STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Muds and Completion Fluids 651
Functions
The functions of drilling fluid muds are:
a. To remove rock bit cuttings from the bottom of the hole and carry them
to the surface.
b. To overcome formation fluid pressure.
c. To support and protect the walls of the hole.
d. To avoid damage to the producing formation.
e. To cool and lubricate the drill string and the bit.
f. To prevent drill pipe corrosion fatigue.
g. To allow the acquisition of information about the formation being drilled
(e.g., electric logs, cuttings analysis).
Classification
The classification of drilling muds is based on their fluid phase, alkalinity,
dispersion, and type of chemicals used.
Freshwater Muds-Dispersed Systems. The pH value of low-pH muds may
range from 7.0 to 9.5. Low-pH muds include spud muds, bentonite-treated muds,
natural muds, phosphate-treated muds, organic thinned muds (red muds, lignite
muds, lignosulfate muds), and organic colloid-treated muds. The pH value of
high pH muds, such as alkaline tannate-treated mud, is above 9.5.
Inhibited Muds-Dispersed Systems. These are water-base drilling muds that
repress the hydration and dispersion of clays. There are essentially four types
of inhibited muds: lime muds (high pH), gypsum muds (low pH), seawater muds
(unsaturated saltwater muds, low pH), and saturated saltwater muds (low pH).
Low Solids Muds-Nondispersed Systems. These muds contain less than
3-6% solids by volume, weigh less than 9.5 lb/gal, and may be fresh or saltwater
base. The typical low solids systems are flocculent, minimum solids muds,
beneficiated clay muds, and low solids polymer muds. Most low solids drilling
fluids are composed of water with varying quantities of bentonite and a polymer.
The difference among low solids systems lies in the varying actions of
different polymers.
Emulsions. Emulsions are formed when one liquid is dispersed as small droplets
in another liquid with which the dispersed liquid is immiscible. Mutually
immiscible fluids, such as water and oil, can be emulsified by stirring. The
suspending liquid is called the continuous phase, and the droplets are called the
dispersed (or discontinuous) phase. There are two types of emulsions used in
drilling fluids: oil-in-water emulsions that have water as the continuous phase
and oil as the dispersed phase, and water-in-oil emulsions that have oil as the
continuous phase and water as the dispersed phase (invert emulsions).
Oil-Base Muds. Oil-base muds contain oil as the continuous phase and water
as the dispersed phase. Oil-base muds contain less than 5% (by volume) water,
while oil-base emulsion muds (invert emulsions) have more than 5% water in
mud. Oil-base muds are usually a mixture of diesel fuel and asphalt; the filtrate
is oil.