Drilling Overview
Bottom Hole Assembly (BHA)
Contents
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Introduction
Components & Functions
Design
Types
Summary
References
Components & Functions
List of components:
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Drilling bits(the major DS component)
Drill collars
HWDP
Stabilizers
Reamers
Subs
Thrusters
Shock sub
Jars
Motors
Turbines
Components & Functions
• Drilling bits
➢ Function
➢ Types
1) Rock-Bits
▪ According to cutters:
✓ Milled tooth bit
✓ Tungsten carbide insert bit
(TCI)
▪ According to no. of cones:
✓ Two cone bit
✓ Three cone bit
✓ Four cone bit
Components & Functions
• Drilling bits
2) Fixed Cutter
▪ PDC bit
▪ Impregnated bit
▪ Composite bit
Types
Components & Functions
• Drilling bits
➢ Bit selection
▪
▪
▪
▪
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Formation type
Mud type
Well profile
Drilling parameters
Bit size
Components & Functions
• Drilling bits
▪ Hole opener
▪ Core bits
Components & Functions
• Drill collars
➢Description and Function
➢Types
1) Spiral
▪ Reduce possibility of differential wall stuck
2) Square
▪ Prevent deviation from existing hole
3) NMDC
▪ Shield directional survey instruments
Components & Functions
• HWDP
➢Description
➢Function
1) Transition between DC and DP is more
resistant to fatigue than DP
2) Recommended than DC for slim drilling
3) Recommended than DC for high angle
drilling
Components & Functions
• HWDP
➢Advantages
1) Permits high speed drilling with less torque
2) Reduces differential stuck and torque (less
contact area than DC)
3) Increases the buckling load (central upset
limit)
4) Acquires faster tripping
5) No loss of BOP control
Components & Functions
• Differece between HWDP & DP
Components & Functions
• Stabilizers
➢ Description
➢ Types
1)
2)
3)
4)
5)
Sleeve
Replaceable blades
Integral
Long integral
Nipple with welded-on blades
Components & Functions
• Stabilizers
Types
Components & Functions
• Stabilizers
➢ Role in
1)
2)
3)
4)
5)
6)
7)
Directional drilling
BHA Centralizing
Prevention of buckling
Rock cutting
BHA Vibration
Pipe stuck
Torque and drag
Components & Functions
• Reamers
➢ Description
➢ Types of Rollers
1) Milled teeth roller
2) Tungsten carbide roller
3) Slick roller
Components & Functions
• Reamers
➢ Functions
1) Reduce torque by reducing the friction on
the BHA contact points
2) Cut gauge hole by the rollers
3) Reduce vibration
Components & Functions
• Subs
➢ Types
1) Crossover Sub
2) Float sub
✓ Advantages and
disadvantages
Components & Functions
• Subs
Types
3) Circulating Sub
▪ Position
✓Above the MWD or the
motor in directional drilling
✓Above the bit in rotary
drilling
▪ Function
✓Used for LCM pumping
✓Used for fast hole cleaning
Components & Functions
• Subs
Types
4) Dart Sub
▪ Description
✓It is a landing sub for
drop in back pressure
valve
▪ Function
✓Used in Kick prevention
during POOH
Components & Functions
• Subs
Types
5) Bent Sub
▪ Description
✓Small collar provides an
offset (fixed angle 1-3° in
½° increment)
▪ Function
✓Used as a deflection tool
in the directional drilling
Components & Functions
• Thrusters
➢ Description
➢ Applications
1) In difficult transfer of DC wt. to the bit
due to high drag
2) In high vibrations
3) In WOB control to optimize ROP
Components & Functions
• Shock Sub
➢ Function
➢ Choice constraints
▪ Spring rate should be such that the build
up of a tri-lobed bottom hole pattern is
prevented for the operated speed range.
▪ Should never be run fully closed or opened
but in its mid position.
Components & Functions
• Jar
➢Description
➢Operation & Function
➢Types
1) Drilling jar
2) Fishing jar
Components & Functions
• Mud motor
▪ Rotating the bit while the drill string is stationary
depends on rev/gallon (no. of loops).
Components & Functions
• Turbine motor
▪ Rotating the bit while
the drill string is
stationary depends on
rev/gallon (no. of
stages).
▪ Turbine motor gives
more rpm than the
mud motor.
