Drilling Fluid Classification

The importance of terminology properly defining a drilling fluid is becoming of greater significance. This includes the drilling contractor, the operator, the mud service engineer, and even the well completion service specialists who
must work with the well after the hole is down. As the number of products added to a given drilling fluid formulation increases, so does the importance of proper mud classification, proper metering and measurement of fluids and special additives.

Drilling Fluid Systems

Water-Based Mud (WBM)

Fresh Water, Low pH

Fresh Water, Low pH – Systems with a liquid phase of water containing only small concentrations of salt, and with a pH ranging from 7.0 to 9.5. This would include spud muds, bentonite treated muds, phosphate muds, and polymer
nondispersed muds.

Brackish Water, Low Ph

Brackish Water, Low Ph – Includes 7.0 to 9.5 Ph sea and brackish, or hard water muds. Source water here may be from open sea or bays (5000 – 15,000 mg/l chlorides).

Saturated Salt Water, Low Ph.

Saturated Salt Water, Low Ph – Liquid phase of these muds is saturated with sodium chloride, although other salts be present. These may be prepared from fresh water of brine water (192,000 mg/l chlorides).

Gyp Treated, Low Ph

Gyp Treated, Low Ph – These gypsum-treated or gyp-base muds are formulated by conditioning the mud with plaster (commercial calcium sulfate), lignosulfonates, and CMC’s.

Lime Treated, High Ph

Gyp Treated, Low Ph – Consist of adding caustic soda, lime, and clay, and an organic thinner. These normally have a Ph greater than 11.0 and contain an excess of lime equal to or greater than Pf.

Fresh Water High Ph

Fresh Water High Ph – Muds having a liquid phase of fresh water but with which have been treated with products which bring the Ph level above 9.0. These would include most alkaline tannate treated muds.

Low Solids

Low Solids – A drilling fluid where solids content is less than 5 percent by volume or a weight less than 9.0 per gal. may have either oil or water base.


Emulsions – Are formed by the physical mixture of two immiscible liquids such as oil and water. One liquid is broken up into droplets in the other liquid by mechanical action. The droplets are so small that they are not able to separate from the suspending liquid even though there might be a great difference in the specific gravity of one with respect to the other. The suspending liquid is called the continuous phase and the droplets are called the dispersed phase. Emulsions are often used in the liquid phase of drilling muds.


Oil-In-Water. Droplets of oil are dispersed in water in these emulsions. Water is a continuous phase and is often the only liquid recovered in a falter loss test.


Water-In-Oil (invert emulsions). The oil is the continuous phase having droplets of water as the dispersed phase. These emulsions contain up to 50% of the total volume as water in the dispersed phase.

Oil-Based Mud

Oil Muds

Oil Muds – Are not emulsions at the start of their use in drilling. They are usually a mixture of diesel fuel and asphalt. Viscosity is controlled by the additions of diesel fuel to thin and asphalt compounds or organo clays to
thicken. Weight is increased by the addition of barites. As a rule, oil muds will form water-in-oil emulsions with formation water or from other sources of water contamination.

Air, Gas, Mist

Air, Gas, Mist – These include aerated and gaseated mud systems in this classification.


Foam – Mixture of air, water and detergent foaming agent.

Related Article:  Gypsum (Gyp) Muds
Water-base Drilling Fluids
type of oil-based mud

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