Page 186 - Air and Gas Drilling Manual

5-28 Air and Gas Drilling Manual
Most shallow air drilling operations utilize either tri-cone drill bits, or air
hammers equipped with air hammer drill bits. Air hammers and their drill bits will
be discussed in detail in Chapter 11.
Tri-cone drill bits used in shallow drilling operations have a large single
opening in the center of the bit body. These drill bits are less expensive to fabricate
than their oil field counterparts which are fabricated with sealed bearings and three
water course orifices that can be fitted with nozzles of various inside diameters.
Since shallow well tri-cone bits have a large center opening, they can be used for
both direct and reverse circulation operations. With the large center opening in the
drill bit, the pressure change in the air flow from the bottom of the inside of the drill
string (inside the drill bit) to the bottom of the annulus is assumed to be negligible.
Therefore, for this illustrative example the inside of the bottom of the drill
string just above the drill bit is assumed to have the same pressure as that at the
bottom of the annulus. Thus,
p bhi = 37 9 psia
.
P bhi = p bhi 144
2
,
P bhi = 5 456 lb/ft abs
The inside of the drill string is approximated by the inside diameter of the drill
pipe. The inside diameter of the API 2 3/8 inch, 4.85 lb/ft nominal, EU, NC 26,
Grade E drill pipe is 1.995 inches (see Table B-4).
d = 1 995. inches
i
d
D = i
i
12
D = 0 166. ft
i
The injection pressure, P in, into the top of the inside of the drill string is
determined from
05 .
  2 aH  
i
 2 + 2 e  T av 
 P bhi b T av  −  1  
i
   
P in =  2 aH  (5-7)
 i 
 e T av 
 
 
where