Development and Testing of Low Friction Coated Centralizers for Ultra-ERD Completions
Authors: Wang Lei , Bailey Jeffrey R. , Rajagopalan Srinivasan , Ozekcin Adnan , Prim Matthew 
Published: November 2018

Abstract
Friction can be one of the major limiters in ultra-ERD (Extended-reach drilling) well completions. For some completion strings, friction running into the hole can limit the length of well to be drilled. Centralizers coated with novel Diamond-Like Carbon (DLC) coatings have been developed to provide operational advantages for these ultra-ERD applications. This paper will document the development, laboratory and field testing, and lessons learned from a project to evaluate the coated centralizer opportunity.
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 prior to coating. Next we selected the coating chemistry, layer hierarchy and functionality, and interlayer properties, leveraging 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 novel centralizer with selected coatings were tested against commercially-available centralizers in a lab environment, and field tests were subsequently conducted in two wells. These lab tests were conducted to assess the friction reduction potential of the coated centralizers. Tests were performed to measure the COF (Coefficient of Friction) 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 open hole. In these tests, the DLC coated centralizers yielded the lowest COF overall when compared to existing tools.
With promising lab results, the first field test was conducted in an onshore unconventional horizontal well with high doglegs. In this well, the data showed very low friction over a 580-m (1900-ft) open-hole interval. A second field trial was then 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 4000-m (13-kft) deviated section at 75°, measurably lower than the offset FF (Friction Factor) of 0.15. Both tests therefore demonstrated intervals of very low friction, however both experienced drag that might be the result of cuttings. To address this problem, a redesign has been prototyped based on a slicker centralizer profile. This new centralizer also requires less polishing and offers an inherently hard substrate that is beneficial to the coating durability.
These were the first field applications of DLC coated centralizers. 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. In one effort to advance the industry capability to run casing strings in extended-reach wells, multilayered DLC coatings were successfully applied to cast-iron based centralizers and demonstrated in lab and field tests, and a prototype was developed to improve the centralizer profile.