Bottom hole temperature (BHT) is a major factor to consider for optimum power section performance. BHT has an impact on power section fit, elastomer integrity, and downhole tool reliability. If downhole temperatures are not accounted for during the mud motor selection and assembly process, the life of the power section may be at risk, potentially leading to premature chunking and even debond issues.
In the lower 48, distinctly defined basins have varying ranges of BHTs. Static BHT is determined by the formations in-situ, or original, geological characteristics. Generally, the earth’s temperature rises as you drill deeper towards its core. However, in some areas the temperature gradient of the earth varies, creating some basins with higher temperature gradients. In these areas, extreme temperature conditions may occur, increasing the stress level on drilling tools.
As the Major Shale Play Operating Parameters table indicates, higher temperature ranges recorded in the Haynesville and some fields of the Eagle Ford basin make these particularly challenging drilling areas. If you’re operating in these basins, those high BHTs will affect your power section. Understanding how to mitigate the negative effects is essential.
The effective temperature of the bottom-hole assembly (BHA) is driven not only by the ambient formation temperature, or static BHT, but also by frictional forces within the wellbore. Based on a recent report from Baker Hughes, 88% of the wells drilled in North America include a lateral hole section. Today, lateral sections can extend from one to two miles in length or more. Longer hole sections generate higher stresses on downhole equipment and allow more time for heat generation to build up.
The following are some of the key factors that can influence these dynamic heating effects:
- Mechanical wellbore configuration (hole size, lateral section length, DLS, borehole quality, etc.)
- BHA geometric configuration (OD, stabilizer placements, motor bend, bit design, etc.)
- Drilling fluid system (OBM, WBM, mud weight, rheology, etc.)
- Drilling parameters (surface RPM, flow rate, drilling mode slide/rotate, etc.)
All of these factors will have a combined effect on the overall heat generation. When coupled with in-situ temperatures in excess of 300°F, the impact is even more substantial.
The challenge is to ensure that downhole equipment performs optimally under these harsh conditions. Improper configuration of the power section, and/or selection of the elastomer, will have a detrimental effect on the downhole mud motor.
Fit plays a key role in the performance of the power section. PV was one of the first companies to design the power section fit based on specific application criteria, minimizing RPM slip at the expected operating conditions downhole.
These days, some operators are trying to minimize costs by pushing equipment to its limits in some areas, drilling longer laterals and one-run, vertical-curve-laterals. In one run operations, selecting the right power section fit is even more important, because of the temperature variance downhole in some applications. A difference of more than 100°F between the start of the section and total depth (TD), may result in a sub-optimal fit for some portions of the run. Despite this challenge, there have been several cases in which single-run well designs have been drilled successfully, utilizing the appropriate power section model, optimal fit and elastomer selection.
In the illustration above, notice the temperature upon entering the curve section compared to the temperature in the lateral section.
In high-temperature applications, fit configuration is only one part of the equation. Remember, standard elastomer properties can become compromised over time in extreme temperatures and under harsh conditions. One way to mitigate this is to implement a differential pressure derating procedure. Derating the maximum differential pressure applied to the power section during operation will extend power section performance.
Lastly, to further extend the life of the power section, it is essential to implement and maintain best drilling practices. The way downhole events are managed during operation goes a long way towards long-term performance for downhole tools and better drilling results.
In conclusion, both static BHT and dynamic heat generation are key factors to consider for optimum performance. There are some extremely harsh environments out there, but as we continue to improve elastomer chemistry and power section designs, it is essential to continue learning and furthering our understanding of what leads to better drilling efficiencies downhole. To improve the performance of power sections when drilling in higher temperature environments, a holistic approach is needed, considering all of the factors we’ve covered here.
It will be interesting to hear from you on this topic, and I invite you to reach out and share your own experiences in reservoirs with high operating temperatures.
At PV, our main focus and expertise is power section performance, but other downhole tools experience similar challenges. Based on your experience, what has made your high-temperature application successful?