Page 187 - Air and gas Drilling Field Guide 3rd Edition

178 CHAPTER 7 Reverse Circulation Models

7.4 STABLE FOAM DRILLING MODEL
As stated earlier, reverse circulation operations are useful in drilling large diame-
ter shallow surface casing boreholes. Stable foam drilling operations are not use-
ful for this type of drilling operation. Stable foam drilling is used in reverse
circulation operations in shallow moderate diameter wells only (e.g., depths less
that approximately 3000 ft or 900 m), such as deep water wells and other nonpe-
troleum industry-related wells. The reason for this limitation is because the
reverse circulation drill bit with its large open orifice does not create the high
shear rate on the drilling fluid needed to generate the stable foam as the foam
enters the bottom of the drill string with the rock cuttings. Therefore, the stable
foam must be generated at the surface and injected into the top of the annulus;
this foam must not break down as it circulates through the entire system. This
is not possible in deep large diameter boreholes.
However, it should be noted that stable foam reverse circulation operations
are used in petroleum industry deep well work over and related production
operations. These operations are feasible because these operations are carried
out in moderate diameter wells (e.g., in production casing and production tubing
strings).

7.5 AIR AND GAS DRILLING MODEL
Unlike the aerated and stable foam drilling fluid models, the air and gas drilling
model requires no special empirical correlations to adjust the results to provide
additional results that agree more closely to field data. Chapter 8 will give illustra-
tive examples for this model.
Air (or gas) drilling is a special case of the theory derived in Section 7.2. The
governing equations for air (or gas) drilling operations can be obtained by setting
Q m ¼ 0 in the equations derived in Section 7.2. The aforementioned assumption
restricts the flow in the annulus to two-phase flow (gas and rock cuttings).
Setting Q m ¼ 0 in Equation (7-25) yields
2 3
6 _ w t 7
6 7
dP ¼
P g T av
4 5
Q g
P T g
8 9
32
2
P g
> T av >
> >
> Q g >
f 6 P T g 7
< =
1 þ 6 7 dh; (7-43)
2gD i 4 p 2 5
> D >
> i >
4
> >
: ;
where
_ w t ¼ _ w g þ _ w s :