One key measurement that can be used to “predict” elastomer swell is the “aniline point” of the hydrocarbon based fluid. The main rule of thumb is:
- Fluids with an aniline point higher than the bottom hole circulating temperature will minimize the amount of swelling in the stator rubber.
- Fluids with an aniline point lower than the bottom hole circulating temperature will result in greater amounts of swell.
The reason for this is the relationship between the hydrocarbon and the nitrile rubber. By definition, aniline point is defined as:
“Temperature at which equal volumes of a hydrocarbon fluid becomes miscible with aniline”
The aniline point is really a direct measurement of the polarity (‘polar’ refers to the positive and negative charge separation in a chemical bond) of the aniline fluid and the hydrocarbon which can then be extrapolated to correlate with swell of the rubber. In the “aniline point” test, the more polar the hydrocarbon, the lower the temperature will be in which it becomes miscible in the aniline. Because “aniline” and hydrocarbons are “polar” compounds, when they are mixed and heated, they want to “react” when a certain temperature is met.
In relating this to how the stator rubber reacts with the drilling fluid it is necessary to understand the polar relationship of the compounds. Nitrile and HSN are both polar compounds so when they are immersed into a hydrocarbon based fluid, which is also a polar compound, and heated, the aniline point of the fluid then becomes critical. If the bottom hole drilling and circulating temperature is greater than the aniline point of the fluid, the elastomer and fluid will begin to react and swelling occurs. The greater the difference between the aniline point and the bottom hole temperature, the greater degree of reaction will take place.