STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

842 Drilling and Well Completions
Unstable Foam (Mist)
In order to increase the formation water-carrying capacity of the air and
natural gas drilling fluids, water is often injected at the surface just after the
air has been compressed and prior to the standpipe (water injector is shown in
Figure 4-186). An amount of water is injected that will saturate the compressed air
when it reaches the bottom of the hole. Thus, if the water-saturated returning
airflow encounters formation water, internal energy in the airflow will not be
required to change the formation water to vapor. The formation water will be carried
to the surface as water particles much like the rock cuttings. If only water is injected
at the surface, then the drilling fluid is called “mist.” Usually a surfactant is injected
with the injected water. This surfactant will cause the air and water to foam. This
foam, however, is not continuous (Le., it will have large voids in the annulus section
because of the high velocity of the returning airflow). This is the reason why this
type of drilling operation is also denoted as unstable foam.
Stable Foam
Stable foam drilling operations are used when even more formation watercarrying
capability is needed (relative to air and gas and unstable foam). Also,
stable foam provides significant bottomhole pressure that can counter formation
pore pressures and thus provide some well control capabilities. Stable foam
drilling operations provide a continuous column of foam in the annulus from
the bottom of the borehole annulus to the back pressure valve at the end of
the blooey line. Air and natural gas and unstable foam require large compressors
to produce a fixed volumetric flowrate of air. Stable foam drilling requires far
less compressed air, and the compressed air is provided by a flexible system.
The air compressors used in stable foam drilling should be capable of supplying
air at various pressures and volumetric flowrates. In general, the back pressure
valve at the end of the blooey line is adjusted to ensure that a continuous foam
column exists in the annulus. However, if the back pressure is too high, the foam
at the bottom of the borehole (in the annulus) will break down into the individual
phases of liquid and gas. Foam quality at the bottom hole in the annulus
should not drop below about 60% [67-691. Engineering calculations for determining
the appropriate parameters for stable foam drilling operations are quite
complicated. There are a few stable foam simulation programs available for well
planning [70]. Those interested in stable foam engineering calculations are
advised to consult service companies specializing in stable foam drilling operations.
Aerated Mud
Aerated mud drilling operations are used throughout the drilling industry,
onshore and offshore. Aerated mud drilling is usually employed as an initial
remedy to loss-of-circulation problems. To aerate water-based mud or oil-based
mud, air is injected into the drilling mud flow at the surface prior to the mud
entering the standpipe (primary aeration) or in the return annulus flow through
an air line set with the casing string (parasite tubing aeration) [71,72]. Primary
aeration is the most commonly used technique for aerating mud. But because
of the high resistance to flow of aerated liquids, as aeration is needed at depth,
parasite tubing aeration offers a usable alternative.
The relative advantages and disadvantages of the various types of air and gas
drilling operations discussed are listed as follows: