Coiled Tubing Friction in Extended-Reach Wells

Abstract The lateral reach and residual bottom hole assembly (BHA) loads in extended-reach wells strongly depend on the coiled tubing (CT) mechanical friction. Detailed CT friction modeling becomes crucial in the pre-job planning stage to ensure successful job predictability. However, current numerical simulators consider constant coefficients of friction (CoFs) that are determined from similar operations without taking into account the effects of the operational and downhole parameters on the CoF for a specific operation. This study outlines the modeling of CT friction force, CoF, and axial BHA loads depending on the operational and downhole parameters when a fluid hammer tool is used. Recent theoretical, laboratory, and field data have established how CoF depends on the downhole parameters (Livescu and Watkins 2014; Livescu et al. 2014; Livescu and Craig 2015). Previously, these effects have not been considered in the CT numerical models, leading to significant CoF differences among available commercial simulators. For instance, the default CoFs in the current pre-job simulations for cased holes, when no lubricant or friction reducing tools such as fluid hammer tools and tractors are used, vary between 0.24 and 0.30 or even higher. This makes it extremely difficult to consistently evaluate and compare the friction reduction effects of lubricants, fluid hammer tools, and tractors in extended-reach wells, especially when the field operator may be consulting with several service companies utilizing different commercial force modelling software. This study presents the CT force matching and fundamental physics based on modeled fluid forces, including radial forces, drag forces, and most importantly pressure forces on the CT friction forces due to fluid hammer tools. Extending the Method of Characteristics, regularly used for studying pressure pulses in straight pipes, the Perturbations Method also accounts for the helical shape of the CT. The new CT fluid hammer model is validated against laboratory data. This rigorous method for calculating the axial BHA load and reduced CT friction force due to radial vibrations can be easily implemented in currently available tubing force analysis (TFA) software for CT operations. This novel approach, which uses detailed CT mechanical friction modeling to take into account parameters such as temperature, internal pressure, pumping rate, etc., improves predictions for CT reach in lateral wells. These findings broaden the current industry understanding of the CT mechanical friction modeling in extended-reach wells and show benefits for the industry when considering variable friction modeling in commercial CT simulators.