Last month, my colleague Matt McDermot-Fouts posted an article about differential pressure and how it applies to motor performance. If you haven’t gotten a chance to read his write-up, here’s a link to the article.
It is generally understood that a downhole motor’s differential pressure rating is a key contributor to drilling performance. RPM output is another key contributor to the drilling performance. As Matt discussed, a power section’s differential pressure rating is determined by the length, geometry, and elastomer used in the power section. The power section’s speed, or displacement, is driven by the geometry of the rotor and stator and is reported as some amount of revolution per unit of flow. For the purposes of this article, I will use revolution per gallon, or rev/gal, as the unit of speed.
Power sections are imperfect systems and the RPM output is not always equal to rev/gal multiplied by the flow rate. RPM drop, or RPM slip, occurs as a portion of the drilling fluid bypasses the seal lines between the rotor and stator. Once this bypass occurs, the motor is no longer operating at its designed displacement. This is important to understand because this RPM reduction can have a significant impact on the motor’s energy output (horsepower), which then affects drilling performance. The amount of fluid bypass, and therefore RPM drop, is determined by several factors.
Fit is the amount of clearance or interference between the rotor and stator, and it is one of the main elements to consider when examining RPM drop. The elastomer in the stator will expand as its temperature increases, or if it reacts to chemicals within the drilling fluid. This expansion of the elastomer makes the fit tighter, which provides more power but could also compromise stator life if not managed properly.
The power section fit is managed at motor assembly, accounting for elastomer swell, so that the amount of compression between the rotor and stator is optimal once the motor is operating downhole. A motor expected to run in hotter downhole temperatures, such as the Eagle Ford and Haynesville basins, would be built looser than one expected to run at low temperatures, like in the Permian.
Selecting and manipulating fits is a science unto itself and something we will discuss in more detail with a later post. For now, let’s assume all motors are built to provide the best mix of performance and life in each specific application.
The final amount of compression between the rotor and stator is driven by the fit selected at assembly and temperatures the motor sees downhole. The more compression between the rotor and stator, the less RPM drop you should expect to see. Higher compression makes it more difficult for the drilling fluid to bypass the seal lines in the power section. With enough compression and no external load, the motor would produce its specified amount of revolution per gallon.
However, drilling motors never really operate without an external load. Motors are required to provide torque, as well as RPM, to the bit box. The higher the torque load, the more RPM drop you will have.
As fluid is pumped into the power section, the only way for it to get from the top to the bottom of the power section is to push, or displace, the rotor out of the way. As the rotor rotates, the fluid progresses to the next cavity and out of the motor to the drill bit. With minimal load and adequate compression, the fluid’s path of least resistance is to turn the rotor.
As the torque load increases, the fluid pressure in each of the pockets increases, and the path of least resistance becomes less obvious. Most of the fluid works to turn the rotor, but some may escape across the seal line and into the next pocket without doing any work to turn the rotor. Now the power section is no longer providing its design specification rev/gal, but rather something less.
This is why RPM curves on a motor specification sheet show reduction as differential pressure increases. The trend continues until the external torque requirement exceeds the pressure holding, or sealing, capability of the power section. At this point the path of least resistance leads the fluid to completely bypass the seal lines and perform zero work to turn the rotor, causing a motor stall.
Power section designers provide a maximum differential pressure rating for each design and elastomer combination so the end-user knows where the motor can operate with an acceptable amount of RPM drop without compromising the integrity of the power section.
As we can see, the integrity of the seal lines in the power section is critical to proper functionality. The seal lines in the power section are created by the rotor and stator profiles meshing together. Stators are relined frequently and due to their manufacturing processes, have a brand-new profile each time.
Rotors, however, often represent a much more significant financial investment. It is not feasible to replace them each time repairs are necessary. Due to the rework processes required, the profile of the rotor may deviate from the new condition and become distorted over time. If the rotor profile has become distorted as a result of multiple repair and recoat cycles, it will no longer correctly mesh with the as-new stator profile. Seal lines in the power section may be less effective and provide a path of even less resistance for the drilling fluid leading to even more RPM drop than what is indicated on the motor spec sheet.
Inspection of the full rotor profile is required to see this type of deviation since the rotor C-to-V’s may still measure within specification, even though other parts of the profile are out of spec.
With the increased capability and availability of downhole RPM measurement devices recently, there has been significant learning about what is happening at the bit box beyond the theoretical topics discussed above. There have already been multiple papers and presentations discussing motor RPM output and what it means to drilling operations. The implications of motor RPM variability on mechanical specific energy (MSE) or depth of cut (DoC) modeling can be significant and should be accounted for in your planning and analyses.
While we have a thorough understanding of the advanced concepts used in our design work, such as the Moineau principles, we continue to learn more about what is occurring downhole. As a power section OEM, PV will watch this space and stay up to date with the latest findings. Our Applications Engineering team is focused on helping our customers get the best performance out of power sections to improve drilling performance.
Feel free to comment or reach out if you have any questions about the post above, or to share any of your own thoughts on the topic. If not, thanks for stopping by and stay tuned for future posts from the PV team.