7 hydraulics

1. Drilling Hydraulics
♦ Overcoming formation pressure
♦ Stabilizing the borehole
♦ Cooling and lubricating the bit and drill string
♦ Transport cuttings to the surface
Drilling Fluid Circulation
Basic Functions
Fundamental Drilling Fluids
There are 3 basic categories of drilling
fluids :
♦ Gas
♦ Foam
♦ Liquid
Drilling Fluids Properties
♦ Gas is highly compressible. Its volume is dependent on pressureand
temperature
♦ Foam is compressible, mixture of air/gas+water+chemicals
♦ Liquid is slightly compressible, properties are changing with pressure and
temperature
Drilling Fluids Properties
Density ppg kg/dm3 SG
Funnel Viscosity sec
Viscosity (PV-YP) cP -lb/100 ft2
Gel Strength lb/100 ft2
Solids Content vol%
Basic data:
Mud Rheology
Properties that determine mud rheologyinclude :
♦ Density
♦ PlasticViscosity
♦ Yield point
♦ Gel strength
♦ Acidity (pH)
♦ Filtration
♦ Chloride Content
♦ Alkalinity
♦ Calciumcontent
♦ Solidscontent

2. How do you measure therheological properties ?
♦ Marsh Funnel
The time required for 1 U.S. quart (or 946 cm3) of fluid to drain through a
funnel.
Example: Water has a funnel viscosity of 26 seconds.
♦ Rotating Cylinder Viscosimeter
Measures torque (usually at 600,300,200,100,6 and 3 rpm). Figures on the
dial are degrees of rotation. A conversion factor is required toconvert the dial
readings to units of shear stress.
Shear Rate, Shear Stress
d
v F
D =Shear rate :
v (m/s)
d (m) = … 1/s
Shear stress :
A
T =
F (lb)
A (100 ft2)
= … lb/100ft2
Principle of
Rotational Viscosimeter
6-speed
Rotating Cylinder
Viscosimeter
Fann Model 35
viscosimeter
Fann Model 35
viscosimeter
6-speed
Rotating Cylinder
Viscosimeter
Rheological Models (1)
What the models do :
Mathematical formulae, allows us to calculate the shear stress
at any shear rate we wish
T = f (D)
The parameters of this function depend on the selected fluid model :
µ for newtonic fluidmodel
PV, YP for the Bingham plastic fluid model
n, K for the Power Law fluid model

3. Rheological Models (2)
From the shear stress and shear rate the viscosity can be calculated :
µ = T/D
This is the dynamic viscosity.
Viscosity of the water at 20C is 1centiPoise : cP (1 dyn-s/cm2).
RheologicalModels(3)
♦ Newtonic
♦ Bingham-plastic
♦ Power Law (pseudoplastic)
♦ Yield-Power Law
♦ Casson
Newtonic Model
T = µ x D
µ viscosity, P, cP
D shear rate,1/s
T shear stress,lb/100ft2
♦ Relationship :
Newtonic Flow Curve
1/s
lb/100 ft2
1022511
R600 - R300
511
rpm300 600
Newtonic viscosity = tan a
a
t
D
Bingham-Plastic Model
♦ Relationship :
♦ Yield Point is an initial stress at 0 shear rate, measure of electrical
attractive forces in the mud under flowing conditions
♦ Once this yield stress is exceeded, equal increments of shear rate
produce equal increments of shear stress.
Shear Stress = (Plastic Viscosity)x(Shear Rate)+Yield Point
T = PV x D + YP
Bingham Model Parameters
When using a Fann viscometer with R1-B1-F1
rotor-bob-spring combination :
PV = (600rpm reading)- (300rpm reading) [ cP]
YP = 2(300rpm reading) - (600rpm reading) [lb/100 ft2]
Note : For field use, a conversion factor is neglected

