The document discusses drilling cost and rate. It provides equations to calculate drilling cost based on rig operating cost, bit cost, rotating time, connection time, and trip time. The cost per foot is lowest when the bit is pulled at the optimal time, such as when the cost curve begins to increase. Bit selection is based on past performance and predicted geology. Drilling rate is affected by factors like bit type, weight, speed, fluid properties, and formation characteristics. Maintaining proper fluid pressures and using appropriate fluids also impacts drilling time.

1. OG 321 DRILLING 1
LECTURE SIX
DRILLING COST AND DRILLING RATE

2. DRILLING COST ANALYSIS
Drilling cost predictions are made so that sound economic
decision can be made
The drilling engineer recommends
• drilling procedures that will safely drilled and complete the
well at the minimum cost possible
• routine rig operations such as
Drilling fluid treatment
Pump operation
Bit selection
Any problem encountered in drilling operation

3. 3
The Drilling Cost Equation
Cf = drilling cost, $/ft
Cb= cost of bit, $/bit
Cr = fixed operating cost of rig, $/hr
tb = total rotating time, hrs
tc = total non-rotating time, hrs
tt = total trip time (round trip), hrs
D

ft
D
t
t
t
C
C
C
t
c
b
r
b
f
$
)
(





= footage drilled
with bit, ft/bit
Also known as cost per foot equation

4. 4
Example
A recommended bit program is being prepared for a
new well using bit performance records from nearby
wells.
Drilling performance records for three bits are shown
for a thick limestone formation at 9,000 ft.
Determine which bit gives the lowest drilling cost if the
operating cost of the rig is $400/hr, the trip time is 7
hours, and connection time is 1 minute per connection.

5. 5
Assume that each of the bits was operated at near
the minimum cost per foot attainable for that bit.
Mean
Bit Rotating Connection Penetration
Cost Time Time Rate
Bit ($) (hours) (hours) (ft/hr)
A 800 14.8 0.1 13.8
B 4,900 57.7 0.4 12.6
C 4,500 95.8 0.5 10.2
Example cont’d
Which bit would you select?

6. 6
Solution:
The cost per foot drilled for each bit type can
be computed using drilling cost equation for
Bit A, the cost per foot is
/ft.
81
.
46
$
)
8
.
14
(
8
.
13
)
7
1
.
0
8
.
14
(
400
800





f
C
ft
D
t
t
t
C
C
C
t
c
b
r
b
f
$
)
(






7. 7
Solution:
Similarly, for Bit B,
/ft.
56
.
42
$
)
7
.
57
(
6
.
12
)
7
4
.
0
7
.
57
(
400
900
,
4





f
C
ft
D
t
t
t
C
C
C
t
c
b
r
b
f
$
)
(






8. 8
Solution, cont’d
Finally, for Bit C,
/ft.
89
.
46
$
)
8
.
95
(
2
.
10
)
7
5
.
0
8
.
95
(
400
500
,
4





f
C
ft
D
t
t
t
C
C
C
t
c
b
r
b
f
$
)
(






9. 9
Solution, cont’d
Bit A: $46.81 /ft
Bit B: $42.56 /ft
Bit C: $46.89 /ft
The lowest drilling cost was obtained using Bit
B.
Regardless of highest bit cost …it has
intermediate bit life and ROP...

10. 10
Drilling Costs
• Tend to increase exponentially with depth.
Thus, when curve-fitting drilling cost data, it is
often convenient to assume a relationship
between total well cost, C, and depth, D, given
by
C = aebD
• Constants a and b depend primarily on the
well location.

11. 11
Drilling Costs, cont’d
Shown on the next page is a least-squares
curve fit of the south Louisiana completed well
data given in Table 1.7 (ADE book)
Depth range of 7,500 ft to 21,000 ft.
For these data,
a = 1 X 105 dollars
b = 2 X 10-4 ft -1.
C = aebD

12. 12
Fig. 1-65. Least-square curve fit of 1978 completed well costs for wells below
7,500 ft in the south Louisiana area.

