STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

860 Drilling and Well Completions
Injected Water Requirements and Formation Water
If water-bearing formations are expected while drilling with air, then it is
necessary to make sure the air entering the bottom of the annulus section of
the borehole is saturated with moisture. If the circulating air is saturated then
the air will not lose internal energy absorbing the formation water. The loss of
internal energy would affect its potential to expand and thus reduce the kinetic
energy of air flow in the annulus. The loss of kinetic energy will reduce the
lifting capability of the circulating air.
Once the circulating air is saturated and it enters the borehole annulus, the
air will carry the formation water as droplets. Thus the formation water will
be carried to the surface in much the same manner as the rock cuttings.
To saturate the circulating air with water so that the air cannot absorb
formation water, the water must be injected into the compressed air at the
surface prior to the standpipe (see Figure 4-185). If only water is injected into
the circulating air, then the drilling operation is called mist drilling. Usually,
however, a foaming agent (or surfactant) is injected with the water. This allows
a foam to be created in the annulus, which aids in transportation of the cuttings
to the surface. These foaming agents are pumped together with the water
injected on the basis of about 0.2% of the injected and projected formation
water. When water and a foaming agent are injected the drilling operation is
called unstable foam drilling
The volumetric flowrate of water to be injected into the compressed air
depends upon the saturation pressure of the water vapor at the bottomhole
temperature. The saturation pressure p,,, (psia) at the bottom of the hole
depends only on the bottomhole temperature and is given by [76,77].
1750.286
log,, psat = 6.39416-
217.23 + 0.555tb
(4- 135)
where t, = bottomhole temperature (OF)
Knowing the saturation pressure of the water vapor, the amount of injected
water can be determined. The amount of injected water, qi (gal/hr), to provide
saturated air at bottomhole conditions is
qi = 269.17 -
[ 1. P.P.P.
where pb = bottomhole pressure in psia
G = weight rate of flow of (dry) air in lb/s
Example
(4- 136)
Using the data and results from the Examples on pp. 856 to 859, determine
the approximate amount of surface injected water and foaming agent needed
to saturate the air at bottomhole conditions and to provide an unstable foam
in the annulus of the borehole.
The saturation pressure can be found from substitution of the bottomhole
temperature of 137.6'F into Equation 4-135. This yields