Design
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Design involves determination of
1) Drill pipe length
2) Drill pipe weight
3) Drill pipe grade
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Design depends on
1)
2)
3)
4)
Hole depth and size
Mud weight
Safety factor (SF)
Drill collar length and weight
Design
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BHA design criteria are
1)
2)
3)
4)
Tension
Collapse
Shock Loading
Torsion
Components & Functions
• Drill collars
➢Design
1) Buoyancy factor method
▪
ABW=BF * Wt.DC * SF
2) Pressure area method
BF1= - P A
BF2= + P A
▪ ABW=BF1+BF2+ Wt.DC
Design
Design Criteria
1) Tension
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P= (Ldp * Wdp + Ldc * Wdc) * BF
where BF= (1- ρm/ρs)
J
So the Dp strength expressed in
terms of yield strength should stand
this load.
P
J
Dp
Dc
Bit
Design
Design Criteria
1) Tension
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Max. tensile design load (Pa) is 90%
of the theoretical yield strength (Pt).
J
Pa= Pt*0.9
Margin of overpull (MOP)= Pa-P
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The design values of MOP ranges
from 50000-100000 lb
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SF= Pa/P
= Pt*0.9/ (Ldp * Wdp + Ldc * Wdc) * BF
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P
J
Dp
Dc
Ldp= [Pt*0.9/ SF*Wdp*BF]- Wdc*Ldc/Wdp
Bit
Ldp= [(Pt*0.9-MOP)/Wdp*BF]Wdc*Ldc/Wdp
Design
Design Criteria
2) Collapse
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It is the external pressure required to
cause yielding of drillpipe or casing.
This depends on the differential
pressure across the pipe body.
The differential pressure Δp can be
expressed by the following equation
Δp= (L* ρ1/144)- [(L-Y)* ρ2/144]
Case 1: Empty pipe
Case 2: ρ1=ρ2=ρ
Y=0 & ρ2=0
Δp=Y* ρ/144
Δp=L* ρ1/144
Note: Length is in ft. and ρ is in pcf
Design
Design Criteria
3) Shock Loading
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It arises whenever slips are set on moving
drillpipe and can contribute to parting of pipe in
marginal designs.
Fs= 3200 Wdp;
Wdp: weight per unit length in air
Design
Design Criteria
4) Torsion
Case 1: Drillpipe
subjected to
pure torsion
Case 2: Drillpipe
subjected to both
torsion and tension
Q=0.096167*J*
Ym/D
Qt=[0.096167*J/D]*(Ym2P2/A2)
Where,
Q= torsional yield strength (lb-ft)
Qt= torsional yield strength (lb-ft)
J= polar moment of inertia =0.098175*(D4-d4)
D= outside diameter (in), d= inside diameter (in)
Ym=yield strength (psi)
P= total load in tension (lb)
A= cross-sectional area (in2)
Types
1) Slick BHA
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defined as one in which no stabilizer is used.
consists of bit, drill collars and drillpipe.
is suitable only for mild crooked hole
formations.
are seldom used except to drill to Kick-off point
in directional drilling.
Types
2) Pendulum BHA
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is used to reduce hole deviation as in the dropoff section of an S-type directional well.
is used to drill soft, unconsolidated formations.
Pendulum technique:
It relies on the principle that
the force of gravity may be
used to deflect the hole
back to vertical
θ
F
Types
2) Pendulum BHA
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Pendulum BHA consists of several drill
collars to provide the pendulum force.
Maximum pendulum force can be
obtained by placing the first stabilizer as
high as possible. However, this exposes
the pendulum technique to failure.
Main disadvantage is obtaining
misaligned hole difficult for running the
casing.
Solution: Min. drill collar OD
=2(csg coupling OD)- bit
OD
Types
3) Packed hole assembly
It relies on the principle that two points will contact
and follow a sharp curve, while three points will
follow a straight line.
Types
3) Packed hole assembly
It got three types depending on the severity of the crooked
hole tendencies.
Rest of string
Zone III
stabilizer
stabilizer
30 ft drill collar
30 ft drill collar
Vibration
dampener
Vibration
dampener
Zone II
stabilizer
Short drill collar
Stabilizer or
reamer
Zone I
bit
Stabilizer or
reamer
bit
bit
Mild crooked hole
Medium crooked hole
Severe crooked hole
Packed BHA
Packed BHA
Packed BHA
Summary
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BHA main components are bits, DC, DP, HWDP,
stabilizers, reamers, subs, thrusters, jars,
motors and turbines.
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BHA design criteria are tension, collapse, shock
loading and torsion.
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BHA main types are Slick, Pendulum and
Packed.
References
• CH.3, Part 1, Section 6,Vol 2, Well engineering
knowledge, Shell round one
• Rabia, Hussain. Oil well Drilling Engineering.
1985