13. 13
Penetration Rate
When major variations are not present in the
subsurface lithology, the penetration rate
usually decreases exponentially with depth.
Under these conditions, the penetration rate
can be related to depth, D, by
where K and a2 are constants.
D
a
Ke
dt
dD 2
303
.
2



14. 14
Drilling Time
The drilling time, td , required to drill to a given
depth can be obtained by separating variables
and integrating. Separating variables gives
dD
e
dt
K D
a
303
.
2
D
0
t
0
2
d



Integrating and solving for td yields
)
1
(
303
.
2
1 2
303
.
2
2

 D
a
d e
K
a
t

15. 15
Drilling Time cont’d
Plotting depth vs. drilling time from past drilling
operations:
• Allows more accurate prediction of time and cost for
drilling a new well
• Is used in evaluating new drilling procedures
(designed to reduce drilling time to a given depth).

16. 16
EXAMPLE - Cost per ft
t R D Total Cost Cf
hr fph ft $ $/ft
5 90 475 36,950 77.80
10 80 900 47,800 53.10
20 60 1,600 69,200 43.30
25 50 1,875 79,750 42.50
30 40 2,100 90,200 43.00
35 30 2,275 100,550 44.20
40 20 2,400 110,800 46.20
These cost data are plotted below.

17. 17
Cost per ft for one entire bit run
Minimum Cost
80
70
60
50
40

18. 18
Economic Procedure in previous Table
 Can pull bit after about 25 hr. ($42.50/ft)
- the precise pulling time is not critical
Note that the cost in dollars per foot
was $43.00 after 30 hr.
 Primarily applicable to tooth-type bits
where wear rate is predictable.

19. 19
Economic Procedure in above Table
 Also used with tungsten carbide insert
bits when inserts are broken or
pulled out of the matrix.
 Unfortunately, wear rate with insert
bits is unpredictable.
 Economically, the insert bit should be
pulled when the cost in $/ft begins to
increase.

20. 20
Economic Procedure in Table
– Bits pulled for economic reasons make it
hard to obtain wear information.
– Operator might pull bit after 120 hr of use
but part of bit might get left in hole.
Recovery is very difficult. Avoid!
– 75% of rock bits are pulled green or
before the bit is worn out.

21. 21
An increase in
TORQUE may
indicate that a bit
should be pulled.
Experience often
dictates when to
pull bit (footage or
hours).

22. 22
Factors that affect Penetration Rate
 Type of Drill bit
 Bit weight
 Rotary speed
 Bottom-hole cleaning
 Mud properties
 Rock hardness
 Formation pore pressure
Variables:
Fixed Factors:

23. 23
Bit Selection is based on
 Past bit records
 Geologic predictions of lithology
 Drilling costs in $/bit...
 Drilling cost in $/ft

24. 24
Bit Weight and Rotary Speed
• Increasing bit weight and rotary speed
boosts drilling rate
• These increases accelerate bit wear
• Field tests show that drilling rate
increases more or less in direct
proportion to bit weight

25. 25
Consider 10” hole
(don’t overdo!!)
Bit Weight x 1,000 lb/in
40,000 lbf

26. 26
Rotary Speed, RPM
Don’t overdo!
Casing wear,
bit life ...

27. 27
EFFECT OF BACK PRESSURE
Keep P_bit = const.= 550 psi

28. 28
Hydrostatic Pressure, 1,000’s of psi
EFFECT OF BACK PRESSURE
0 - 5,000 psi

29. 29
Drilling Time, days
Drilled with gas
Drilled with mud
EFFECT OF DRILLING FLUID
mud vs. gas

30. 30
Rotating Time, hours
EFFECT OF DRILLING FLUID
water vs. air

31. 31
Old style water course bits

32. 32
EFFECT OF SOLIDS IN THE MUD