Page 379 - Air and Gas Drilling Manual

8-62 Air and Gas Drilling Manual
“Mist” injection is the old term which was the injection of water with basically
no additives. Modern air drilling defines injection of water with additives as
unstable foam drilling operations.
8.5.1 Saturation of Gas at Bottomhole Conditions
Water is injected into the air or other circulation gases at the surface in order to
saturate the gas with water vapor at bottomhole conditions. The reason this is done
is to assure that the circulation gas as it flows out of the drill bit orifices into the
annulus will be able to carry formation water coming into the annulus as whole
droplets.
If water is not injected into the gas at the surface and the gas is dry when it
comes out of the drill bit orifices, a portion of the formation water will be absorbed
by the gas as water vapor. Thus, the dry gas will be saturated by the formation
water. This saturation process decreases the internal energy (i.e., enthalpy) in the gas
as the gas enters the annulus. This reduction in internal energy at the bottom of the
annulus dramatically reduces the kinetic energy per unit volume of the gas as it
flows from the bottom of the annulus to the surface (this reduction in kinetic energy
per unit volume is mainly due to a reduction in velocity). The reduction in kinetic
energy per unit volume in the annulus reduces the carrying capacity of the gas
throughout the annulus. This reduction in carrying capacity of the circulation gas
occurs when the circulation system needs all of its carrying capacity to carry to the
surface the additional load (beyond the rock cuttings load) of the formation water
flowing into the annulus.
By injecting water at the surface into the gas, the gas becomes saturated with
water vapor inside the drill string as it flows down the string. Therefore, the gas is
saturated with water vapor when it reaches the bottom of the inside of the drill string
and flows into the annulus. As this saturation process occurs inside of the drill
string the compressor systems at the surface continue to maintain compression in the
gas column and, thus, maintain the internal energy of the column. Therefore, when
this gas enters the annulus it is already saturated with water vapor and cannot absorb
the formation water as water vapor. Further, the gas has the internal energy to carry
the new load of formation water as droplets. This is the most efficient method of
carrying an influx of formation water from the annulus of a well. In general, it is
not efficient to try to use dry gas to absorb formation water as a water vapor and, in
essence, “dry-out” a well.
The empirical formula for determining the saturation of various gases including
air can be found in a variety of chemistry handbooks and other literature. The
empirical formula for the saturation pressure of air, p sat, can be written as [14]

  1 750 286  
,
.
6 39416 −   
.
  217 23 + 0 555 t bh   
.
.
p sat = 10  (8-5)
where p sat is saturation pressure of the air at annulus bottomhole conditions (psia).
The approximate volumetric flow rate of injected water to an air drilling operation is
determined by the relationship between the above saturation pressure and the