STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Air and Gas Drilling 853
drilling, the entry of formation fluids to the well is considered a kick. The well
is not considered to be a blowout until the formation fluids fill the annulus
and are flowing uncontrolled from the well. In air (or gas) drilling, once
formation fluids enter the annulus, the well is in a blowout condition. This is
due to the fact that the formation fluids that enter the well can flow to the
surface immediately.
Unstable Foam (Mist). The addition of water and a foaming agent into the
circulating air (or gas) fluid will slightly increase the bottomhole pressure during
drilling operations. However, this slight increase in bottomhole pressure does
not alter the situation with regards to the potential well control capability of
this circulating fluid. Again, as above, once formation fluids enter the well, the
well is in a blowout condition.
Stable Foam. When a well is drilled with stable foam as the drilling fluid,
there is a back pressure valve at the blooey line. The back pressure valve allows
for a continuous column of foam in the annulus while drilling operations are
under way. Thus, while drilling, this foam column can have significant bottomhole
pressure. This bottomhole pressure can be sufficient to counter formation
pore pressure and thus control potential production fluid flow into the
well annulus.
Aerated Mud. In aerated mud drilling operations, the drilling mud is injected
with compressed air to lighten the mud. Therefore, at the bottom of the well
in the annulus, the bottomhole pressure for an aerated mud will be less than
that of the mud without aeration. However, an aerated mud drilling operation
will have very significant bottomhole pressure capabilities and can easily be used
to control potential production fluid flow into the well annulus.
Blowout Prevention Equipment
Air and gas, unstable foam and stable foam techniques are used almost
exclusively for onshore drilling operations, rarely in offshore applications.
Aerated mud, however, is used for both onshore and offshore drilling operations.
The minimum requirements for well blowout prevention equipment for drilling
with air and gas, unstable foam and stable foam techniques are shown in Figures
4-187 and 4-188. The BOP stack arrangement in Figure 4-187 is for a rather
standard rotary rig that will accommodate at least two sets of pipe rams in
addition to the rotating head and the blind rams. Figure 4-188 shows a BOP
stack arrangement used for smaller rotary rigs that do not have sufficient cellar
height to accommodate the second set of pipe rams.
The minimum requirements for well blowout prevention equipment for
aerated mud drilling operations are basically the same as those for normal mud
drilling operations.
Air, Gas and Unstable Foam Calculations
Pertinent engineering calculations can be made for air and gas drilling
operations and for unstable foam drilling operations.
Volumetric Flowrate Requirements
There is a minimum air (or gas) volume rate of flow that must be maintained
in order to adequately clean the bottom of the hole of cuttings and carry these