BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
Treatment of Contamination in Gypsum Fluids
Salt – A properly conditioned gypsum fluid can be expected to tolerate up to 100,000 mg/L salt.
Since an increase in salt increases the solubility of the calcium ion, the filtrate alkalinity should
be increased with caustic soda to limit the level of soluble calcium. UNI-CAL should be added as
needed to control the rheological properties.
Air, Mist, and Foam Drilling
The use of air for drilling, with or without a fluid phase, is a technique that has been used in many
areas of the world to increase penetration rates, overcome lost circulation zones, and improve
lifting capacity. Because of inherent limitations, equipment requirements, and handling
techniques, this is considered a highly specialized technique and is used only in selected
applications.
There are at least four basic methods which utilize air alone or in combination with water or a
drilling fluid. They can be classified as follows:
Table 3-14
Classifications of Air / Water Drilling Fluids
Classification Components Lift Medium
Dusting Air alone Air
Water misting
Air
Foaming agent
Air
Water
Aerated fluid
Air
Fluid
Fluid
Stiff foam
Air
Foaming agent
Viscosifier
Foamed fluid
As indicated, the systems can be classified further according to the lift media. In classifications 1
and 2, air is the lift medium for the generated cuttings, therefore, the formation is in contact with
air. In systems 3 and 4, the drilling fluid or foam is the external phase of the air fluid dispersion.
Therefore, the formation is in contact primarily with the fluid phase. These two distinguishing
factors are important with respect to their effect on the borehole. The classifications and their
distinguishing factors are discussed below.
Dusting
This drilling system injects only air down hole at sufficiently high rates to carry the cuttings from
the hole. Dusting provides minimum back pressures and produces the fastest penetration rates
attainable with air drilling techniques. As long as the hole is dusting, there are usually few
problems, but when water is encountered, several problems can occur.
• A water column can build up causing loss of circulation if the formation is not competent.
• Formation water can mix with the drill dust causing what are commonly called fluid rings.
These rings can restrict air flow and, in some instances, cause tight hole problems.
• The water will wet shale formations and possibly create sloughing and borehole instability.
Where only small amounts of water are encountered during drilling operations, a drying
agent such as silica gel may be injected into the air stream.
Reference Manual
Baker Hughes Drilling Fluids
3-46 Revised 2006