PEEK-blend rod guides extend service life in high-temperature wells
In the early 1990s, oil & gas exploration companies improved operations by including in their downhole systems sucker-rod guides constructed of synthetic polyphthalamide, or PPA. PPA was effective in these applications because it has greater resistance than many plastics to a broad range of chemicals, along with high tensile strength and stiffness at high temperatures, better fatigue resistance, increased dimensional stability and high sensitivity to water absorption. PPA was a preferred choice over metal and other plastics, especially in deeper wells that featured operating temperatures that could reach up to 300˚F.
However, while PPA performs reliably at higher temperatures and is rated to 400˚F, it is not rated for continuous-service at that temperature. This characteristic was discovered after PPA sucker-rod guides were tested using the continuous use temperature (CUT) testing scale, which showed that the service life of continuous-use sucker-rod guides constructed of PPA would begin to be adversely affected in temperatures as low as 220˚F.
The test results indicated that various characteristics of the downhole production environment, such as bottom-hole temperature, chemicals used, side loads, duration of operation, water cut, fluid viscosity, and well deviation, can reduce the CUT for PPA guides. Additionally, these well characteristics in combination with the varying types and amounts of fines, solids, and corrosive elements encountered in the production tubing will cause PPA guides to wear faster than in a well with lower temperatures.
In fact, the CUT tests revealed that PPA-based compounds can experience rapid degradation, with greater than 50% loss of strength, after only six weeks of continuous exposure to water at 250°F and complete disintegration after one week of continuous exposure to water at 400°F. The combination of chemicals in an aqueous solution at elevated water temperatures causes oxidative degradation of the PPA guide’s molecular structure.
Premature and excessive wearing of sucker-rod guides at high temperatures will pose serious problems in downhole recovery systems. Sucker-rod guide failure will lead to premature tubing wear. In extreme cases, sucker-rod guides can begin to disintegrate, which allows small fragments of plastic to enter into the production fluids. When this occurs, the fragments also can enter the pump and plug flow lines, causing substantial operational issues. Both cases of fragment incorporation will result in expense, downtime, and deferred production rates, all caused by the need to repair or replace downhole equipment.
A PEEK into the future
CUT tests indicated that PPA sucker-rod guides were not capable of performing as reliably as thought in applications with higher downhole temperatures. Knowing that PPA guides were beginning to fail prematurely in high-temperature wells, Norris, a product brand of Apergy Corp., began a lengthy process to identify a better material for use in sucker-rod-guide construction. They found one in a colorless thermoplastic polymer known as polyether ether ketone, or PEEK.
PEEK, developed in the early 1980s, possesses excellent mechanical and chemical-resistance properties that are retained in high-temperature operating environments, up to 480˚F. PEEK also is highly resistant to thermal degradation and attack by aqueous solutions, which are two notable PPA shortcomings.
"The industry needed a sucker-rod guide constructed of a material that could not only handle higher operating temperatures, but also supply the long-life service that synthetic polyphthalamide sucker-rod guides had previously demonstrated," said Frosty Gray, director of sales and operations, Norris. "Norris’ goal was to develop a new extended-life sucker-rod guide that could perform at elevated temperatures."
After extensive research and nearly two years of field testing, the new PEEK-blend sucker-rod guide is a replacement for PPA models in shale wells, primarily because PEEK retains its mechanical and chemical-resistance properties even at very high temperatures when compared with most other thermoplastics. Its performance capabilities fit perfectly into the sweet spot for sucker-rod guides used in continuous-duty operations in deep, high-temperature shale wells.
Addressing a problem
In 2015, a petroleum and natural gas exploration and production company that is a major operator in U.S. shale plays, experienced a series of sucker-rod string failures at its wellpads in the Eagle Ford shale field in Texas. In just the third quarter of 2015 alone, they reported 11 string failures, with a mean time between failure (MTBF) of 461 days, an unacceptably low rate of reliability.
