Effects of Solids Loading on Drag Reduction in Polymeric Drilling Fluids Through Straight and Coiled Tubing
Authors: Shah S.N. , Tareen M. , Clark D. 
Published: March 2010

Abstract
An experimental study has been conducted to evaluate the effects of solids on the drag reduction characteristics of polymeric drilling fluids, loaded with solids, through straight and coiled tubing. The polymers investigated were PHPA and XCD. These polymers were dissolved in brines which were prepared by mixing potassium chloride (KCl) and sodium formate (Naformate) in water. Various concentrations of polysaccharide gum (XCD) and partially hydrolyzed polyacrylamide (PHPA) polymers were dissolved in the KCl and Na-formate solutions. These fluids were pumped through straight pipes. Based on the results, it was established that the PHPA/KCl solution exhibited better drag reduction than PHPA/Na-formate, XCD/KCl, and XCD/Na-formate solutions in straight pipes. Thus, the PHPA/KCl solution was selected for further study.
This polymer solution was then loaded with barite and bentonite to simulate solid cuttings. The fluid system was pumped through different sized coiled tubing and straight pipes. A correlation in terms of Fanning friction factor and generalized Reynolds number was developed, and percentage drag reduction was calculated. It was observed that, when clear polymeric solutions were loaded with solids, drag reduction decreases drastically. Finally, the effect of curvature on the flow of solidladen fluids was studied. It was observed that frictional losses in coiled tubing were almost twice those of frictional losses in straight tubing for the solid-laden polymeric fluids under investigation.

Introduction
In the past, various studies have been conducted on the flow of clean Newtonian and non-Newtonian fluids through straight pipes in turbulent flow. For the flow of Newtonian fluids through smooth pipes, von Karman(1), Nikuradse(2), and Prandtl(3) suggested correlations, but the most commonly used correlation was proposed by Drew et al.(4). Their correlation is valid for Reynolds numbers between 3,000 and 3,000,000. Similarly, for Newtonian fluids flowing through rough pipes in a turbulent flow regime, Moody(5), Nikuradse(2), Churchill(6), and Chen(7) developed correlations. Among these, Chen's correlation is the most widely used in the industry, as Chen's correlation is explicit for Fanning friction factor and correlates the Fanning friction factor, pipe roughness, pipe diameter, and Reynolds number. Although non- Newtonian fluids have not been studied as often as Newtonian fluids, they have been under intense examination for some time for flow effects through smooth and rough pipes. Dodge and Metzner(8), Melton and Malone(9), and Shah(10) presented extensive theoretical and experimental studies. Similarly for non- Newtonian fluids through rough pipes, Dodge(11) and Shah(10) correlations are widely used in the industry.
As shown, numerous studies have been conducted in the past, especially in the case of Newtonian fluids. However, no study could be found in the literature regarding the effects of solids on the flow of non-Newtonian fluids through straight or coiled tubing. The objective of this project was to study the effects of solids loading on the drag reduction characteristics of commonly used polymeric drilling fluids flowing through straight and coiled tubing. This was achieved by first studying the polymeric fluids flowing through straight pipe without solids.