apparatus was equipped with a
static loading device which used an air-actuated piston to simulate constant force
impacts similar to those produced in rotary drilling. Strain gauges and a linear
potentiometer were used to obtain force displacement curves.
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Drilling Engineering
Rock Failure Mechanisms of Rolling Cone Bits
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Drilling Engineering
Rock Failure Mechanisms of Rolling Cone Bits
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Drilling Engineering
Rock Failure Mechanisms of Rolling Cone Bits
As load is applied to a bit tooth (A), the constant pressure beneath the tooth
increases until it exceeds the crushing strength of the rock and a wedge of finely
powdered rock then is formed beneath the tooth (B). As the force on the tooth
increases, the material in the wedge compresses and exerts high lateral forces on
the solid rock surrounding the wedge until the shear stress exceeds the shear
strength of the solid rock and the rock factures (C).
The force at which fracturing begins beneath the tooth is called the threshold
force. As the force on the tooth increases above the threshold value, subsequent
fracturing occurs in the region above the initial fracture, forming a zone of broken
rock (D).
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Drilling Engineering
Rock Failure Mechanisms of Rolling Cone Bits
At low differential pressure, the cuttings formed in the zone of broken rock are
ejected easily from the crater (E). The bit tooth then moves forward until it reaches
the bottom of the crater, and the process may be repeated (F, G).
At high differential pressures, the downward pressure and frictional forces
between the rock fragments prevent ejection of the fragments (E’). As the force on
the tooth is increased, displacement takes place along fracture planes parallel to
the initial fracture (F’, G’). This gives the appearance of plastic deformation, and
craters formed in the manner are called pseudo plastic craters.
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Drilling Engineering
Factors Affecting Penetration Rate
The most important variables affecting penetration rate that have been identified
and studied included: bit type, formation characteristics, drilling fluid properties, bit
operating conditions (WOB, and ROP), bit tooth wear, and bit hydraulics.
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Bit Type
For rolling cutter bits, the initial ROP is often highest in a given formation when
using bits with long teeth and a large cone offset angle. However, these bits are
practical only in soft formations because of a rapid tooth destruction and decline in
penetration rate in hard formations.
Drag bits are designed to obtain a given penetration rate. Drag bits give a wedging
type rock failure in which the bit penetration per revolution depends on the number
of blades and the bottom cutting angle. The diamond and PCD bits are designed
for a given penetration per revolution by the selection of the size and number of
diamonds or PCD blanks.
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Formation Characteristics
The elastic limit and ultimate strength of the formation are the most important
formation properties affecting ROP. The shear strength predicted by Mohr failure
criteria sometimes is used to characterize the strength of the formation. To
determine the shear strength from a single compression test, an average angle of
internal friction of 350 was assumed. The angle of internal friction varies from
about 30 – 400 form most rocks
The mineral composition of the rock also has some effect on ROP. Rocks
containing hard, abrasive minerals can cause rapid dulling of the bit teeth. Rocks
containing gummy clay minerals can cause the bit to ball up and drill in a very
inefficient manner.
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Formation Characteristics
The permeability of the formation also has a significant effect on the ROP. In
permeable rocks, the drilling fluid filtrate can move into the rock ahead of the bit
and equalize the pressure differential acting on the chips formed beneath each
tooth. This would tend to promote the more explosive elastic mode of crater
formation. it also can be argued that the nature of the fluids contained in the pore
spaces of the rock also affects this mechanism since more filtrate volume would
be required to equalize the pressure in rock containing gas than in a rock
containing liquid.
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Drilling Fluid Properties
The properties of drilling fluid reported to affect the ROP include: density,
rheological flow properties, filtration characteristics, solids content and size
distribution, and chemical composition.
Penetration rate tends to decrease with increasing fluid density, viscosity and
solids content, and tends to increase with increasing filtration rate. The density,
solid, and filtration characteristics of the mud control the pressure differential
across the zone of crushed rock beneath the bit. The fluid viscosity controls the
system frictional losses in the drillstring and thus the hydraulic energy available at
the bit jets for cleaning. The most important factor out of the drilling fluid properties
is the density. Changing density will change the overbalance. The ROP decreases
as the overbalance increases.
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Operating Conditions
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Operating Conditions
When plotting ROP vs. WOB obtained experimentally with all other drilling
variables held constant has the characteristic shape as shown:
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Operating Conditions
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Operating Conditions
No significant ROP is obtained until the threshold bit weight is applied (point a).
ROP then increases rapidly with increasing values of WOB. For moderate value of
bit weight, a linear curve is often observed (segment bc). However, at higher
values of bit weight, subsequent increase in bit weight causes only slight
improvements in ROP (cd). In some cases, a decrease in ROP is observed at
extremely high value of WOB (de). This type of behavior often is called bit
floundering. This poor response of ROP at high values of bit weight usually is
attributed to less efficient bottomhole cleaning at higher rates of cuttings.
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Operating Conditions
A typical plot of ROP vs. rotary speed obtained with all other drilling variables held
constant is shown. ROP usually increases linearly with low RPM. At higher values
of RPM the response of ROP to increase RPM diminishes. The reason is due to
the poor hole cleaning.
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Bit Diameters
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Drilling Engineering
Factors Affecting Penetration Rate
Effecting of Bit Diameters
Generally, as the bit diameter
increases, the applied weight
on the bit is distributed over
the
larger
consequently
area
reduces
which
the
rate of penetration of the bit.
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