4. Bingham-Plastic Flow Curve
YP
1/s
lb/100 ft2
1022511
R600 - R300
511
rpm300 600
a
Plastic viscosity = tan at
D
Bingham-Plastic Flow Curve
YP
1/s
lb/100 ft2
1022511
rpm300 600
a ß
Apparent viscosity = tan a (at 300 rpm)
tan ß (at 600 rpm)
t
D
Bingham Plastic Model
♦ Apparent viscosity varies with shear rate for non-Newtonian fluids
♦ Apparent viscosity decreases with increased shear rate (called
shear thinning)
♦ As shear rates approaches infinity, apparent viscosity reaches a
limit (the plastic viscosity).
♦ Advantage: easy to use, and represents most drilling fluids
♦ Disadvantage: does not accurately represent drilling fluids at low
shear rates
Power Law Model
♦ Water-based mud systems are typical
♦ n and K values are used in the model
♦ Drilling fluids behave more plastic than water when subjected
to increasing pressure and temperature
Power Law Model
♦ Relationship (pseudoplastic fluid) :
Shear Stress = (K Consistency Factor) x (ShearRate)n
♦ Consistency Factor (K) describes the thickness of the fluid
and somewhat analogous to apparent viscosity
(i.e., as K increases, mud becomes thicker)
♦ Flow behavior index (n) indicates the degree of non-Newtonian behavior.
If n = 1, the fluid is newtonic.
If n > 1, viscosity increases as shear rate increases ( dilatant fluid)
If 0 < n < 1, viscosity decreases as shear rate increases (pseudoplastic fluid)
T = K x Dn
Power Law Flow Curve
1/s
lb/100 ft2
1022511
511
rpm300 600
t
D

5. ♦ To calculate n :
n = 3.32 log (600rpm reading / 300rpm reading) [-]
♦ To calculate K :
K = (300rpm reading)/ 511n [lb-sn/100 ft2]
Power Law Model Parameters
Power Law Flow Curve
1/s
lb/100 ft2
3.00952.7084
rpm300 600
log t
log D
K
f
n = tan f
Power Law Model
l Advantage of the model : represents the behavior of drilling fluids at low
shear rates more accurately
♦ Most drilling fluids are pseudoplastic
♦ The viscosity is decreasing with increasing shear rate : shear thinning is
desirable
Yield Power-Law Flow Curve
1/s
lb/100 ft2
1022511
511
rpm300 600
Y
t
D
Flow Curves of Different Fluids
1/s
lb/100 ft2
1022511
rpm300 600
Ps.plastic
Newtonic
Dilatant
shear rate
shear
stress
n < 1
n = 1
n > 1
Drilling Fluid Model Application
♦ Power Law model for water-, synthetic-oil or petrofree
fluid based mud systems
♦ Bingham Plastic model for oil-based mud systems
Best fits :

6. Time-Dependent Behavior
♦ Most drilling fluids exhibit time-dependent
behavior.
♦ Shear Stress is dependent on duration of
shear. Why?
♠ Clay plates or fibers are broken into smaller
particles at higher rates of shear.
♠ These small particles aggregate into layer
units as shear rate is decreased again.
♠ Both of these events take a considerable
length of time.
Gel Strength
Gel strength describes the time-dependent flow behavior of a drilling
fluid.
♦ Measures the attractive forces in the fluid under static conditions.
♦ Increases steadily with time (strong) or only slightly with time
(weak gel).
♦ Strong gels are the result of high clay concentration which may
require excessive pressure to break circulation.
♦ May lead to lost circulation (i.e., strong gels are undesirable.)
♦ Fluid essentially moves as a single, undisturbed solid body orplug
♦ Movement occurs by a thin layer of fluid “slipping” along a conductor
surface
♦ Plug flow generally occurs at very low flow rates
♦ Generally accepted upper limit for plug flow is at Nre= 100
Plug Flow
Flow Regimes Flow Regimes
♦ Laminae are concentric cylindrical shells which slide or extend past
one another like sections of a telescope
♦ The velocity at the pipe wall is 0; maximum velocity is at the center
of the pipe
Laminar Flow
3-D View of Laminar Flow in a Pipe
(Newtonian Fluid)
2-D Velocity Profile of Laminar
Flow in a Pipe (Newtonian Fluid)
Velocity
Radius
r r
dv
dr
= 0
MAXV
0

