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Reduced-Friction Centralizers Field-Tested for Ultraextended-Reach-Drilling Completions

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Friction can pose major limitations on well length in ultraextended-reach-drilling (ultra‑ERD) well completions. Centralizers coated with diamond-like carbon (DLC) coatings have been developed to provide operational advantages for these ultra-ERD applications. While drilling, pipe can be rotated to release friction, but in completion operations this may not be possible. Although pipe can be air-filled to provide buoyancy, there are many examples of screens and perforated pipe that cannot be floated. The complete paper documents the development, laboratory- and field testing, and lessons learned from a project to evaluate coated centralizers.

Project Summary

The project began with evaluation of currently available centralizers made of a hard material to support the thin, low-friction-coating layer. This led to selection of an inexpensive cast-iron centralizer that required machining and polishing before coating. Next, the project team selected the coating chemistry, layer hierarchy and functionality, and interlayer properties, applying knowledge from an artificial-lift application. Multiple coating compositions had previously been evaluated for friction and wear characteristics in block-on-ring laboratory tests.

Prototypes of the new ­centralizer with selected coatings were tested against commercially available centralizers in a laboratory environment to assess the friction-reduction potential of the coated centralizers. Tests were performed to measure the coefficient of friction (COF) of the coated centralizers against other types of uncoated tools made of cast iron, zinc alloy, pressed spring steel, and polymer under wet conditions in simulated cased and openhole conditions. The DLC-coated centralizers yielded the lowest COF overall when compared with existing tools.

Following the promising laboratory results, the first field test was conducted in an onshore, unconventional horizontal well with high doglegs, for which the data showed very low friction over a 1,900‑ft openhole interval. A second field trial was conducted in an offshore ERD well in a liner-running application. In this test, a stable low friction factor of 0.10 was observed over a 13,000-ft deviated section at 75°, measurably lower than the offset friction factor (FF) of 0.15. Both tests demonstrated intervals of very low friction; however, both experienced drag that might be the result of cuttings. To address this problem, a redesign based on a slicker centralizer profile has been prototyped. This new centralizer also requires less polishing and offers an inherently hard substrate that is beneficial to coating durability.

Introduction

An idea to use the low-friction properties of diamond-like surfaces in upstream applications took root more than 10 years ago. Through various cycles of planning, testing, development, and field trials, the evaluation of multilayered thin-film coatings progressed from concept development to laboratory testing and, ultimately, field trials. One application to sucker rod couplings has progressed to early commercialization. This application primarily benefits from the wear resistance of the DLC coating and uses “counterface friendliness” by which the coating protects the counterface surface from wear.

ERD was identified as an application area for these coatings because of the low friction they exhibit. Tests in 2011 on hardbanding of drilling tools showed that the rotary-drilling-application space may still be a stretch for this technology because of application-process requirements and perceived wear performance. Test subs were used in a well to evaluate coating longevity. The subs were located in the heavyweight transition pipe between the collars and drillstring. In one sub, a few slivers of commercial coating remained after 93,000 ft of total travel (rotation plus translation). Three other subs showed no remaining coating, but this was a harder coating applied to the harder of two hardbanding materials. This test showed the importance of the substrate hardness.

Tests were repeated under similar conditions in 2013 with substantially improved coatings. Coating retention increased to 95–99% after similar travel distances that averaged 100,000 ft. It was realized that this total travel distance might be sufficient for casing-running applications, even if greater wear resistance would need to be demonstrated for a drilling application.

Running casing and liners in the hole can be challenged by friction. When friction can be rotated out of the system, there may be little residual friction benefit from the coatings. Some completions—for example perforated liners and sand-control screens—may not be rotated or floated. For these applications, a low-friction centralizer may be beneficial. Today, the industry has several product offerings purported to lower pipe-running friction, and one objective of the development program described in the paper was to benchmark coated centralizers against some of these other products.

The initial research effort that began several years ago uncovered the possible benefits of multilayered coating systems, but the technology and applications required some development effort. Requirements for both interface strength and coating toughness could best be satisfied by optimizing layer chemistry and composition and by including multiple layers optimized for various functionalities. Compositional grading is also created across interfaces to provide multiple functionalities, including the primary benefit of enhanced interfacial strength. The development of higher-durability coatings was a combined exercise in contact mechanics, tribology, and materials design. Testing has indicated that the overall abrasive wear performance of the coating increased approximately 100‑fold from the first commercial coatings tested to the newer coatings.

