I always like to kick off these types of discussions with the “why factor.” Why does this matter to you and why should you bother to read this post? My general observation is that directional drillers/engineers like to drill fast and avoid downhole failures. Having a quality rotor coating and profile is key to both.
It’s important that rotors are manufactured with stringent quality control procedures, regulating nominal coating thickness, strict rotor profile tolerances and optimal surface finish. For example, in order for a PV rotor to pass inspection, it must match the rotor’s design profile within a tolerance of plus or minus five-thousandths of an inch – about the thickness of a sheet of standard printer paper.
The rotor’s surface is made to be strong, but a rotor is only new once. As soon as it goes downhole, it can be subjected to chemical attack, abrasion or impact damage from different types of solids and debris, as well as high fluid velocities; all of which can lead to damage of the rotor’s surface.
If a rotor sustains damage, the surface can become rougher, corroded, and cracked. This can compromise the integrity of the rotor-stator seal, resulting in a loss in performance, and perhaps even elastomer chunking which may necessitate an unplanned trip.
We will come back to the effects of the drilling environment on the coating condition shortly but first, let’s take a step back and talk about the coatings themselves.
Like stator elastomers, new coating technologies are constantly being developed; however, most rotor coatings can be divided into two categories: electroplated (chrome) or high-velocity oxygen fuel (HVOF) spray carbide coatings.
The image below shows a picture of a typical chrome and carbide coating under magnification. By the nature of electrolysis, the chrome adheres itself to the substrate in “layers” and natural microcracks are common. Carbide coatings, on the other hand, are characterized by very small pores with poor connectivity.
So, which type of coating is better? There are pros and cons to both, so it ultimately depends on your application.
Carbide coatings have a higher resistance to corrosion and wear. However, compared to chrome, carbide is relatively brittle and prone to cracking or fracturing upon impact. As a result, carbide can be highly susceptible to handling damage and impact damage from large solids and foreign debris being pumped through the power section. Another drawback to carbide is its cost which can often be three or four times more expensive than chrome.
Chrome has a relatively low coefficient of friction and tends to maintain its surface finish better than carbide which can become increasingly rough with repeated use. As stated above, chrome is a less expensive solution as compared to carbide.
Unfortunately, chrome coatings have an Achilles heel, one that has a significant impact. While chrome coatings have a relatively low corrosion resistance compared to carbide, the microcracks naturally present in chrome coatings (shown above) make chrome more prone to corrosion, allowing drilling fluids to damage the coating. Depending on the level of corrosiveness, fluids can eventually penetrate the coating and cause damage to the substrate. The image below shows severe corrosion on a chrome rotor after a single run, a painfully common sight in motor shops across North America.
So, what drilling fluids cause the most corrosion and when should I use chrome vs carbide? The science of drilling fluids can be complex and is beyond the scope of this post but the main determining factor on whether or not a chrome coating is suitable is the chloride content of the mud. In water-based mud (WBM) applications, chrome coated rotors are generally acceptable up to approximately 30,000 ppm (mg/L). Once beyond that limit, the chrome coating will start to deteriorate. With chloride contents in the 30,000 – 100,000ppm range, the rotor life will be shortened, and damage can be expected. Chloride contents above 100,000 ppm will reduce the rotor life significantly and coating failures after just a few hours are possible.
Generally, in high chloride applications we recommend the use of a carbide coated rotor, which has a much denser porosity than chrome and has greater resistance to chlorides and other corrosive agents. However, this recommendation comes from a technical standpoint only and assumes that impact damage is not a major concern. Using chrome in a high chloride environment may be a smart move if the alternative is cracking an expensive carbide rotor after relatively few hours.
I’m always wary of the oversimplification of complex systems for the sake of digestibility. Mud systems can be complex and chloride content is not the only factor that drives corrosion damage.
Corrosion is a chemical process, and the corrosion rate will also generally be increased by other factors, including:
- dissolved gasses in the drilling fluid
- lower PH levels
- increased bottom hole temperatures
- stress concentrations and localized coating damage
One question I commonly get during technical sessions is, “can I apply carbide to my chrome rotor or vice versa?” The answer is: it’s not recommended. As can be seen in the image below, the electroplating process for chrome results in an uneven coating thickness, with a thicker coating located at the rotor majors and a thinner coating at the rotor minors. The unevenness of chrome deposition can magnify with repeated chrome re-coats, each time increasing the major-to-minor thickness ratio. The tungsten carbide thermal spray, however, results in a relatively even coating thickness across the entire profile.
To compensate for these differences, PV specifically manufactures rotors designed for each coating type. This ensures the final coated profile of either a chrome or tungsten carbide rotor will meet the required final specification. So, if a chrome coating were applied to a carbide rotor, the result would be a sub-optimal fit and stress concentrations along the profile, increasing the chances of performance issues and a potential downhole failure.
Now that we’ve covered corrosion, let’s look at the other challenge to rotor coatings: wear and impact damage. The two primary factors that drive these mechanical damages are the solids content in the mud and the pumping of foreign objects through the power section. These solids can lead to wear of the rotor surface as in the image below.
Sometimes, foreign objects (large solids, pebbles, rock chips, LCM material, etc.) can be pumped through the power section. These foreign objects can cause significant localized impact damage to both chrome and carbide. An example of impact damage on a chrome rotor is shown in the image below.
It’s important to note that corrosion, wear and impact damage are often synergistic. For example, if a rotor surface is compromised by local damage from a foreign object, the substrate may be exposed and may corrode rapidly. Therefore, what may look like pure corrosion upon motor teardown may have started as impact damage.
As you can see, the life of a rotor is pretty rough. It starts out as this beautiful, shiny piece of manufacturing art and then spends the rest of its days getting pummeled by solids, debris and chlorides. Ouch!
When rotors see enough damage (often after a single run downhole) they get sent for re-coat. The coating vendors strip the old coating and patch/weld/polish up the damaged rotor to get it ready for re-coat. Coating vendors can work miracles but as I stated above a coating is only new once and every time it is re-coated it loses profile. The figure below shows what can happen to a rotor profile after several re-coats. Since the tolerances on rotors are so crucial and tight, it is inevitable that the rotor’s profile will deviate from spec. This can lead to stress concentrations and a sub-optimal fit with the stator, increasing the chances of reduced ROP and a potential failure.
There is no one size fits all solution when it comes to rotor coatings, and the owner of the asset needs to consider the expected application and long-term economics in order to make the best decision. Chrome coatings generally carry a re-coat cost which is a fraction of the cost of the more expensive carbide. As such, chrome will likely offer the most economical solution in applications with oil-based drilling fluids and with chloride content below 30,000 ppm (mg/L), especially if impact damage risk is high. When drilling in water-based drilling fluids with higher chloride content (brines) with a lower probability of impact damage, the ability to re-run carbide rotors may offset the higher re-coat costs of carbide.
Quality translates into performance which ultimately means more money in your pocket. PV has strategically partnered with the top directional service providers in the market, and these customers have some of the highest standards regarding quality control, which includes rotors. So, if you’re an operator running PV power, you’re in good shape and can look forward to faster drilling, less failures and lower well costs!
Hopefully, this article was helpful and if you have any further questions please don’t hesitate to comment or reach out to me personally!