7. Laminar vs. Turbulent
♦ A fluid layer adjacent to the surface of a conductor adheres to
the surface, and each successive fluid layer slides past the
previous with increasing velocity.
This is called laminar (layered) flow.
♦ At higher velocities, the layers lose their order and randomly
crash into each other. This is called turbulent flow.
Flow Regimes
♦ Fluid moves as a plug essentially, due to the chaotic, random shearing
motion
♦ Only near the walls of the pipe does an orderly shear exist (laminar boundary
layer)
♦ Velocity gradient is very steep near the walls but essentially flat elsewhere -
formed by eddies
Turbulent Flow
2-D Velocity Profile of Turbulent Flow in a Pipe
(Newtonian Fluid)
Velocity
Radius
r r
MAXV
0
Flow
Regime
Comparison
F
o
r
m
a
t
i
o
n
F
o
r
m
a
t
i
o
n
D
r
i
L
L
P
i
p
e
The fluid flow can be either laminar, transitional or turbulent. How do we
determine which one?
♦ Calculate Nreand compare to the critical Reynolds no. Nrec
♦ Calculate critical velocity - based on the Nrec number - and
compare to the actual flow velocity
♦ Note: Nrec= 2100 for water and Bingham plastic fluids
Nrec= 3470 - 1370n forpseudoplastic fluids
Determining Flow Regime
To calculateNre you need to know the following :
♦ Pipe diameter
♦ Average fluid velocity
♦ Fluid density
♦ Fluid viscosity
Determining Flow Regime

8. Reynolds Number
Pipe Diameter x Average Fluid Velocity x Fluid Density
Nre =
Fluid Viscosity
Nre =
1000 d v MW
µ
d pipe inside diameter, mm
v fluid velocity, m/s
MW fluid density, kg/liter
µ viscosity, cP
Drilling Fluid Circulation
♦ Drilling fluid is pumped under pressure through the manifolds,
standpipe, kelly hose, swivel, kelly, drillpipe,drillcollars and bit
Calculate the mean or bulk velocity through pipe as follows:
Typically, this is 1000 ft/min (300 m/min). Is it laminar or turbulent ?
♦ The pressure required to circulate a fluid in turbulent flow is
approximately 1.8 times the power of flowrate
♦ Pressure drop across drillstring represents 35% of total pump
pressure
v = q/A
Mud Pumps
l Pump output- duplex pump
Qt = 1.5715 x 10-6 nL(D2 - d2/2) liter/min
Qt theoretical flow rate, l/min
n strokes per minute
L length of stroke, mm
D liner diameter, mm
d piston rod diameter, mm
Mud Pumps
l Pump output- triplex pump
Qt = 2.356 x 10-6 nLD2 liter/min
Qt theoretical flow rate, l/min
n strokes per minute
L length of stroke, mm
D liner diameter, mm
♦ Volumetric efficiencies
♠ Single Acting pumps = 95% - 98%
♠ Double Acting pump = 90%
♦ Efficiency is affected by :
♠ Discharge pressure
♠ Pump operating speed
♠ Suction line design
♠ Fluid aeration
♠ Mechanical wear
Mud Pumps Mud Pumps
l Volumetric efficiency
ηv = Qout/Qt
Qout measured output flow, l/min
Qt theoretical flow rate, l/min

