02 cost control
DESCRIPTION
Cost ControlTRANSCRIPT
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1 © 2005 PetroSkills LLC, All Rights Reserved
COST CONTROL
The object of any drilling operation
is to drill a usable well at minimum
cost
In most drilling operations, faster is
cost effective
Minimize non-drilling operations
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2 © 2005 PetroSkills LLC, All Rights Reserved
Cost Comparisons Cost per foot when comparing bit
runs, type of bits, downhole motors, bit
weight and rotary speed, hydraulics,
etc.
Total cost or total cost per foot when
comparing different types of muds,
casing programs, drilling practices,
etc.
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COST PER FOOT
Equation for calculating cost per foot
Where CT is the cost per foot or
meter
CB is the cost of the bit
Cr is the hourly cost of the drilling
operations
CC C t T
FT
B r ( )
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COST PER FOOT
Equation for calculating cost per foot
T is the trip time in hours
t is the rotating time in hours
The nomenclature for equations is
at the end of each chapter
F
TtCCC rB
T
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COST PER FOOT
The hourly cost of the drilling
operations include
The daywork cost of the drilling rig
Rental equipment
Mud logger
Average mud maintenance cost
Work boats
Helicopters
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COST PER FOOT
Directional drilling cost that reoccur
every day
Supervision
Standby personnel
Logging while drilling
Anything that is charged by the day
or hour
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COST PER FOOT
The hourly rig cost does not include
one time charges or items that are
not time dependent
Casing
Cementing
Wireline open hole logs
Wellhead equipment
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COST PER FOOT Example Problem 2-1 shows how to
calculate the cost per foot
Bit # 4
Rig cost = $2,000 per hour
Bit cost = $4,000
Rotating time = 100 hrs
Footage = 3000 feet (914 m)
Depth = 10,000 feet (3048 m)
Trip time = 2 hr per 1000 feet (305 m)
Rotary speed = 100 rpm
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COST PER FOOT
Calculate the cost per foot
F
TtCCC rB
T
ft 3000
100001000
hr 21002000$4000$
TC
/ft33.81$TC
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COST PER FOOT
Example Problem 2-2 shows how to
calculate the cost per foot
Bit # 5
Rig cost = $2,000 per hour
Bit cost = $4,000
Rotating time = 63 hrs
Footage = 1300 feet (396 m)
Depth = 11,300 feet (3444 m)
Trip time = 2 hr per 1000 feet (305 m)
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COST PER FOOT
Calculate the cost per foot
F
TtCCC rB
T
1300
113001000
26320004000
TC
/ft77.134$TC
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COST PER FOOT
Calculate the cost per foot
F
TtCCC rB
T
1300+3000
11300100001000
210063200040004000
TCC
/ft49.97$TCC
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CLASS PROBLEM Calculate cost per foot for
each
Insert bits
Number or bits = 2
Cost per bit = $10,000
Cost of rig = $1250/hr
Rotating hours = 180
Total time for two trips = 21
hrs
Footage = 4500’ (1372 m)
25.00 feet per hour
PDC bit and motor
Number or bits = 1
Cost per bit = $50,000
Cost of rig = $1250/hr
Cost of motor = $300/hr
included in drilling and trip
time
Rotating hours = 135
Trip time = 12 hrs
Footage = 4500’ (1372 m)
33.33 feet per hour
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Class Problem
Insert Bits
Cost of two bits = 10000 x 2 = 20,000
Cost per foot calculation
/ft28.60$
4500
21180125020000
TC
F
TtCCC rB
T
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Class Problem
PDC bit and motor
Assume the motor cost is included in
the trip time
Cost per foot calculation
/ft74.61$
4500
12135300125050000
TC
F
TtCCC rB
T
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Class Problem
PDC bit and motor
Assume the motor cost is not
included in the trip time
Cost per foot calculation
CC C t T
FT
B r ( )
/ft94.60$
4500
121250135300125050000
TC
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Class Problem
Insert bits assuming the rig rate is
48,000 per day or 2000 per hour
Cost of two bits = 10000 x 2 = 20,000
Cost per foot calculation
CC C t T
FT
B r ( )
/ft78.93$
4500
21180200020000
TC
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Class Problem
PDC bit and motor assuming the rig
rate is 48,000 per day or 2000 per hour
Assume the motor cost is included in
the trip time
Cost per foot calculation
CC C t T
FT
B r ( )
/ft69.83$
4500
12130300200050000
TC
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Factors Affecting Penetration Rate
Bit type and formation
hardness
Bit weight and rotary speed
Bottomhole cleaning or
hydraulics
Mud properties
Auxiliary practices
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Factors Affecting Penetration Rate
Bit type will affect penetration rate
Bit selection will be covered later in
the book
Formation hardness
Can we do anything about
formation hardness?
NO!
