Reduced-Friction Centralizers Field-Tested for Ultraextended-Reach-Drilling Completions
Authors: Feder Judy 
Published: November 2019

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 The paper has not been peer reviewed.
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 up-stream 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.