As we’ve discussed in previous posts in our article series, there are numerous items to consider when aiming for peak mud motor performance, and thereby peak drilling performance. Before a motor is selected, specifications are reviewed and the best combination of speed, differential pressure capacity, and torque slope is selected for the application. One final lever that we can pull to affect the performance of the motor is the fit of the power section.
Fit is a measure of the amount of interference or clearance between the rotor and stator after they are assembled into a power section. An ideal power section fit provides enough compression to limit RPM slip during operation but not so much compression that it leads to failure due to excessive stress in the elastomer. Too little compression allows the drilling fluid to bypass the seal lines in the power section without imparting energy on the rotor. Without this energy, the bit will lose RPM, leading to reduced ROP.
The main factors that influence fit are the profile size and condition of the rotor and stator, the expected downhole temperature of the run, and any effects the drilling fluid might have on the elastomer. Fit is typically determined by choosing one stator from a group of various stator profile sizes to pair with a rotor. The fit must be chosen to provide the best balance of performance and stator life to achieve the objectives of the application.
Applications with high downhole temperatures will require a looser fit at assembly to account for the thermal expansion of the elastomer. Applications with mild expected downhole temperatures can be assembled with tighter fits to maximize performance without much concern for the thermal expansion of the elastomer creating too much compression and potentially causing chunking.
Downhole fit is estimated based on temperature correction guidance from the power section manufacturer. Each power section manufacturer will have unique temperature correction guidelines. Some provide the amount of fit change per 1°F, while others simply report expected fits at various temperatures across a wide range to serve as estimates for what to expect once the motor is downhole.
PV’s fit chart outlines the recommended temperature ranges for each stator size of a given model. It will also indicate the temperature correction factor for that model. PV’s temperature correction factor is the number of degrees Fahrenheit (or Celsius) required to affect the stator elastomer by 0.001”. The downhole fit is calculated by following the steps below.
First, find the difference between the expected downhole temperature and 70°F. For this example, we will use 300°F as the anticipated downhole temperature.
300°F – 70°F = 230°F
Next, divide the temperature difference (230) by the stator temperature correction factor. The stator temperature correction factor can be found on the fit chart supplied by the power section provider. In this case, it would be 4.
230 ÷ 4 = 57.5
Then, multiply by 0.001″, and round to the nearest thousandth.
57.5 x 0.001 = 0.058
Next, add the estimated stator profile change to the shop (assembly) fit. The shop fit can be found in the fit chart. In this case, it would be -0.019, or 0.019 loose.
-0.019 (loose) + 0.058 = 0.039 (tight)
The temperature ranges for each stator size provided by the manufacturer are guidelines, as stated above, to provide the best balance of performance and stator life. Local field experience is vital in choosing the proper fit for an application. Motor providers will know which fits are successful at different temperatures in different areas. Some may even be able to provide recommendations of how different fluid systems affect fit, based on their experience.
Proper fit selection requires an understanding of the drilling objectives, an eye on what fits have performed well historically, and the correct product offering. Fit recommendations will vary from model to model and manufacturer to manufacturer.
One key item that dictates how much temperature range a power section can handle is the elastomer. Elastomer development is an ongoing process with manufacturers looking for ways to improve their products’ performance and reliability in challenging high-temperature environments.
Achieving 100% success in our drilling applications remains the goal. Operators, motor providers, and power section manufacturers must continue working together to choose models and fits that provide the best motor performance and reliability.