9. Mud Pumps
l Hydraulic power
Ph =
Ph =
P pump pressure, bar
Q pump output, flow rate, l/min
PQ
600
kW
PQ
447.50
hp
Mud Pumps
l Engine power required
ηm mechanical efficiency, 0.85 - 0.90
ηv volumetric efficiency, best case is 0.98
Pm = Ph / (ηmηv )
Hydraulic Design
• Select flow rate
• Calculate annular velocity
• Calculate annular pressure loss
• Calculate string pressure losss
• Calculate motor, MWD pressure losses
• Calculate bit pressure loss available
• Calculate TFA, nozzle sizes and jet velocity
• Calculate SPP
• Check HHP, HHP/sq.in
• Calculate surface system pressure loss
• Calculate pump pressure, spm, liner size
• Calculate slip velocity and cuttings transportratio
Flow Rate Selection (1)
• Crucial factor forsuccesful hole cleaning, especially in deviated holes.
• The operating requirements of the extra downhole tools must be considered
Min-max flow rate range specified for motors turbines and MWD tools.
• Rough guide : use 2 x AV in holes between 50-60° than in vertical holes.
• Use 100 fpm ( 30 m/min) annular velocity as a starting point, and adjust it,
if necessary.
Flow Rate Selection (2)
• Laminar - transitional - turbulent flow considerations :
- pressurelosses
- ECD
- hole erosion
Motor and MWD Hydraulic Requirements
Mud motor : a) Flow rate range ( gpm, liter/min)
b) Max. operating differential pressure (psi, bar)
c) Bit Pressure Range (psi, bar)
MWD : a) Flow rate range ( gpm, liter/min)
b) Bit (back) pressure (psi, bar)
--------------------------------------------------------------------------------------
Example : 6-3/4” SperryDrill 4/5 lobe, 4.8 stage motor
a) 300 - 600 gpm (1135 - 2270 liter/min)
b) 590 psi (40.7 bar)
c) 200 - 1200 psi (13.8 - 82.7 bar)

10. Annular Flow Velocity
To calculate average annular velocity ...
3.14(Dh2
- Dp2
)
Vaverage
=
4000 x Q
m/min
Q flow rate, liter/min
Dh diameter of hole, mm
Dp diameter of pipe, mm
Annular Flow Velocity
To calculate critical velocity in annulus :
3.04 {PV +√ PV2 + 40.05YP (Dh- Dp)2 x MW }
Vc =
(Dh - Dp) x MW
m/min
PV plastic viscosity, cP
YP yield point, lb/100 sq.ft
Dh,Dp hole size, pipe diameter, inch
MW mud weight, kg/l
Pipe Flow Velocity
To calculate critical velocity in pipes :
2.48 {PV +√ PV2 + 73.57YP Dpi2 x MW }
Vc =
Dpi x MW
m/min
PV plastic viscosity, cP
YP yield point, lb/100 sq.ft
Dpi pipe internal diameter, inch
MW mud weight, kg/l
Flow Regimes
Vaverage
< Vcritical
= Laminar flow
To determine if flow is laminar or turbulent ...
Vaverage
> Vcritical
= Turbulent flow
Annular Flow Pressure Loss
To calculate pressure loss for annular laminar flow ...
PaL =
PV x L x Q
40863(Dh+Dp)(Dh - Dp)3
YP x L
1326 (Dh - Dp)
+
Note: Calculate pressure drops for each section of annulus
bar
L length of section, m
Q flow rate, l/min
Dh diameter of hole, inch
Dp diameter of pipe, inch
Pipe Flow Pressure Loss
To calculate pressure loss for pipe laminar flow ...
PpL =
PV x L x Q
61295 D4
YP x L
1326 D
+ bar
PV plastic viscosity, cP
YP yield point, lb/100 sq.ft
L length of section, m
Q flow rate, l/min
D inside diameter of pipe, inch

