A formation at a depth of 6,400 ft is to be drilled using 6,000 ft, 5-in. drill
ID: 1713953 • Letter: A
Question
A formation at a depth of 6,400 ft is to be drilled using 6,000 ft, 5-in. drill pipe (wt. = 19.5 lb/ft) and 400 ft 8-in. drill collar (wt. = 220 lb/ft) with a 13.3 ppg mud. Bit records for 3 different bits are shown below. Find the suitable bit if the rig rental cost is $36,000/day, hook HP = 280 HP and it takes 25 sec. to screw or unscrew one stand.
Bit
Bit Cost, $/hr
Interval, ft
ROP, ft/hr
Motor Cost, $
Bit Salvage Value, $
A
800
830
14
1200
0.0
B
400
780
25
1000
0.0
C
450
1050
22
1500
800
Bit
Bit Cost, $/hr
Interval, ft
ROP, ft/hr
Motor Cost, $
Bit Salvage Value, $
A
800
830
14
1200
0.0
B
400
780
25
1000
0.0
C
450
1050
22
1500
800
Explanation / Answer
by looking at data
Bit A will never be economical because its ROP is very low compared to bit B and Bit C
Time taken Bit B= 6400/25 = 256 hr =10.66 days
time taken by Bit C=6400/22 =290.90 hr=12.12 days
bit C cost in this duration = $450*290
bit B cost = 400*256
diffrence = 450*290 - 400*256 =$28100
which is much more the moter cost difference and salvage value of bit C combined
hence Bit B is economical option
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