STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Air and Gas Drilling 849
above the surfactant-water mixture), the surfactant mixture cannot be
properly swept out of the hole.
9. With the bit directly on bottom, start the air down the hole. Straight air
should be pumped at normal drilling volumes until the surfactant sweep
comes to the surface, appearing at the end of the blooey line and foaming
like shave cream.
10. Continuously blow the hole with air for about 30 min to 1 hr.
11. Begin drilling. After 5 or 10 ft have been drilled, the hole should dust
(although it is sometimes necessary to drill 60 to 90 ft before dust appears
at the surface). If the hole does not dust after these steps have been
carried out, pump another surfactant slug around. If dusting cannot be
achieved, mist drilling may be required to complete the operation.
Depending on the hole depth, the entire procedure requires 2 to 6 hr.
Holes of over 11,000 ft have been successfully unloaded using the aeration
method. A well can be dusted, mist-drilled, dried up and returned to dust
drilling. To dry a hole properly, it is important that it be kept clean.
Drying agents have been tried but without much success. The best drying
agent available at the present time is the formation itself.
12. It should be noted that when drilling with natural gas as the drilling fluid
from a pipeline source with limited pressure, nitrogen is often used to
unload the hole.
It is quite desirable to place water into an open-hole section prior to running
a casing or liner string and cementing. The water will provide a hydraulic head
to hold back any formation gas in the open-hole section that could cause a fire
hazard at the rig floor.
However, if the operator feels that the open-hole section would slough badly
if water were placed in the hole, then the casing or liner string may have to be
run into the dry open hole. This means that great care must be taken in running
a casing or liner string into the open-hole section if the subsurface formations
are making gas.
There are procedures that can be followed to allow the safe placement of
casing or liner string in a dry open-hole section that is making gas. Figures 4-188
and 4-189 show the typical blowout prevention (BOP) stack arrangements used
for air (or gas) drilled boreholes [74]. Figure 4-188 shows the BOP stack
arrangement for rotary rigs that have rather high cellars. Such a rig would be
appropriate for drilling beyond 8,000 ft of depth. Figure 4-189 shows a more
typical BOP stack arrangement for rotary rigs used in air and gas drilling
operations. These rigs typically drill boreholes of 8,000 ft in depth or less. These
low cellar rigs cannot fit the larger BOP stack (i.e., the one shown in Figure 4-188)
into the cellar. Small BOP stacks shown in Figure 4-189 give rise to safety
problems when lowering casing or liners into the borehole during well completion
operations. The safety problems arise when a well, which is making gas,
is allowed to be opened to the surface when running a casing or liner string.
If proper procedures are not used when the stripper rubber is changed to
accommodate the outside diameter changes in drill pipe or casing or liner,
formation gas can escape to the rig floor where it can be ignited.
An example of the typical safety problem that can occur when completing
these wells is when a 6+ in. open hole has been drilled below the last casing
(or liner) shoe. The open section of the borehole is usually to be cased with a
4+ in. liner. The liner is to be lowered into the open hole on a liner hanger
that is made up to 34 in, drill pipe. It is assumed that prior to the liner
operation, drilling had been under way; thus the stripper rubber in the rotating