STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Air and Gas Drilling 857
Knowing the bottomhole pressure, the number of bit orifice openings and
the inside diameter of these openings, the pressure inside the drill pipe just
above the bit and the surface injection pressure can be found.
The total area of the orifice openings A,(ft2) is
(4-1 29)
where n = number of orifices
dn = diameter of the orifice openings in in.
The weight rate of airflow through the system G(lb/s) is
G=- Qr s
60
(4-130)
where y, = specific weight of air at the surface location in lb/ft3
The pressure above the bit Pa (lb/ft2 abs) can be found from
(4-131)
where k = ratio of specific heat of air (or gas) for air k = 1.4
g = acceleration of gravity (32.2 ft/s2)
The surface injection pressure Pi(lb/ft2 abs) can be determined if knowing
the pressure above the bit. The surface injection pressure is determined from
P: + b'Tiv (ezanpav -
(4-132)
and
S
a' = -
53.3
(4-1 33)
C,Q2
b'pF
P
(4-134)
where Dip = inside diameter of the drill pipe, ft
Example 1
Using the data given below and results from the Example on p. 856, determine
the real air volumetric flowrate and the expected surface injection pressure. The