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Factors Affecting Penetration Rate
Bit weight will affect penetration rate
Generally, if the bit weight is
increased, penetration rate will
increase if hydraulics are adequate
Equation 2-5 gives the relationship
of bit weight versus drilling rate
D KWR
d
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Factors Affecting Penetration Rate The exponent “d” is
usually close to 1.0 at the higher bit weights, which means that drilling rate is proportional to bit weight
Straight line on graph
It may be exponential at the lower bit weights
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Factors Affecting Penetration Rate
The slope, K is not
a constant
Function of
Hole size
Drilling fluid
Formation
Bit type
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Factors Affecting Penetration Rate
Double bit weight
in mud from 2000
to 4000 lbs/in
3.5-25’/hr 714%
Double bit weight
in air from 2000 to
4000 lbs/in
28-62’/hr 221%
3.5
25.0
28
62
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Factors Affecting Penetration Rate
Laboratory data
showing how
fluid type will
affect
penetration rate
Clear Brine
in Berea
Sandstone
Clear Brine
in Mancos
Shale
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Factors Affecting Penetration Rate
PDC bit
performance
Exponent is
less than one
in the lab (one
in the field)
Drag bits drill
differently than
roller cone bits
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Factors Affecting Penetration Rate
Performance
of PDC bit at
various rpm
Slope
increases as
the rpm
increases
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Factors Affecting Penetration Rate
Bit life is a
function of the bit
weight
With modern bits,
bit life is difficult to
predict and does
not follow the
equation
LW Nb
1
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Factors Affecting Penetration Rate
Bit life
Bit life is a function of the seal
Seal life is a function of the conditions in the wellbore
Journal or friction bearings are even harder to predict
Once the seal fails in a friction bearing, the bearing does not last long
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Factors Affecting Penetration Rate
The bit weight which yields the
lowest cost per foot is the optimum
bit weight
Bit weight is the most significant
factor affecting penetration rate
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Factors Affecting Penetration Rate
Rotary speed
Generally, as rpm is increased,
penetration rate will increase
It is a function of the exponent “a”
which varies from 0.5 to 1.0 in field
data
D NR
a
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Factors Affecting Penetration Rate
In soft
formations the
exponent “a” is
closer to one
In harder
formations, the
exponent “a”
decreases to as
little as 0.5
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Factors Affecting Penetration Rate
Field data in the
Atoka shale
show the
exponent “a” is
less than one
Also depends
upon the type of
fluid used
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Factors Affecting Penetration Rate
In harder formations, it is usually
better to increase bit weight rather
than rotary speed
In soft formations, rotary speed can
be an alternative to bit weight
PDC bits do well with increased
rotary speed
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Factors Affecting Penetration Rate
Example 2-3
Determine the cost per foot for bit
#4 in Example 2-1 if the rpm is
reduced to 50 rpm from 100 rpm
and the bit life is doubled
Assume the exponent “a” is 0.5
fph 2.21fph 30100
505.0
50
RD
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Factors Affecting Penetration Rate
Example 2-3
The footage drilled at 100 rpm was
3000 feet
The footage drilled at 50 rpm
would be:
feet 4243hours 200 x fph 2.21 F
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Factors Affecting Penetration Rate
Example 2-3
The cost per foot would be:
As compared to $81.33 per foot
Even though the bit lasts longer,
the cost per foot increases
ftCT /65.104$4243
100001000
220020004000
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Factors Affecting Penetration Rate
Generally, it is better to drill faster
as long as the rig cost does not
increase too much
The more expensive the daily
operating cost, Cr, the more money
you can spend to make it drill faster
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Factors Affecting Penetration Rate
Bottom hole cleaning or hydraulics
Bottom hole cleaning is defined as
cleaning the cuttings from the
bottom of the hole (below the bit)
It is not concerned with hole
cleaning in the annulus
Penetration rate is a function of bit
weight and rotary speed if bottom
hole cleaning is adequate
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Factors Affecting Penetration Rate
Inadequate
bottom hole
cleaning is
termed
“hydraulic
flounder” or
“hydraulic
founder”
60
160
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Factors Affecting Penetration Rate
“Hydraulic
flounder” can
also occur with
rotary speed
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Factors Affecting Penetration Rate
Bottom hole cleaning is a function of
the hydraulics and is often
measured in hydraulic horsepower
per square inch or HHP/in2
Softer formation require more
hydraulics than harder formations
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Factors Affecting Penetration Rate
Drill off test
can be
used to
determine if
hydraulics
are
adequate Bit Weight
Pen
etra
tion R
ate
Flounder Point
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Factors Affecting Penetration Rate
In softer
formations,
hydraulics may
even help the bit
drill faster
Note the
increase in ROP
with increased
HHP
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Factors Affecting Penetration Rate
Extended nozzle
bits can be used
to increase
penetration rate
by using
hydraulics to
erode the
formation in
softer formations
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Factors Affecting Penetration Rate
Extended nozzle
bits can be used
to increase
penetration rate
by using
hydraulics to
erode the
formation in softer
