Estudo da perda de carga e troca térmica no escoamento de fluidos Newtonianos e não-Newtonianos em coiled tubing

Abstract

Coiled tubing is a system consisting of a long, continuous and flexible steel pipe used in many processes, especially in the oil industry. Measuring over 6000 meters, part of the pipe is directed to the well from an injector and a rotary table, while the other part remains wrapped in a reel subdivided in many layers. During well abandonment, different fluids, such as water and cement slurry, are pumped through coiled tubing to ensure well isolation and sealing. Fluid flow in curved pipes generates significant energy dissipation compared to a straight pipe, and it is necessary to predict the pressure drop to obtain the pumping pressure used in the operation. The energy dissipation by friction and the heat exchange between the fluid and the environment change the fluid temperature and, consequently, its physicochemical properties, affecting the rheology and the cement setting time. An excess of set retarders is added to the slurry formulation to increase the time required for setting, resulting in increased process time and cost. It is essential to predict pressure drop and heat transfer in coiled tubing in order to optimize fluid formulation and process control. The objective of this work was to experimentally and mathematically evaluate the flow of Newtonian and non-Newtonian fluids in coiled tubing in order to simulate the pressure and temperature profile along the tube. Filtered water and an aqueous solution of xanthan gum, with a rheological behavior similar to a cement slurry, were used in experimental tests. The experimental unit is 375 meters long divided into 8 layers with pressure and temperature measurements in each layer. Fluids were pumped at different volumetric flow rates, curvature ratios and initial temperature. Mathematical models were proposed to calculate pressure drop and heat transfer, considering permanent and transient regime. Based on experimental results obtained in the laboratory, the parameters of a friction factor correlation for non-Newtonian fluids present in the literature were reestimated. To predict the temperature variation as a function of time and length, an energy balance was proposed considering heat transfer by friction and the heat transfer of the fluid with the pipe and the environment. Parameter modeling and estimation were solved using FORTRAN language, based on laboratory and field data. The partial differential equations were spatially discretized using the finite volume technique. Additionally, the DASSL integrator was used to obtain the dynamic solution of the set of equations. The mathematical modeling assumes the existence of a sequential pumping of different fluids along the coiled tubing, with different curvature ratios and internal transverse flow area. A case study was conducted, considering experimental data obtained from well abandonment operations with a real coiled tubing system. The developed software was used to simulate the real conditions, where a good approximation to the field data was observed, with a percentage error between the experimental and calculated values below 7%.