Page 477 - Air and Gas Drilling Manual

10-2 Air and Gas Drilling Manual
operations can be adequately described and examined. In Chapter 6 the basic
equations have been derived and their auxiliary friction factor, and nozzle flow
equations presented. These equations form the foundation for both methodologies as
they are discussed in this treatise. Foam quality is given by Equation 6-87. This is
Q g
Q g Q f
3
where Q g is the volumetric flow rate of the compressible gas (ft /sec),
3
Q f is the volumetric flow rate of the incompressible fluid (ft /sec).
The foam quality value is a function of the pressure in the annulus since the
volumetric flow rate of the compressed gas in affected by the pressure magnitude.
Laboratory and field experiments have been conducted that show that stable foam
will exist within certain limits of the foam quality value. These are approximately
the foam qualities of 0.60 and 0.98 [1 to 3]. If the foam quality value falls below
approximately 0.60, the foam will separate into its two phases. If the foam quality
value is above 0.98, the foam also separates into two phases (and is denoted as a
“mist”). For a stable foam drilling operation the lower foam quality value is usually
found at the bottom of the annulus and the higher foam quality value at the top of
the annulus.
Stable foam drilling fluids are used primarily to counter formation water or loss
of circulation problems (above the target depth) in boreholes that have formation
damage sensitive production formations. The effectiveness of this type of drilling
fluid in countering these downhole problems resides somewhere between the
effectiveness of air drilling and traditional aerated drilling. The actual selection of
stable foam drilling fluids over either air drilling or aerated drilling fluids is not a
distinct analytic process. Such a selection is usually made after investigating the
drilling problem experiences in number of offset wells in a given drilling province.
Further, stable foam drilling fluids can be preferable to other low bottomhole
pressure producing circulation systems in areas where surface water is difficult to
acquire. Stable foam circulation systems were first used in production workover
operations [1 to 3]. The first engineering discussion of stable foam drilling
operations for oil and gas recovery applications was given in 1971 [4].
Although stable foam drilling fluids are a special class of aerated drilling fluid,
the physical properties of drilling foam is unlike nearly all other drilling fluid
classes. The actual injection weight rates of flow of both the incompressible fluids
and compressible gases are low relative to flow rates in air and gas drilling and
aerated drilling operations. The stable foam is generated as the mixture flows
through drill bit nozzles to the annulus. The stable foam creates an intricate bubble
structure that flows through the annulus space some what like saving foam from a
can. Although the bubble structure has a low specific weight, the structure has been
found to have plastic viscosities and a yield point similar to the plastic property of
drilling muds [4, 5].
Stable foam drilling fluids were initially used to drill through rock formations
that had fracture and/or pore systems that would drain traditional incompressible
drilling fluids (e.g., fresh water, water and oil based drilling muds, formation water,