Page 344 - Air and Gas Drilling Manual

.
Equation 8-3 becomes
   A b = 0 358 ft 2  Chapter 8: Air, Gas, and Unstable Foam Drilling 8-27  05 .

2

53 36) (
(.
  2 635) (
504 41)
.
.
P =  0 014 )  200  + 25 + 2 0 2   2  + 1 827) 2 
(  .
(,
(.)
b
0 358) ( .
 0 668
    .     ( 32 2 . ) ( . 1 0)   

2
P = 2 163 lb/ft abs
,
b
P b
p =
b
144
.
p b = 15 02 psia
Geometry in the Annulus
Major and minor friction losses must be included in order to obtain accurate
bottomhole and injection pressures. Therefore, it is necessary to include the
geometric dimensions of the drill pipe tool joints. When drilling at 10,000 ft, the
drill string is composed of 9,500 ft of API 4 1/2 inch, 16.60 lb/ft nominal, EU-
S135, NC50 (IF) from the surface to the top of the drill collars. The 4 1/2 inch
outside diameter body of the drill pipe has an inside diameter of 3.826 inches (see
Table B-3). Approximately every 30 ft there are tool joints which are about 1 1/2 ft
in length. The outside diameter of these tool joints is 6 5/8 inches with an inside
diameter of 3 3/4 inches. For the calculations that follow, the drill pipe tool joint
lengths will be “lumped” together as a continuous length to approximate their
contribution to the overall major (wall friction) loss in the flow system. Thus, the
drill pipe tool joints of the drill pipe in the 7,000 ft cased section of the borehole are
calculated as a “lump” at the bottom of this cased section. The drill pipe tool joints
of the drill pipe in the 2,500 ft openhole section of the borehole are calculated as a
“lump” at the bottom of this openhole section.
These lumped approximations for the drill pipe tool joints are somewhat rough
approximations, but will give quite accurate bottomhole and injection pressures.
Using this lumped approximation, the pressure terms along the annulus around the
drill pipe and inside the drill pipe are in error by a few percent. However, this short-
coming can obviously be relieved by calculating a short 1 1/2 ft long tool joint
every 30 ft along the entire drill pipe length of the drill string. This can easily be
accomplished with a sophisticated computer program. But these lumped
approximations are very useful in demonstrating the calculation technique steps.
These lumped approximations are very easy to incorporate in an engineering
calculation program and it has been found that these approximations are quite
adequate for most engineering practice applications. The lumped length
approximations will be denoted by the numbers 1 through 5. These numbered
lengths will start at the top of the borehole (1) and continue to the bottom (5).