BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
coalescence or aggregation. Surfactants are our most important tool for controlling these destabilizing
forces and processes.
Non-Aqueous external phase
Base Oils
Hydrocarbon oils are the continuous phase in oil-base fluids. They are non-polar, low-surface
energy/tension liquids and interact only weakly with mineral solids. This characteristic is the basis for
the use of oil-base fluids as non-reactive, inert drilling fluids. Hydrocarbon oils will not allow clays to
swell, which makes them ideal for drilling hydratable shales.
The most commonly used oils today are synthetics where certain environmental regulations prevail,
low-aromatic-content, low-toxicity mineral oils, and No. 2 diesel oil. Crude oil has been used in the
past but finds little application in today’s modern day oil-mud drilling fluids. Crude is relatively
cheap, often available, but may need topping to minimize flammability since a flash point greater than
180°F (82°C) is advised.
• Crude contains native asphaltenes and resins which can interfere with other additives.
• Crude is usually used in “poor boy” oil-base fluids.
No. 2 diesel oil is a moderately-priced, commonly available distillate which contains none of the
native asphaltenes or resins in crude and is the most commonly used oil for mixing oil-base fluids.
Diesel based systems are generally used today only in land drilling operations. Offshore drilling
operations limit the use of diesel base systems due to the toxicity of the base oil. The aromatics in
diesel oil can swell rubber gaskets, seals, and pipe rubbers, however, an aniline point greater than
140°F (60°C) (the higher the aniline point, in general, the lower the concentration of aromatics) is
recommended. Mineral oils are recommended due to lower toxicity relative to diesel oil. Most of the
toxicity of diesel oil is associated with the aromatics, particularly the multi-ring or polynuclear
aromatics. Low-toxicity mineral oils have low to very low concentrations of aromatics and those
present are predominantly single-ring. Synthetics are the preferred oil in offshore drilling operations
where environmental regulations prohibit the discharge of cuttings and/or whole mud to the sea.
All hydrocarbon oils currently used in oil-base fluids are more viscous than water. This probably
accounts for the lower penetration rates observed with oil-base fluids (all things being equal) relative
to water-base fluids because of the higher viscosity at the bit. The higher viscosity oil also partially
accounts for the lower filtration losses observed with oil-base fluids relative to water-base fluids
because of Darcy's Law (assuming identical permeability filter cakes). See Table 5-3 for the
properties of several oils.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 5-17