Mud Compatibility & How It Impacts Motor Performance

An important topic that comes up frequently in meetings with Operators and mud motor providers is mud compatibility testing and the effects drilling fluids have on stator elastomers.

There are two main categories of drilling fluids, water-based and oil-based. How the fluid reacts to the power section is the main concern today. Fluid properties can have a direct effect on the performance and life of the power section. When a mud compatibility test is performed the results will be reported back to the customer but what do those results really mean to you and your power section performance?

The drilling fluid’s effects on the rubber will be analyzed and reported per the ASTM-D471 testing standard, developed by the American Society for Testing and Materials (ASTM). Five key variables used in the report include changes in durometer “hardness”, volume swell, modulus, elongation, and the elastomer’s tensile strength. Below is an example of test results showing low mud compatibility.

Sample Fluid Compatibility Results


Power section companies are continuing to develop new elastomers and coupled with the wide variety of fluid systems, no two compatibility tests are likely to be the same. However, no matter what the rubber is the effects will translate into the same effect on motor performance.

When interpreting the results of mud compatibility testing, it’s important to look at the changes to all of these key variables as they relate to each other. Now, let’s take a look at each of these test variables in more detail.

Rubber material hardness is measured on a durometer scale. A rubber’s change in durometer affects how much force must be applied before rubber will deform and, in operation, be able to hold its sealing capabilities.

In the fluid compatibility testing results above, the Change in Hardness indicates the rubber is softening significantly due to a severe chemical reaction. Extreme softening of the rubber can lead to premature stator chunking due to the initial properties of the rubber being considerably weakened. This weakening prevents the rubber from holding the required differential pressure, resulting in a weak motor and causing the power section to stall.

Rubber durometer can become softer, but it can also become harder. This can be a benefit up to a certain point. As rubber hardens, it can become stronger, allowing it to better hold differential pressure which results in less RPM drop and an increase in horsepower. However, hardening rubber can eventually become brittle due to heat aging which can lead to stator chunking.

The next test result reported is volume swell. Drilling fluids can cause rubber to swell or shrink. The Volume Swell results above show the rubber had a 15% change in volume swell from its original state. If a stator elastomer experiences volume swell, it means the rubber is absorbing one or more components found in the drilling fluid. The amount of volume swell depends on the composition of the rubber, type of hydrocarbons in the drilling fluid, rubber-to-fluid exposure time, and temperature. The drilling fluid’s lubricating additives, weighting material, pH control, viscosifiers, and fluid loss additives can also cause volume swell and property changes in the elastomer. The below animation shows the potential effect of elastomer swell on the power section fit.

Another variable that can cause rubber swell is the aniline point of the drilling fluid. The aniline point is the temperature at which equal amounts of the hydrocarbon-based fluid and aniline become miscible, or able to mix completely. This temperature will vary based on the aromatics in the fluid. Fluids with higher aniline points will cause less swell in nitrile compounds, but as the aniline point increases past the recommended range, the rate of change could increase. An aniline point between 160°F to 195°F will typically minimize swelling. As you get below 160°F, the likelihood of rubber swell will start to increase. It is important to note that each elastomer will react differently due to its chemical composition.

Volume swell can be significant due to what will happen to the fit of the power section downhole. When the rubber experiences volume swell, the interference between the rotor and stator rubber will increase. This can lead to premature rubber failure due to increased friction causing more heat buildup, or hysteresis, in the stator lobes.

Increased friction causes heat buildup, or hysteresis, in the stator lobes.


So, what about when rubber shrinks in volume? This can be a benefit to some extent. As temperatures increase down hole and inside the power section a small amount of rubber shrinkage can counteract the natural thermal expansion of the elastomer downhole, reducing the amount of stress on the elastomer. However, a significant amount of shrinkage can lead to a fit being too loose creating an amplified RPM drop which will lead to stalling and premature rubber chunking.

Modulus, elongation, and tensile tests are done by submerging a rubber sample in drilling fluid and stretching it to the point of breaking. Results are reported as a percentile change against the baseline data of a standard rubber sample.

Modern nitrile butadiene rubber (NBR) elastomers used in power sections are often referred to as “hard rubbers”. A rubber’s hardness is directly related to its modulus, the measurement of how much force is needed to stretch the rubber. For example, hard rubber is typically high modulus rubber. A reduction in a rubber’s modulus means it has become easier to deform or stretch. This translates into a reduced ability to hold pressure.

Typically, with a reduction in modulus, you will see an increase in elongation. Elongation is reported as a percentage of how far the rubber was able to stretch relative to its original length. It is essentially a measure of how elastic or brittle the elastomer has become by reacting with the drilling fluid. A reduction in elongation indicates that the elastomer has become more brittle whereas an increase indicates that the elastomer has become more elastic. Elongation and modulus are closely related with elongation often decreasing as the modulus increases and vice versa. A tensile test measures the force it took to break the rubber.

In summary, all five key measurements in drilling fluid compatibility testing – Hardness, Volume Swell, Modulus, Elongation, and Tensile – are important for extending motor performance.

  • Some drilling fluids cause volume swell, which leads to sub-optimal fits and a higher chance of downhole failure.
  • A decrease in modulus and increase in elongation can reduce the pressure-holding capacity of your elastomer, making it weaker and less capable of generating horsepower at the bit due to a significant RPM drop.
  • In addition to this, adverse effects of shrinkage can also lead to premature failure due to stiffening and embrittlement of the elastomer.
  • It’s important to know what additives are in the drilling fluid and to maintain a proper aniline point to achieve the highest performance possible from the power section.

Directional drillers and operators alike expect high performance from their motor, avoiding downhole failures at all costs. Understanding how drilling fluid can affect stator elastomers and knowing how to read a fluid compatibility chart can be useful towards maintaining desired results from your motor.