STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Muds and Completion Fluids 679
Tables 4-51
Gaseous Drilling Mud Systems
Properties
Qpe of Mud Density, ppg pH Temp. Limit O F Application Characteristics
Airlgas 0 - 500 High energy type system.
Fastest drilling rate in dry,
hard formations. Limited by
water influx and hole size.
Mist
Foam
0.13-0.8
0.4-0.8
7-11
4-10
300
400
High energy system. Fast
penetration rates. Can
handle water intrusions.
Stabilize unstable holes
(mud misting).
Very low energy system. Good
penetration rates. Excellent
cleaning ability regardless
of hole size. Tolerate large
water influx.
Air-Gas Drilling Fluids
This system involves injecting air or gas downhole at the rates sufficient to
attain annular velocity of 2,000 to 3,000 ft/min. Hard formations that are
relatively free from water are most desirable for drilling with air-gas. Small
quantities of water usually can be dried up or sealed off by various techniques.
Air-gas drilling usually increases drilling rate by three or four times over that
when drilling with mud as well as one-half to one-fourth the number of bits are
required. In some areas drilling with air is the only solution; these are (1) severe
lost circulation, (2) sensitive producing formation that can be blocked by drilling
fluid (skin effect), and (3) hard formations near the surface that require the
use of an air hammer to drill.
There are two most important limitations on using air as a drilling fluid: large
volumes of free water and size of the hole. Large water flow generally necessitates
converting to another type of drilling fluid (mist or foam). Size of the hole
determines a volume of air required for good cleaning. Lift ability of air is
dependent upon annular velocity only (no viscosity or gel strength). Therefore,
large holes require an enormous volume of air, which is not economical.
Mist Drilling Fluids
Misting involves the injection of air and mud or water and foamer. In case
of “water mist” only enough water and foamer is injected into the airstream to
clear the hole of produced fluids and cuttings. This unthickened water causes
many problems due to wetting of the exposed formation which results in sloughing
and caving. Mud misting, on the other hand, coats the walls of the hole
with a thin film and has a stabilizing effect on water-sensitive formations. A mud
slurry that has proved adequate for most purposes consists of 10 ppb of
bentonite, 1 ppb of soda ash, and less than 0.5 ppb of foam stabilizing polymer
such as high viscosity CMC. If additional foam stability is needed, additional