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156 Part II Gas Drilling Systems

Illustrative Example 7.2
If the air injection rate of 1,200 scf/min is not available for the well in Illustrative
Example 7.1, but surfactants are available to reduce the water‒air interfacial
tension from 60 dynes/cm to 40 dynes/cm, what is the minimum air injection
rate required by water removal?
Solution
This problem can be solved using the chart in Figure B.12. The answer is
1,000 scf/min.

removal. This approach can be used to generate engineering charts for var-
ious well conditions. Some of the charts are presented in Appendix B.
The second way to remove water is by reducing the water‒gas interfacial
tension by adding surfactant solutions to the gas stream. This reduces the
gas kinetic energy threshold required for lifting water. Various types of
surfactants/foamers are available in the industry, although the cheapest
surfactant is still the detergent.

7.3.4 Bit Balling
Gas drilling has the same problem with bit balling as when bits ball with
mud. This happens when there is too much solids and not enough gas
flow rate. Reservoirs and other low-permeability formations “weep” fluid.
This leads to bit balling and mud rings. In addition to increasing the gas
injection rate, adding detergent, adding a drying agent, or switching to
mist can help solve these problems. Weeping often stops when the near
wellbore fluids are depleted. Because they are so dry, nitrogen and natural
gas are especially effective at drying a damp or weeping formation.
Another type of bit balling that is not well documented in the litera-
ture is ice balling or “frozen” bits. This was discovered when bits looked
as if they were the victims of mud balling but no mud was present. The
temperature of gas at the bit can be much lower than expected. This low
temperature is due to the Joule–Thomson cooling effect, where a sudden
gas expansion below the bit orifice causes a significant drop in tempera-
ture. The temperature can easily drop to below the ice point, resulting in
ice balling of the bit if water exists. Even though the temperature can
still be above the ice point, it can be below the dew point of water
vapor, resulting in the formation of liquid water that causes mud ring
problems in the annulus. If natural gas is used as the drilling fluid, it can
form gas hydrates with water around the bit, known as hydrate balling.