formations
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Factors Affecting Penetration Rate
As long as the
formation can be
eroded, extended
nozzle bits may
drill faster
By 6000 feet
(1829 m), the
higher pressures
no longer drilled
significantly faster
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Factors Affecting Penetration Rate
Conventional bits at
6000 psi (422
kg/cm2) did not drill
any faster than 2000
psi (141 kg/cm2) in
sandstone
Below 8000 feet
(2438 m), there is no
improvement with
extended nozzle bits
in this well
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Factors Affecting Penetration Rate
Conventional
bits at 6000
psi (422 kg/cm2)
did drill faster
than 2000 psi
(141 kg/cm2)
while drilling
shale in this
well
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Factors Affecting Penetration Rate
Extended
nozzles
increased
penetration rate
down to 10,000
feet (3048 m) in
this well
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Factors Affecting Penetration Rate
Extended nozzles can break off
leaving junk in the hole
The nozzles do not clean the
cutting structure of the bit as well
as conventional nozzles
Bit balling can be a problem
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Factors Affecting Penetration Rate
Hydraulics and PDC bits
Hydraulics may be more important
with PDC bits than with roller cone
bits to clean the cutting structure
Originally all PDC bits had three
jets similar to roller cone bits
PDC bits ball up easier and require
jets to clean all the cutters
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Factors Affecting Penetration Rate Hydraulic
threshold for PDC
bits while drilling
with water based
mud
Minimum
hydraulics are
required to
prevent bit balling
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Mud Properties
Mud weight
Mud type
Solids content
Other properties
Viscosity
Filtration Rate
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MUD PROPERTIES
Mud weight
It is the overbalance between
hydrostatic imposed by the mud
weight and formation pressure that
affects penetration rate
As mud weight increases,
penetration rate will decrease if the
formation pressure remains
constant
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MUD PROPERTIES
As the amount
of over balance
increases, the
drilling rate
decreases
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MUD PROPERTIES
Penetration rate
is less affected
by the over
balance in
harder
formations
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MUD PROPERTIES
Effect of filter cake differential on
penetration rate
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MUD PROPERTIES
The mud type makes a difference in
penetration rate
Air drills fastest
Water drills faster than mud
Water is a good drilling fluid but not
a very good hole cleaning fluid
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MUD PROPERTIES
As the solids
content
increases, the
penetration rate
decreases
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MUD PROPERTIES
Solids
Once solids are introduced into the
system, the activity, size and
numbers of solids will affect the
drilling rate
Solids also contribute to mud
weight so penetration rate will
decrease due to higher hydrostatic
pressure
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MUD PROPERTIES
The more
reactive the
solids, the
more affect
they have on
penetration
rate
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63 © 2005 PetroSkills LLC, All Rights Reserved
MUD PROPERTIES
Solids
Dispersed clays will generally drill
slower than non-dispersed clays,
which drill slower than flocculated
clays
Dispersed clays act like a higher
concentration of solids and smaller
particle size
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64 © 2005 PetroSkills LLC, All Rights Reserved
MUD PROPERTIES
Viscosity
The viscosity of the drilling fluid in
the mud tanks has little or no affect
on penetration rate
Drilling fluids are shear thinning
As the velocity of the fluid
increases, the viscosity decreases
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65 © 2005 PetroSkills LLC, All Rights Reserved
MUD PROPERTIES
At the bit, the
shear rates are
very high so the
viscosity is
relatively low
Some drilling
fluids shear
thinner than
others
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66 © 2005 PetroSkills LLC, All Rights Reserved
MUD PROPERTIES
Viscosity
Viscosity is usually increased by
the addition of solids in the mud
Higher solids content and viscosity
will decrease penetration rate
It is difficult to isolate any one mud
property
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MUD PROPERTIES As the filtration
rate decreases, the penetration rate decreases
It is harder to equalize the pressure in the fractured formation when filtration rate is reduced
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68 © 2005 PetroSkills LLC, All Rights Reserved
MUD PROPERTIES
In general, the more mud you have
in the hole, the slower the well will
drill
After air, water is the best drilling
fluid
Unfortunately, all wells cannot be
drilled with air or water
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Auxiliary Practices Short trips and wiper trips
Reaming connections
Trip time
Rig selection
Bottomhole assembly
Stabilizers
Shock subs
Jars
Motors
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70 © 2005 PetroSkills LLC, All Rights Reserved
Auxiliary Practices Casing strings must be
economically justified,
running the casing must
save money
The bigger the casing,
the more expensive
Must look at total cost
including future
operating costs
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Summary
Efficiency in drilling must be a
team effort
Involve top management to the
roughneck
Good new ideas must be given an
opportunity to be successful
Use experience and sound
engineering
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Summary
Drilling faster minimizes cost
Stay out of trouble
Keep an open mind and don’t be
afraid to try something new
Look at every aspect of the drilling
operation
Honestly evaluate changes in the
drilling program
Keep it as simple as possible