Page 185 -

Gas Drilling Operations 153

7.3.3 Water Removal
Liquids (water and/or oil) from wet formations accumulate at the
bottomhole when the air/gas injection rate is not high enough to carry
them to the surface. The accumulation of liquids increases the bottom-
hole pressure, which compresses the gas and reduces the gas velocity,
resulting in reduced carrying capacity of the gas and, in turn, solid and
more liquid accumulation at the bottomhole. This cycle will create
drilling complications such as mud ringing and pipe sticking. Adding
foamers (surfactants) to the gas stream can ease this problem to a certain
extent. If the liquid production rate is significantly high, additional gas
injection capacity is required, or the air/gas drilling needs to be converted
to foam drilling. Switching to foam drilling will result in a much lower
rate of penetration, and waiting for compressors with greater capacities
will also reduce the overall drilling performance due to added nonrotat-
ing time. A guideline is highly desirable for drilling engineers who are
making decisions about whether to convert to foam drilling.
A traditional way to determine the effect of formation fluid influx on
hole cleaning is using the effective rate of penetration obtained from the
equivalent rate of penetration of the influx rate (GRI, 1997; Guo and
Ghalambor, 2002). Guo and colleagues (2008) conducted a comprehensive
study of liquid carrying capacity of gases. They developed a systematic
method for predicting the gas volume requirement necessary to remove
formation fluids of various influx rates.
Starting with Turner’s (Turner et al., 1969) theory of liquid loading in
gas production wells, Guo and colleagues used the minimum kinetic
energy criterion to establish the following theory:

E km = 0:0576 σρ L (7.1)
ffiffiffiffiffiffiffiffi
p
where
E km = minimum kinetic energy required to carry up liquid by flowing
3
gas, lbf-ft/ft or N-m/m 3
σ = interfacial tension between liquid and gas phases, dynes/cm
3
ρ L = density of liquid, lb/ft or kg/m 3
The typical values for water‒gas interfacial tension and water density
3
are 60 dynes/cm and 65 lbm/ft , respectively. Equation (7.1) yields the
3
minimum kinetic energy value of 3.6 lbf-ft/ft . Since this kinetic energy
3
value is greater than the kinetic energy value of 3 lbf-ft/ft required for