11. Annular Flow Pressure Loss
To calculate pressure loss for annular turbulent flow ...
PaT =
PV 0.2 x L x Q 1.8 x MW0.8
70696(Dh+Dp) 1.8 (Dh-Dp)3
bar
PV plastic viscosity, cP
L length of section, m
Q flow rate, l/min
MW mud weight, kg/liter
Dh diameter of hole, inch
Dp diameter of pipe, inch
Pipe Flow Pressure Loss
To calculate pressure loss for pipe turbulent flow ...
PpT =
PV 0.2 x L x Q 1.8 x MW0.8
90163 D4.8
bar
PV plastic viscosity, cP
L length of section, m
Q flow rate, l/min
MW mud weight, kg/liter
D inside diameter of pipe, inch
Equivalent Circulating Density
To calculate equivalent circulating density ...
ECD = MW +
Σ Pa
0.0981 x L
SG or kg/liter
MW mud weight, SG or kg/liter
Σ Pa total annular pressure loss, bar
L length of annulus, m
Bit Nozzle Selection
• 300 fps (100 m/s) jet velocity (or less in soft formations)
• 48-65% of the pump pressure as bit pressure loss
• HSI = 2.5 - 7 (as per bit supplier’s recommendation)
• Sufficient pressure drop for downhole motor operation
• Adequate back pressure for MWD, or other dh. tools
♦ To calculate jet velocity ...
Bit Hydraulics
16.666 Q
TFA
Vj = m/s
Q flow rate, liter/min
TFA total flow area, mm2
♦ To calculate bit pressure drop ...
Bit Hydraulics
Pb =
C2
TFA2
1.465 Q2
MW
bar
Q flow rate, liter/min
MW mud weight, kg/liter
C orifice coefficient, 0.95 for jet nozzle
TFA total flow area, mm2

12. Bit Hydraulics Optimization
Hydraulic Optimization is maximizing ...
♦ Bit Hydraulic Horsepower
♦ Jet Impact Force
or
Bit Hydraulics Optimization Criteria - BHHP
Maximizing Bit Hydraulic Horsepower requires
65% of available pump pressure to drop in the bit
l Bit Hydraulic Horsepower
BHHP =
Pb pressure loss in the bit, bar
Qj flow rate through the nozzles, l/min
Pb xQj
447.50
hp
Bit Hydraulics
l Bit Hydraulic Horsepower per Square Inch :
HSI =1.836
Pb pressure loss in the bit, bar
Qj flow rate through the nozzles, l/min
Db bit size, mm
Pb x Qj
Db2
hp/ in2
Maximizing Jet Impact Force requires 48% of
available pump pressure to drop in the bit
♦ Jet Impact Force
IF = 2.24 x Vj x Pb x Qj x MW N
Vj jet velocity, m/s
Pb pressure loss in the bit, bar
Qj flow rate through the nozzles, l/min
MW mud weight, kg/liter or SG
Bit Hydraulics Optimization Criteria - IF
Surge & Swab Pressure
PIPE
MOVEMENT
MUD FLOW
• Annular Pressure Loss
SWAB
SURGE
Surge & Swab Pressure
l Calculation Procedure
– Calculate average pipe speed
– Calculate maximum mud velocity each section
– Calculate equivalent circulation rates
– Calculate pressure loss gradients
– Multiply pressure loss gradients times section lengths
– Correct pressure losses to tripping mud weight
– Add all section pressures for total surge or swab pressure

