Effect of Coiled Tubing Buckling on Flow Characteristics of Annular Flow of a Non-Newtonian Drilling Fluid: Comparison of Numerical Calculations with Experiments

Abstract Coiled tubing (CT) technology has become increasingly integral to modern oilfield operations, including well intervention, logging, drilling, perforation, and fishing. Accurate prediction of frictional pressure losses in the annulus is critical for determining pump horsepower requirements and for designing efficient hydraulic programs. However, tubing buckling—particularly within the annular space—can significantly alter flow behavior and complicate hydraulic analysis. This study presents a comprehensive numerical and experimental investigation of steady-state, isothermal, laminar, transitional, and turbulent flow of non-Newtonian (Herschel–Bulkley) fluids in an annulus with a non-rotating, buckled inner tubing string. The research focuses on the effect of tubular buckling on horizontal annular flow using three complementary approaches: computational fluid dynamics (CFD) simulations, laboratory experiments conducted on a custom-built flow loop, and a narrow-slot analytical model. A helical buckling configuration—commonly observed near the lower section of coiled tubing strings—was modeled. CFD simulations were performed using ANSYS Fluent (2020 R2), which solves the Navier–Stokes equations via the finite volume method to determine frictional pressure losses and velocity distributions. Experimental tests were conducted under laminar, transitional, and turbulent regimes for non-Newtonian fluids across various buckling geometries, including sinusoidal, transitional, and helical configurations, to validate the numerical predictions. Both CFD and experimental results demonstrated that frictional pressure losses decrease with increasing axial compressive load on the inner pipe. This reduction was most pronounced for fluids exhibiting higher yield stress and stronger shear-thinning behavior, while the effect was minimal for fluids with low yield stress, low consistency index, and high flow behavior index—particularly under laminar flow conditions. The developed CFD model showed strong agreement with the experimental data, confirming its capability to reliably predict hydraulic performance in buckled annular geometries. The insights from this research enhance understanding of fluid dynamics around helically buckled coiled tubing and have direct applications in optimizing bottom-hole pressure control, improving bit hydraulics, refining drilling fluid programs, and guiding efficient pump selection for coiled tubing operations.