02 Cost Control

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


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.


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 ( )


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


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


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


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


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


COST PER FOOT

Calculate the cost per foot

F

TtCCC rB

T

ft 3000

100001000

hr 21002000$4000$

TC

/ft33.81$TC


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)


COST PER FOOT


F

TtCCC rB

T

1300

113001000

26320004000

TC

/ft77.134$TC


COST PER FOOT


F

TtCCC rB

T

1300+3000

11300100001000

210063200040004000

TCC

/ft49.97$TCC


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


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


Class Problem

PDC bit and motor

Assume the motor cost is included in

the trip time


/ft74.61$

4500

12135300125050000

TC

F

TtCCC rB

T


Class Problem

PDC bit and motor

Assume the motor cost is not

included in the trip time


CC C t T

FT

B r ( )

/ft94.60$

4500

121250135300125050000

TC


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


CC C t T

FT

B r ( )

/ft78.93$

4500

21180200020000

TC


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


CC C t T

FT

B r ( )

/ft69.83$

4500

12130300200050000

TC


Factors Affecting Penetration Rate

Bit type and formation

hardness

Bit weight and rotary speed

Bottomhole cleaning or

hydraulics

Mud properties

Auxiliary practices



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!



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


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



The slope, K is not

a constant

Function of

Hole size

Drilling fluid

Formation

Bit type



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



Laboratory data

showing how

fluid type will

affect

penetration rate

Clear Brine

in Berea

Sandstone

Clear Brine

in Mancos

Shale



PDC bit

performance

Exponent is

less than one

in the lab (one

in the field)

Drag bits drill

differently than

roller cone bits



Performance

of PDC bit at

various rpm

Slope

increases as

the rpm

increases



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



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



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



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



In soft

formations the

exponent “a” is

closer to one

In harder

formations, the

exponent “a”

decreases to as

little as 0.5



Field data in the

Atoka shale

show the

exponent “a” is

less than one

Also depends

upon the type of

fluid used



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



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



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



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



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



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



Inadequate

bottom hole

cleaning is

termed

“hydraulic

flounder” or

“hydraulic

founder”

60

160



“Hydraulic

flounder” can

also occur with

rotary speed



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



Drill off test

can be

used to

determine if

hydraulics

are

adequate Bit Weight

Pen

etra

tion R

ate

Flounder Point



In softer

formations,

hydraulics may

even help the bit

drill faster

Note the

increase in ROP

with increased

HHP



Extended nozzle

bits can be used

to increase

penetration rate

by using

hydraulics to

erode the

formation in

softer formations



Extended nozzle

bits can be used

to increase

penetration rate

by using

hydraulics to

erode the

formation in softer

formations



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



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



Conventional

bits at 6000

psi (422 kg/cm2)

did drill faster

than 2000 psi

(141 kg/cm2)

while drilling

shale in this

well



Extended

nozzles

increased

penetration rate

down to 10,000

feet (3048 m) in

this well



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



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


Factors Affecting Penetration Rate Hydraulic

threshold for PDC

bits while drilling

with water based

mud

Minimum

hydraulics are

required to

prevent bit balling


Mud Properties

Mud weight

Mud type

Solids content

Other properties

Viscosity

Filtration Rate


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


MUD PROPERTIES

As the amount

of over balance

increases, the

drilling rate

decreases


MUD PROPERTIES

Penetration rate

is less affected

by the over

balance in

harder

formations


MUD PROPERTIES

Effect of filter cake differential on

penetration rate


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


MUD PROPERTIES

As the solids

content

increases, the

penetration rate

decreases


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


MUD PROPERTIES

The more

reactive the

solids, the

more affect

they have on

penetration

rate


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


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


MUD PROPERTIES

At the bit, the

shear rates are

very high so the

viscosity is

relatively low

Some drilling

fluids shear

thinner than

others


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


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


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


Auxiliary Practices Short trips and wiper trips

Reaming connections

Trip time

Rig selection

Bottomhole assembly

Stabilizers

Shock subs

Jars

Motors


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


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


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

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02 Cost Control

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