Shale plays usually require artificial lift systems, mostly because of rapid depletion rates during the first year. Sucker rods with more resistant connections that overcome the limitations of conventional designs provide numerous advantages in unconventional wells that use beam pumping.

In the first stages of oil production, oil and gas flow easily from the reservoir to the surface. The well’s high reservoir energy ensures high production rates. However, pressure soon and rapidly begins to fall, making it necessary to resort to artificial lift systems. For a period of time, transitional methods are enough to keep production going at acceptable levels, but beam pumping soon becomes the best option.

Beam pumping applications for shale wells

Shale operations have been consistently growing in number in recent years, posing increasing challenges such as deeper wells, deviated wellbore geometries, corrosive environments and higher production rates.

In this scenario, beam pumping applications can offer many advantages. They are a flexible form of artificial lift as well as a cost-effective method able to handle different well conditions since they can be used in a wide variety of production rates and depths.

To design a beam pumping application, operators must take into consideration the costs as well as the expected run life of the system as a whole. A beam pumping installation for shale wells is typically characterized by:

  • Tubing size: 23/8 in. or 27/8 in.;
  • Pump plunger diameter: up to 1¾ in.;
  • Rodstrings: 86 (1-in. SH-7/8-3/4) or 76 (7/8 SH-3/4);
  • Rod steel grades: D or D Special (due to H2S presence);
  • Pump depth: 2,134 m to 3,048 m (7,000 ft to 10,000 ft); and
  • Additional loads due to well deviation and use of guided rods.

Sucker rod challenges

Beam pumping applications pose several challenges for sucker rods, strongly increasing the risk of pumping failures.

Due to their characteristics, some shale installations require 23/8-in. diameter tubing strings. This impacts the size of the sucker rods that can be installed in the column since 1-in. rods do not fit inside this size of tubing. Diameters no larger than 7/8 in. with slimhole couplings can be used.

To maintain the pumping and production capabilities of the system, rods must be able to provide maximum tolerance to cycle loads. API rods are limited in this capacity, so high-strength rods are usually required. However, corrosive environments make it difficult to use high steel grades as their mechanical properties increase the risk of facing body failures due to H2S or CO2 corrosion.

Shale wells present deviated wellbore geometries and severe doglegs. Pumps must be set beyond the kickoff point (KOP), and this increases tubing-rod contact force. To ensure the required rod and tubing life, molded guides must be included in certain sections of the string with a very severe dogleg or with a constant inclination. Conventional rods do not have the increased fatigue resistance required to withstand the extra axial load generated in the pumping cycle by the contact of the guides with the tubing.

Rods with standard connections frequently back off, are easily over- or under-torqued, and present high stress concentration areas because they distribute stress unevenly along the thread profile.

In addition, API connections make it necessary to oversize beam pumping installations in every level because of the deeper depths and common well conditions of shale wells. This situation causes several difficulties, including a heavier rodstring.

New design for high loads

Tenaris’s BlueRod premium sucker rods are especially suitable for shale operations that require beam pumping. The rod connection was particularly designed for high loads, improving field performance as well as the rod’s fatigue life. Therefore, they maximize the results of rod pumping systems applications.

Main features include the cut-tapered trapezium profile thread with diametrical interference, which reduces the pre-tension in the pin makeup. The flank-to-flank contact eliminates the gap that is present in the conventional thread profile and increases the interference level, reducing the tendency to loosen. The lower displacement during makeup and uniform contact between the flanks ensures a better stress distribution and a reduction in the permanent deformations created in threads during both makeup and operation. As they were developed for high loads, they provide improved performance in wells facing this challenge.

Tenaris BlueRod premium sucker rods offer the following benefits:

  • Higher working capacity of the rod pumping system to operations that are normally restricted to electric submersible pumps;
  • Improved performance in high-load operations;
  • Reduced stress level and energy consumption in the pumpjack, as small diameter sucker rods reduce the rodstring’s weight;
  • Improved performance in mild corrosive conditions with extreme mechanical demands, given that the BlueRod premium sucker rods can be manufactured with 4320 KD steel grade, a softer steel with better toughness and improved fatigue corrosion behavior; and
  • Reduced number of sucker rod failures and therefore lower need of workover operations.

Premium rod connections allow operators to design lighter rodstrings composed of sucker rods with smaller diameters. This ensures additional benefits. For example, there is still a chance to use regular pumping units if the design is appropriate, performance improves in high-load operations, and lighter rodstrings reduce the stress level and energy consumption in the pumpjack.

Special tools are not required for handling and makeup operations with Tenaris BlueRod premium sucker rods since conventional power sucker rod tongs can be used. In addition, Tenaris produces all the accessories required for using this product, including reductions and crossovers.

Proven technology

Tenaris BlueRod premium sucker rods already have a history of success. A major U.S. operator chose this technology for a 200-well project in the Eagle Ford Shale. Depletion rates in the Eagle Ford Shale are high. After the first year liquid production tends to drop below 350 bbl/d, so beam pumping is commonly installed.

Tenaris analyzed a representative group of wells from this field to extract conclusions that might help oil and gas companies better choose products for their shale operations. As an example, a representative shale oil well with production requirements of 245 bbl/d and a pump depth of 1,646 m (5,400 ft) was analyzed. The tubing diameter was 23/8 in., and the pumping unit available was a 640 Mark II with a stroke length of 168 in. This 76 guided design was 90% loaded with premium connection Modified Goodman rating with a service factor of 1, what is analogous to a load of 170% Modified Goodman rating of an API D string, SF1. This installation has been running for more than a year without rod failures.

Among the sample of wells analyzed, H2S concentration found in gaseous face was from 800 ppm up to 2,100 ppm. More than 90% of the premium connection sucker rodstrings installed were 4320 KD grade, and they showed an improved fatigue corrosion behavior.

About 96% of the strings analyzed were guided, a condition that increases the drag force and thus the maximum axial load in the pumping cycle, placing API rodstrings above their fatigue resistance. BlueRod premium sucker rods are designed to overcome this challenge.

The maximum depth for a 13/4-in. pump was 3,353 m (11,000 ft) pumping 170 bbl/d. Due to the higher depths and the common shale well conditions, the API connection design sets limits to the beam pumping system, forcing engineers to oversize their installations in every level of the system. Using those limits, Tenaris compared an 86 type API D rodstring with a 76 premium rodstring for the same production requirements.

This experience showed some of the benefits of working with premium connection rods. API D rodstrings would be working at 100% of Goodman ratio when a 76 premium connections rodstring would be working at 80% of its total capacity. A 76 premium connection rod design allows the use of a 23/8-in. tubing by running 7/8-in. with slimhole couplings. In addition, premium rod designs provide a rodstring that is 16% to 20% lighter. Tenaris observed that more than 50% of the pumping units installed with premium connection rodstrings were 640s.

Finally, in those cases where the depth or rate forced the operator to choose an 86 design, premium connection sucker rods offered extra strength using 4320 KD steel with the advantage of providing mild corrosion protection.