Page 270 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 246

[ ] Well Programming
2.5.12

Whole-mud invasion. If the permeability is sufficiently high
and/or differential pressure is excessive, whole mud might be forced
into the formation. This would also happen with a water base mud. All
the damage that would occur with a water-base mud such as migration
of fines, permeability blockage, etc., would occur with an oil mud.
Also, there would be the possibility of emulsion blockage and oil wet-
ting. Further, the addition of properly sized bridging agents will help
to avoid whole mud invasion, as will minimizing differential pressure
to the lowest safe level.

2.5.12. Air, Foamed, and Aerated Systems

Air can be used in several ways to improve rate of penetration and
reduce or eliminate formation damage. However, the circumstances
must be suitable for the system and extra equipment and personnel
may be needed.
With all the techniques summarized here, specialist advice is nec-
essary to evaluate the proposed solutions, costs, and equipment
requirements.
Air drilling. This is only suitable where formations to be drilled
contain no liquids. Large volumes of compressed air are used as the cir-
culating medium. The areas of application are hard, dry formations:
dry geothermal zones and dry gas production zones. However, air
drilling cannot be used if any formations will slough or extrude into
the wellbore in the absence of hydrostatic control (unconsolidated for-
mations, plastic salts, etc.).
Where air drilling is used, several significant advantages are
gained:

1. The rate of penetration may be increased by a factor of five or more
2. Formation damage due to hydrostatic, solids, or chemical reactions
is eliminated
3. Lost circulation is eliminated
4. The cost of the alternative mud system is saved, which offsets the
cost of the air drilling equipment and personnel
5. Bit life may be extended due to lack of erosion and seal damage
from mud solids
6. The well is continuously tested in gas zones while drilling

246