Page 598 - Air and Gas Drilling Manual

11-36 Air and Gas Drilling Manual
Equation 6-62 is used to determine the pressure inside the drill bit just above the
nozzles. The flow through nozzles is sonic. Therefore, Equation 6-62 must be used
to determine the pressure upstream of bottomhole annulus pressure. Thus, Equation
6-62 becomes
41
(.635 ) (604 . ) . 05
2
P iab . 05
. 24
1
2
0
(32 . ) ( . ) ( . ) 2 . 04
4
1
(.00160 )
0
53 .36 . 24
2
,
P 76 220 lb/ft abs
iab
P iab
p iab
144
.
p 529 3 psia
iab
Positive Displacement Motor Performance
Positive displacement pumps and motors are characterized by their displacement
volume. From the basic data in Figure 11-12, the motor displacement can be
obtained. The displacement, s, is [2]
(231 .016 ) (400 )
s
(200 )
s 462 032 in 3
.
Using Figure 11-12, a pressure drop across the motor of 220 psi will give 25
horsepower (at the drill bit speed of 200 rpm). The pressure at the bottom of the
motor cavity, p bm , is approximately equal to p iab . Therefore,
p bm 529 3 psia
.
P bm p bm 144
2
,
P 76 219 lb/ft abs
bm
The pressure above the motor, p am , is
p am p bm 220

.
p am 749 3 psia