Mechanical properties of materials, specifically multiphase and multilayered systems, are almost always determined by structure and composition at the nanoscale. Often, features such as chemistry, structure, interfaces, and boundaries govern the overall fracture behavior—an essential macroscopic mechanical property requirement—in such materials. It is essential to develop capabilities that allow for determination of properties (e.g., fracture toughness, interface strength) with very high site specificity to enable rational design of materials for enhanced performance. It is also necessary to engineer coating architectures to be able to suppress lifetime-limiting failure modes, key among these catastrophic delamination or spallation of the coating, which relates directly to the fracture toughness. The overall fracture toughness of the coating, while driven by toughness within individual layers, is likely more dependent upon the fracture toughness and strength at individual layer interfaces, which typically are the weak links in such layered systems.

To support this research, a nanoscale cantilever indentation technique was developed. These microbeams were subjected to destructive-force-displacement testing to determine material properties and failure response at the nano-micro level. This methodology was used to determine interlayer and intralayer fracture toughness values in multilayer coating architectures with the eventual goal of maximizing layer durability and performance.

The complete paper discusses and illustrates block-on-ring testing and fabrication of the DLC-coated centralizers

Laboratory Coefficient of Friction Test

A centralizer drag test was conducted to compare the COF of coated centralizers with those of cast iron, zinc alloy, polymer, pressed steel, and other centralizer types. The complete paper details the testing procedure and presents a compilation of COF comparison results for seven different types of centralizers under four different running conditions.

According to the laboratory-test results, the DLC coated centralizers demonstrated at least 50% COF reduction compared with the uncoated centralizers in the simulated openhole conditions (i.e., pulling centralizers across wet sandpaper surfaces). The results suggest that DLC-coated centralizers might enhance operation running margin and possibly increase the reach of ultra-ERD wells for running casing, screens, and liners in long openhole sections without rotation or flotation. However, it is important to reiterate that this conclusion holds for the laboratory-test conditions and may not fully extend to actual downhole conditions because of recognized limitations of the laboratory-scale tests, including the following:

  • No wellbore pattern or tortuosity
  • No filter cake
  • No cutting beds or solids
  • No temperature effect
  • 50-lb side force per centralizer is much lower than typical contact side force values in wells

Field Tests

The complete paper describes, illustrates, and summarizes the results of two field tests to confirm measurable friction reduction in actual practice: one for running casing in an unconventional land well (Fig. 1), and another for liner running in an offshore well. Hookload data and model results show that wells had sections of very low resistance as the casing was run in hole. In the horizontal well, extremely low resistance was seen running in the angle build section, with very low loss of hookload. Further in the lateral, the resistance increased and pipe rotation was required at approximately the same depth as in offset wells.

Fig. 1—(a) Well path with an openhole section of 1,500-ft build and turn followed by 8,500-ft lateral, and (b) field installation of coated centralizers.

Next Steps

Data from the two field trials suggest that more than just sliding friction likely impeded the string descent to bottom. The blade profile and inclination angle relative to the borehole axis may have led to some ploughing of cuttings in open hole, and potentially greater force required to overcome vertical steps and ridges on the open borehole surface.

Subsequently, work began on new prototype centralizers manufactured from a harder spring steel than the standard product to improve substrate hardness before coating. The redesigned prototype tool has fewer manufacturing steps and higher specifications for substrate hardness, low porosity, and low roughness. It is expected that this device could maneuver cuttings beds and ledges more readily than the version that was used in the field tests. The next step in this program is to manufacture and field-test the redesigned DLC-coated centralizer for an ERD application.

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 193342,“Development and Testing of Low-Friction-Coated Centralizers for Ultra-ERD Completions,” by Lei Wang, Jeffrey R. Bailey, SPE, Srinivasan Rajagopalan, Adnan Ozekcin, and Matthew Prim, ExxonMobil, prepared for the 2018 Abu Dhabi International Petroleum Exhibition and Conference, 12–15 November, Abu Dhabi. The paper has not been peer reviewed.

Reduced-Friction Centralizers Field-Tested for Ultraextended-Reach-Drilling Completions

01 November 2019

Volume: 71 | Issue: 11

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