Page 504 - Air and Gas Drilling Manual

Chapter 10: Stable Foam drilling 10-29
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 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 adequate for
most engineering practice applications.
The calculation sequence for this illustrative example is divided into five depth
increments. The increments start at the top of the borehole with the first increment
at the top and the fifth increment at the bottom.
The total length of the lumped drill pipe body increment (first), H 1, in this
cased section of the annulus is
7 000
,
,
H 1 7 000 1 5 .
30
,
H 1 6 650 ft
The inside diameter of the casing along this length in this cased section of the
annulus is
.
d 1 8 017 inches
d 1
D 1
12
D 1 0 668 ft
.
and the outside diameter of the drill pipe body along this length is

d 450 inches
.
2
d 2
D 2
12