Stator Shelf Life and the Risk/Reward of Motor Re-Use

If you’ve ever had the chance to walk the floor of a motor shop, chances are you will have seen your fair share of power sections. These power sections consist of a rotor and stator as shown in the video below. The stator is lined with a specialized elastomer and upon close inspection, it looks like a relatively stable material. Come back to that same shop a month later and the elastomer’s properties will have changed very little.

However, the truth is that the physical properties of stator elastomers, like most elastomers, do in fact change with time. This is important to understand since this may present hidden risks to both the Directional Company and the Operator.

Elastomers age when exposed to heat, light, ozone, oxygen, and radiation. Aging causes the elastomer to harden and crack and changes its mechanical properties. As the elastomer is exposed to the elements, it breaks down, becomes more brittle, and cracks.

To compensate for this natural process, PV has devised the following recommendations for stator shelf life (assuming both ends of the stator are covered):

 

It is recommended that stators stored outside are painted a light color and are located out of direct sunlight and have some form of protective covering over each end.

 

 

The above recommendations are for a brand-new stator, but what about a stator that has already done some work in its life?

The drilling environments found in the US shale basins can be very challenging, and our customers demand a lot from our power sections. Between high temperatures, highly reactive, solids-laden drilling fluids, and the sheer physics of what is occurring downhole, our stator elastomers are exposed to some extremely harsh loads and environments. In addition, a downhole failure and subsequent unplanned trips can be very costly to the drilling operation.

One of the benefits of elastomeric power sections is that they can be stripped and relined with a fresh rubber profile between runs. This means, assuming your rotor is in good shape, you basically have a brand-new motor to power your bit after each reline.

Imagine you were the lead mechanic for an F1 Racing crew. You need your car to perform in a harsh environment and a tire failure mid-race will be very costly. So, when possible, changing out the tires with some fresh rubber is a smart move that helps you minimize risk. Similarly, if performance and reliability are your goals on the well site, a freshly relined power section is often a wise investment.

 

 

However, as we all know, some of the biggest challenges to the Shale Industry are economic vs purely technical. Directional companies and operators are under intense pressure to reduce cost and thus may consider re-running a power section. This helps to spread the cost of RELINE over multiple runs. But what are the risks?

As mentioned above, the aging process of elastomers is accelerated by exposure to heat. By storing a brand-new power section outside in the hot sun vs in a climate-controlled facility, you can reduce its recommended shelf life by more than half!

However, this differential in heat pales in comparison to the heat generated when the power section is operated downhole. The combination of ambient downhole temperatures and dynamic heating effects (caused by the nutation of the rotor and stator) can result in elastomer temperatures well above 350°F! The result is that rather than taking years to degrade, the elastomer can age considerably in a matter of days or even hours.

This brings us to an important phenomenon known as “hysteresis.” Power section elastomers are viscoelastic materials which means they exhibit both viscous and elastic characteristics when undergoing deformation. An important characteristic of this viscoelasticity is that these elastomers give off heat when they are deformed. With power sections, the heat concentrates in the middle of the lobes due to friction as the rubber molecules rub back and forth. As the elastomer in the center of the lobes heats up, it ages at a much faster rate than the surrounding elastomer and becomes stiff/brittle. Eventually, a crack will form and propagate until the entire lobe chunks.

 

Okay, so operating a power section accelerates the aging of the elastomer. But what if you still want to re-run it? What important factors could you consider in making the decision to re-run the motor?

Say the driller runs a power section for 50 circulating hours and no chunking is observed and the profile appears in spec upon teardown. The elastomer may physically look as good as new but it has in fact aged due to a combination of hysteresis and heat aging from ambient bottom hole temperatures. You may be able to get away with re-running the stator, especially in less demanding, low temperature, water-based applications, but the risk of failure will be higher as compared to a freshly relined stator.

A second risk of re-running a power section is a loss in ROP due to stator wear and slip. Tight motor fits, sub-optimal rotor condition and especially the pumping of solids through the power section at high flow rates, can all cause the stator profile to wear. This creates the opportunity for power section slip and ROP loss to occur.