13. Surge & Swab Pressure
l Average Pipe Speed
vp = (ft/stand)(60) / (measured time in seconds/stand)
l Example : Calculate the average pipe speed when 93 ft (28.3
m) stands of drill pipe are being pulled at 30 s/stand
vp = (93 ft/stand)(60)/30 s/stand)
vp = 186 ft/min or 56.7 m/min
Surge & Swab Pressure
l Mud velocity maximum
vm = (0.45 + (dp
2 / (Dh
2 - dp
2)))(vp)(1.5)
l Example : Calculate the mud velocity when tripping 5 inch (127 mm) drill pipe
from an 8-1/2 inch (215.9 mm) hole at an average pipe speed of 186 fpm (56.7
m/min).
vm = (0.45 + (52 / (8.52 - 52))) (186)(1.5)
vm = 273 fpm or 83.2 m/min
Surge & Swab Pressure
l Equivalent circulating rate
The circulating rate which produces the same annular mud velocity as caused
by movement of the drillstring into or out of the borehole
l Example : Find the equivalent circulating rate for a 273 fpm (83.2 m/min) mud
velocity inside an 8-1/2 inch (215.9 mm) hole around 5 inch (127 mm) drill pipe.
GPM = 520 gpm
LPM = 1966 lit/min
Surge & Swab Pressure
l Find the surge/swab pressure for an equivalent circulating rate of 520 gpm when
tripping 9,000 feet of 5 inch drill pipe from an 8-1/2 inch hole. The mud weight is
13.0 ppg.
– Find the annular pressure loss gradient for 13.0 ppg mud weight
• Psi/1000 ft = 39
– Calculate the annular pressure loss with 9,000 ft of drill pipe in hole
• (39 psi/1000 ft) (9,000 ft) = 351 psi
– The surge/swab pressure =351 psi
– The equivalent densities :
13.0 + 0.75 = 13.75 ppg
13.0 - 0.75 = 12.25 ppg
Surge & Swab Pressure
l Factors affecting the swab / surge pressures :
– Depth
– Hole size
– Pipe size
– Pipe acceleration and velocity
– Mud weight
– Mud rheology
– Gel strength
– Mud cake thickness
Surge & Swab Pressure
l Maximum recommended pipe speed :
– Establish maximum permissible surge or swab pressure
– Select seconds/stand trip speed
– Evaluate surge or swab pressure
– Adjust pipe speed for change in pressure required and re-calculate
– Continue re-calculating until desired pressure loss is realized
– Record seconds/stand as recommended trip speed
– Create table with trip speed vs. bit depth

14. Slip Velocity
What is slip velocity?
♦It is the rate at which cuttings fall back to the bottom of the hole in
static mud
♦For efficient hole cleaning, the average velocity (or annular velocity)
should be at least twice that of the slip velocity
Particle Reynolds Number
NRes =
d Vs MW
µ eff
d particle diameter, cm
Vs slip velocity, cm/s
MW mud weight, g/cm3
µ eff effective viscosity, P
If NRes < 10 Laminar slip
Slip Velocity
53.3 x (Wcuttings - MW) x d2
cuttings xVavg
6.65 x YP x (Dh - Dp) + PV x Vavg
x 60Sv = fpm
Note : for Bingham-plasticfluids
Slip Velocity
If If NRes > 100 Turbulent slip
dcuttings x(Wcuttings - MW)
MW
Sv= 0.2 x fpm
Note : for Bingham-plasticfluids
Transport Efficiency
TE =
Va - Vs
Va
x 100 %
Drilling Hydraulics Basics
for
Deviated Boreholes

15. Why directional drilling hydraulics is more complex?
Because …
• Downhole motor, turbine and other hydraulically
operated tools are being used in deviated wells
• Mud pulse telemetry is widely utilised in MWD and LWD
tools
• Cuttings transport is problematic and critical in highly
deviated boreholes
l Mud properties, circulating system
l Flow rate selection
l Bit nozzle selection
l Hole cleaning, cuttings transport
l Hole erosion
l ECD
Issues
Mud Properties
• Low solids, and particularly low sand content
• Fresh water mud,or low aromatics OBM
• Good rheologyfor low pressure losses, but efficient cuttings
transport
• Low friction coefficient in high angle/horizontal holes
Requirements :
Circulating System
Requirements :
• Minimum 2 - preferably triplex - pumps
• Properly maintained and charged pulsation dampeners
• Efficient suction filters
• Ditch magnets
• 3 shakers for 16” or bigger hole sizes when motor drilling is
planned
• Selection of different bit nozzles
l Hole inclination ranges :
near vertical 0 - 10°
low 10 - 30°
intermediate 30 - 60°
high 60° <
Hole Cleaning
<30° No beds formed, cuttings are suspended and transported
>30° Cuttings bed formed, bed slides down and cause pack-off
45 to 55 degrees is the most critical !
Cuttings on the low side could …
• form an unstable cuttings bed
• cuttings bed builds up and slides down as a block,or
• be transported at the bed / mud interface as ripples, or dunes
(saltation) - best induced by low viscosity fluids in turbulence
Cuttings Transport Problems in Deviated Wells

