|
The amount of fluid velocity
and fluid volume to be applied are determined by the
major bit hydraulics functions: namely, the cooling
of the bit and the transportation of the cuttings.
Use the following formula to calculate
the maximum and minimum circulation pump rate (Q):
Depending on the measurement
system in use, the formula variables have been defined
in the following table:
|
Variable
|
Description of Variable
|
Units and Constants
|
|
Q
|
Pump Rate
|
US Gallons/min
|
Imperial Gals./min
|
Litres/min
|
|
Va
|
Annular Velocity
|
Feet/min
|
Feet/min
|
Metres/min
|
|
h
|
Unit Height of Annulus
|
12 Inches/foot
|
12 Inches/foot
|
1000 mm/metre
|
|
d
|
Drill Hole Diameter
|
Inches
|
Inches
|
mm
|
|
s
|
Drill String Diameter
|
Inches
|
Inches
|
mm
|
|
c
|
Volume Conversion
|
231 Inch3/US
gal
|
277 Inch3/Imp
gal
|
1 x 106
mm3/Litre
|
Based on the equation given above and
on the maximum and minimum recommended annular velocities,
the maximum and minimum recommended circulation pump
rates (Q) for the most
common drill bit diameter and drill string diameter
combinations are given in the following table:
|
Drill Hole Designation
|
Hole
Diameter (d)
|
Drill
String Dia. (s)
|
Recommended
Circulation Pump Rate (Q)
|
|
.
|
.
|
.
|
US Gals / min
|
Imp Gals / min
|
Litres / min
|
|
E
|
1.485 Inch
37,72 mm
|
1.375 Inch
34,93 mm
|
2.1 (Max)
0.8 (Min)
|
1.8 (Max)
0.6 (Min)
|
8 (Max)
3 (Min)
|
|
A
|
1.890 Inch
48,01 mm
|
1.750 Inch
44,45 mm
|
3.4 (Max)
1.2 (Min)
|
2.9 (Max)
1.0 (Min)
|
13 (Max)
5 (Min)
|
|
B
|
2.360 Inch
59,94 mm
|
2.188 Inch
55,58 mm
|
5.3 (Max)
1.9 (Min)
|
4.4 (Max)
1.6 (Min)
|
20 (Max)
7 (Min)
|
|
N
|
2.980 Inch
75.69 mm
|
2.750 Inch
69,85 mm
|
8.9 (Max)
3.2 (Min)
|
7.4 (Max)
2.7 (Min)
|
34 (Max)
12 (Min)
|
|
H
|
3.782 Inch
96,06 mm
|
3.500 Inch
88,90 mm
|
13.8 (Max)
5.0 (Min)
|
11.5 (Max)
4.2 (Min)
|
52 (Max)
19 (Min)
|
|
P
|
4.827 Inch
22,61 mm
|
4.500 Inch
114,30 mm
|
20.5 (Max)
7.5 (Min)
|
17.1 (Max)
6.2 (Min)
|
78 (Max)
28 (Min)
|
“Q” values
approaching the MAXIMUM limit should be used in cases
where any of the following are applicable:
- The cuttings generated
by the bit are relatively large such as those generated
by surface-set
diamond bits with nominal face
stone sizes of 10 stones per carat or larger.
- The hardness of
the formation causes considerable friction on the
bit face resulting in high heat generation.
- The formation is
relatively consolidated with little or no fluid return
loss.
“Q” values
approaching the MINIMUM limit should be used in cases
where any of the following are applicable:
- The cuttings generated
by the bit are relatively fine.
- The formation being
drilled is susceptible to hydraulic erosion.
- When drilling in
very hard, fine-grained formations, it may be desirable
to operate diamond
impregnated bits with lower circulation rates
to permit some abrasion of the matrix in order to
maintain a satisfactory rate of penetration.
- Where the bit in
use has a waterway configuration with a small number
of narrow fluid passages, the use of a lower circulation
rate will reduce the fluid pressure at the bit face
and will in turn reduce the effect of hydraulic lift
that tends to counteract the applied bit
load
|
