The requirement for intervention operations in extended-reach wells continues to grow. It is estimated that globally around 30-40% of the end sections of the extended-reach wells are inaccessible by the current coiled tubing (CT) friction reduction technologies, such as lubricants, vibratory tools, and tractors. Although many of the extended-reach wells are open-hole, there is a lack of understanding in the industry regarding the predictable and consistent friction reduction performance at downhole conditions of the existing CT technologies in those open-hole wells.

Conventional friction reduction techniques for CT operations have been focused around mechanical or chemical methods for cased wells. For instance, following an extensive laboratory testing research program, a lubricant was recently developed that lowers the CT coefficient of friction between 40 and 60% in new, clean wells (Livescu et al. 2014; Livescu and Craig 2015; Livescu and Craig 2018). Friction reduction of this magnitude roughly translates in doubling the CT lateral reach. However, the friction reduction performance of lubricants is diminished in wells filled with sand of proppant. In addition, very limited studies are available for open-hole wells. To reach the remaining 30-40% un-reachable length of open-hole wells and cased-wells with sand or proppant, lubricants are required to work in conjunction with other technologies such as vibratory tools and tractors.

The instrument previously used for metal-on-metal friction reduction research was modified to mimic the downhole conditions of CT sliding movement in open-hole wells and cased-hole wells with sand or proppant. That is, the coefficients of friction between the CT metal surface and the non-metal surface of a rock and sand or proppant layer can be measured. This instrument was designed for researching the effects of temperature, pressure, CT sliding speed, surface roughness, and fluid composition on the coefficient of friction. For clean cased-hole wells, the effects of pressure and sliding speed were weak in the laboratory tests, while the effects of temperature, surface roughness, and fluid type and composition were strong. For the friction reduction in open-hole wells, several rock samples taken from formations and reservoirs with different properties, such as porosity, permeability, pore size, etc., were used. The tests were performed with several CT coupons of different grades and both proprietary and third-party lubricants, to better understand the factors affecting the lubricity in open-hole wells. It was found that, at downhole conditions, the friction performance of the lubricant previously developed decreases from 40-60% for cased wells to 30-40% for open-hole wells.

This is the first study available in literature consisting of laboratory friction tests performed with lubricants to mimic the CT operations in open-hole wells and sand/proppant-filled cased-hole wells. Detailing the testing procedures and results are of significant help to the industry for understanding the downhole factors affecting the CT friction in extended-reach open-hole wells and for obtaining predictable and consistent friction reduction results for CT operations in those wells.

You can access this article if you purchase or spend a download.