16. Problem Indicators
♦ Tendencious deviation from calculated torque and drag values
♦ Cuttings volume decreasing, or lack of cuttings
♦ Higher standpipe pressure after resuming circulation on connections
♦ Low/Hi vis pills return carries volume of cuttings to surface
♦ Cuttings are small and rounded
Hole Cleaning
l 0 to 30 degrees inclination :
– Conventional methods clean the well good
– Effective suspension of cuttings and barite by shear stress
– No cuttings bed formation tendency
– Application of slip velocity equations okay
– Add 20 to 30% safety factor in circulation time for deviation from vertical
– Use high viscosity sweeps
– Adjust flow profile
l 30 to 60 degrees inclination :
– Problems :
• Cuttings bed formation tendency
• Cuttings move as a bed or by saltation at bed/mud interface
• Back sliding and packing off (at circulation stops)
– Strategies :
• Circulate cuttings above the critical inclination range on connections
• Minimize circulation stops
• Frequent short trips with backreaming
• High pump rates
• Frequent low-vis pills in turbulence followed by high vis sweeps
Hole Cleaning
l 60 to 90 degrees inclination :
– Problems :
• Cuttings bed formation
• Insufficient mud velocity to transport cuttings on low side
• Excessive torque and drag
– Strategies :
• Circulate cuttings above the critical inclination range on connections
• Minimize circulation stops, use top drive not kelly
• Pump out on connections
• Frequent short trips with backreaming
• Low viscosity mud in turbulent flow in open hole annulus
• Frequent low vis pills followed by high vis sweeps
Hole Cleaning
l Source of increased torque and drag in high angle holes :
– Wellbore contact on the HS of the hole
– The wellboreis not clean, drill stem moves in a solid bed
– Drillstring movement across rock fragments and filter cake on the LS
• conventional lubricants offer little relief (use mechanical additives)
• best to keep hole clean in the high angle section
• cuttings form in valleys reducing effective hole size
Hole Cleaning
l Other considerations
– When pumps shut down cuttings bed will slide in sections of holewith critical
deviation angle sticking pipe
• have a back-up pump (or cementing truck) ready to go on-line in case of
main pump failure
• intermittent reciprocation of drillstring with high flow rates well into
turbulence (backreaming the stand prior to connection)
• watch returns when pumping sweeps to evaluate hole cleaning and keep
close watch on density and solids content of the mud
Hole Cleaning

17. Excerpt from 28 Hole Cleaning Rule of Thumbs
RT1
The intermediate inclination range hole is typically the
most difficult to clean.
RT3
Boycott settling can accelerate bed formation, particularly
between 40-50° inclination.
RT5
Cuttings accumulate in intervals of decreased annular
velocity and can unload when circulation stops, if the
inclination is <50°
RT9
Cuttings beds are easy to deposit, difficult to remove.
RT14
An increase in annular velocity improves hole cleaning,
regardless of the flow regime.
RT16
The cuttings transport mechanism is largely a function
of annular velocity.
RT17
Laminar flow is preferred if formations are sensitive to
erosion
RT18
Turbulent flow is effective in high-angle, small diameter
intervals in competent formations
RT19
Hole cleaning capacity in laminar flow is improved by
elevated low shear-rate viscosity and gel strength.
RT25
Pipe rotation and reciprocation can improve hole cleaning
RT26
Mud weight increases the buoyant force on the cuttings
and helps hole cleaning.