1. Lesson 4 Drilling Cost & Drilling Rate
PETE Well Drilling
Lesson 4 Drilling Cost & Drilling Rate

2. Contents The AFE Drilling Cost and Bit Change
Factors Affecting Drilling Rate
Bit Weight, Rotary Speed
Bottom-hole Cleaning
Mud Properties, Solids Content
Hydrostatics

3. Assignments HW #2. ADE 1.12, 1.13, 1.14 Due Friday, Sept. 13, 2002
Read: ADE, Ch. 1 (All) Learn: Rig Components - Definitions
HW #2. ADE 1.12, 1.13, Due Friday, Sept. 13, 2002

4. Before getting approval to drill a well the Drilling Engineer must prepare an AFE - a detailed cost estimate for the well
DRY COMPLETED
HOLE
INTANGIBLE COSTS $ $
TANGIBLE COSTS $ $
TOTAL COST $ $

5. AUTHORIZATION FOR EXPENDITURE (AFE)
EXPENDITURE DRY HOLE COMPLETED
(24.5 DAYS) (32.5 DAYS)
INTANGIBLE COSTS
LOCATION PREPARATION 30, ,000
DRILLING RIG AND TOOLS 298, ,613
DRILLING FLUIDS 113, ,976
RENTAL EQUIPMENT , ,785
CEMENTING , ,369
SUPPORT SERVICES 152, ,648
TRANSPORTATION , ,400
SUPERVISION AND ADMIN , ,791
SUB-TOTAL 814, ,126,581
TANGIBLE COSTS
TUBULAR EQUIPMENT 406, ,529
WELL HEAD EQUIPMENT 16, ,201
COMPLETION EQUIPMENT ,717
SUB-TOTAL 422, ,018,447
SUB-TOTAL ,237, ,145,028
+ CONTINGENCY (15% ??) ,423, ,466,782

6. Drilling Cost vs. Time
DEPTH ft TD
DAYS or DOLLARS

7. Drilling Cost Analysis
The Drilling Engineer:
Recommends drilling procedures that will safely drill and complete the well at the lowest cost possible
Makes recommendations concerning routine rig operations:

8. The Drilling Engineer Examples of routine rig operations
drilling fluid treatment
pump operation
bit selection
handling problems during the drilling process

9. 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
= footage drilled
with bit, ft/bit

10. Example 1.5
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.

11. Example 1.5 cont’d
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
B 4,
C 4,
Which bit would you select?

12. Solution:
The cost per foot drilled for each bit type can be computed using Eq For Bit A, the cost per foot is

13. Solution:
Similarly, for Bit B,

14. Solution, cont’d
Finally, for Bit C,

15. 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. - Highest bit cost …but - intermediate bit life and ROP...

16. Drilling Costs C = aebD …………………..(1.17)
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 …………………..(1.17)

17. Drilling Costs, cont’d
C = aebD
Constants a and b depend primarily on the well location.
Shown on the next page is a least-squares curve fit of the south Louisiana completed well data given in Table 1.7.
Depth range of 7,500 ft to 21,000 ft.
For these data,
a = 1 X 105 dollars
b = 2 X ft -1.

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

19. 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.
WHY?

20. Drilling Time
The drilling time, td , required to drill to a given depth can be obtained by separating variables and integrating. Separating variables gives
Integrating and solving for td yields

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

22. EXAMPLE - Cost per ft These cost data are plotted below.
t R D Total Cost Cf
hr fph ft $ $/ft , ,
,600 69,
,875 79,
,100 90,
, ,
, ,
These cost data are plotted below.

23. Cost per ft for one entire bit run80 70 60
Minimum Cost 50 40

24. Economic Procedure in above 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.

25. 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.

26. 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.

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

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

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

30. 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

31. 40,000 lbf
Consider 10” hole
(don’t overdo!!)
Drilling Rate, ft/hr
Bit Weight x 1,000 lb/in

32. Don’t overdo!
Casing wear,
bit life ...
Drilling Rate, ft/hr
Rotary Speed, RPM

33. Drilling Rate, ft/hr EFFECT OF BACK PRESSURE
Keep DP_bit = const.= 550 psi
Drilling Rate, ft/hr

34. EFFECT OF BACK PRESSURE
0 - 5,000 psi
Drilling Rate, ft/hr
Hydrostatic Pressure, 1,000’s of psi

35. EFFECT OF DRILLING FLUID
mud vs. gas
Drilled with mud
Depth, ft
Drilled with gas
Drilling Time, days

36. EFFECT OF DRILLING FLUID
water vs. air
Depth, ft
Rotating Time, hours

37. Relative Drilling Rate, %
Old style water course bits

38. EFFECT OF SOLIDS IN THE MUD

39. Hydrostatic Pressure Gradient
Fresh Water Pressure Gradient = psi/ft
Density of Fresh Water = lb/gal
Hydrostatic Pressure (at 12,000 ft depth):
with water: p = Gw * Depth (vertical depth)
= psi/ft * 12,000 ft
= 5,196 psi

40. with 14 lb/gal mud: p = GM * Depth
Hydrostatic Pressure
with 14 lb/gal mud: p = GM * Depth
= *14.0 *12,000
= 8,736 psig
(5,196 psi with water)

41. Hydrostatic Pressure Required
What mud weight is required to balance a pore pressure of 10,000 psig at a vertical depth of 12,000 ft?
MW = lb/gal