All the failed strings were equipped with PPA sucker-rod guides. Bottom-hole temperatures in the wells ranged from 270˚F to 295˚F. The failures occurred at depths of 6,950 ft. to 10,756 ft. Levels of hydrogen sulfide (H2S) ranged from five to 2,000 parts per million (ppm). The maximum side load in the failure section was 270 lbs. per rod. After examination, the sucker-rod guide failures were attributed to the exposure of the PPA guides to an aqueous solution at elevated bottom-hole temperatures.
Knowing that PEEK-blend guides could be a reliable solution to the failures of the PPA guides, Norris offered its PEEK-blend NHT Series Sucker-Rod Guides to this company for testing. In creating the PEEK-blend guides, Norris incorporated manufacturing process improvements such as next-generation injection-molding machines that feature chilled molds, and raw material dryers. These process improvements, as well as tighter quality controls, allowed for reliable production of guides that feature a homogeneous microstructure, which improved tensile strength and heat-resistant capabilities.
Specific features and benefits of the Norris PEEK-blend NHT sucker-rod guides include:
- Reduced tubing wear versus other high-temperature plastics with wear characteristics similar to base PPA material
- A 400°F continuous-service rating
- A 480°F maximum service rating
- Superior chemical resistance in corrosive environments
- Reduced porosity versus conventional rod-guide plastics
- Proven Sidewinder design (higher erodible wear volume with minimal fluid turbulence)
- Injection molded for maximum adhesion to rod body
- Separate hoppers for increased melt temperature, resulting in zero fluid contamination
The results "sPEEK" for themselves
The company installed Norris PEEK-blend NHT guides in 17 test wells. Six wells had alternating guides: one rod with PPA guide material, then one rod with PEEK-blend NHT guide material. The other 11 wells were outfitted with NHT guides from a depth of approximately 7,000 ft. up to the pump.
The results have been spectacular. In the wells outfitted with PEEK-blend Norris guides, nearly all premature sucker-rod guide failures in high-temperature environments were eliminated. No rod strings required replacement since initial installation, and they have been running for more than 750 days, which is already a 50% increase in run time over the PPA guides that were failing prematurely.
The company had three goals for the PEEK-blend Norris guides that replaced the PPA sucker-rod guides: first, outperform the PPA guide in high-temperature environments; second, double the MTBF when compared to the PPA guides; and third, perform as well in a high-temperature application as a PPA guide performs in a low-temperature well. The first two goals were met and the third is in the process of being achieved. These results illustrate unequivocally that Norris NHT sucker-rod guides can outperform and outlast PPA models in high-temperature applications.
Similar results have been experienced elsewhere when PEEK-blend NHT guides have been installed:
- Another South Texas operator had wells with operating conditions of 300°F, 20 ppm H2S, with 150-lb. to 275-lb. predicted side load. After the rod string was pulled at 238 days for pump repair, the string was run in with NHT sucker-rod guides. The result: the NHT guides outperformed reaction-set polymers, PPS, PPS-X-all previously tested conventional organic polymers-and thermoplastic resins in terms of material loss and chemical permeability.
- Another of this operator’s wells, with conditions of 300°F, 140-lb. to 200-lb. predicted side load, and 50 ppm H2S, experienced severe pump tagging and fluid pounding. At 261 days the rod string parted. NHT guides installed and outperformed reaction-set polymers, PPS, PPS-X, and thermoplastic resins in terms of material loss and chemical permeation.
These results gave well operators a substantial return: extended sucker-rod service life and prevention of premature tubing wear caused by guide failure. For example, in a typical Eagle Ford or Bakken shale well, remediation costs that include pulling the tubing string can exceed $100,000. And this does not include the value of lost production. Eliminating these remediation costs and keeping the well producing more than offsets the small additional cost required to purchase premium rod guides.
While some operators prefer to run PEEK-blend guides from top to bottom, costs can be minimized by using PPA guides to the high-temperature zone and then using PEEK-blend guides from there. Either way, operators can eliminate costly well intervention, and keep the well online and producing. In the end, selecting a field-proven sucker-rod guide for high-temperature applications reduces rod-pumping expenses.