Page 172 - Drilling Technology in Nontechnical Language

Chapter 7 – DRILLING FLUIDS 163

Air as a circulating medium

It is possible to circulate with compressed air. For this to work, the
following conditions are necessary:
1. The formations to be drilled must remain stable without
hydrostatic mud pressure to support them.
2. There must be no danger of a fluid influx into the well (oil or

salt water).
The areas of application are hard, dry formations, such as dry
geothermal zones and dry gas production zones. While drilling through a
gas-bearing reservoir, the well produces gas while drilling.

Aerated and foamed muds

Aerated mud is simply injecting standard drilling mud with air,
effectively lightening the fluid column. The main advantages are

the following:
1. Maintaining full circulation in loss zones.
2. Increase ROP by reducing chip hold down (as explained in
chapter 6).
3. Reduce the incidence of differential sticking. This occurs when a
permeable formation is exposed in the wellbore and the following
conditions are also present:

▫ The mud hydrostatic pressure is greater than the pore
fluid pressure

▫ Mud solids have built a plaster on the formation face
▫ The drillstring is allowed to remain stationary for awhile.
If these conditions are all present, it is possible for the drillstring
to become stuck. The greater pressure in the wellbore pushes the
pipe into the wall, and friction then makes it hard to move the
pipe up or down or in rotation. If the overbalance is high enough,

the pipe cannot be moved. Reducing the drilling fluid density
reduces the sticking effect.
4. Reduce formation damage.

_Devereux_Book.indb 163 1/16/12 2:09 PM