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What is Aniline Point?

By definition, “aniline point” is the temperature at which equal volumes of a hydrocarbon fluid becomes miscible with aniline.

• Miscible means able to be mixed. Two liquids are said to be miscible if they are partially or completely soluble in each other. Commonly, the term miscible is understood to mean that the two liquids are completely soluble in each other. (See also Solubility.)
• Aniline (an·i·line) is a colorless oily liquid arylamine derived from coal tar or indigo, made commercially by reducing nitrobenzene. It is slightly soluble in water and freely so in ether and alcohol. Combined with other substances, especially chlorine and the chlorates, it forms the aniline colors or dyes. It is an important cause of serious industrial poisoning (anilinism), and high doses or long exposure may be carcinogenic. Called also amidobenzene and aminobenzene.

How does this relate to stators?

It is important to understand that the term “aniline point” is really a unit of measure. It does not mean there is aniline in the drilling fluid.

Relating to the definition of “Aniline Point” in regards to the stator, it is the temperature at which the elastomer “reacts” with the hydrocarbon based drilling fluid. Hydrocarbon based fluids should have an “aniline point” associated with it and this information can be usually be obtained from the mud company if it is not available at the rig site.

As a general rule of thumb, fluids with a “low” aniline point (meaning a lower temperature) produce higher swell in nitrile and HSN elastomers. The opposite is true of “high” aniline point fluids where it tends to induce less swelling.


The Aniline point is really a direct measurement of the polarity of a hydrocarbon. This measurement is extrapolated to correlate with swell in the stator elastomer.

Aniline is a polar compound, so the more polar the hydrocarbon fluid is, the lower the temperature that it will dissolve in aniline. The more similar the polarity of the different compounds is, the more they want to mix together. Nitrile and HNBR elastomers are polar compounds so the more polar the drilling fluid, the more swell will occur in the elastomer.

“polarity” refers to the positive and negative charge separation in a chemical bond.

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