Numerical Simulation for Ball Passing Capacity in Coiled Tubing

Abstract

To prevent a ball becoming stuck in coiled tubing, it is critical to determine the flow velocity of fluid in a hydraulic drive ball-off. The computational fluid dynamics method and standard k-ε turbulence model were applied to simulate the flow within the tubing of a section of coiling block. By analyzing the flow around the ball, the force on the ball exerted by fluid was computed, and the movement trend of the ball at different locations obtained by steady simulation. With the dynamic grid technique, the movement of the ball was identified through transient simulation. The research showed that: the fluid force through static simulation is too large to be used for judging the ball's passing capacity in a section of coiling block. The ball moved around the coiling block with a minimum value of θ = 90°, revealing that the ball could easily reach the highest point if the minimal angular velocity of ball was greater than zero. Moreover, the conclusion that the ball could pass through the whole section of the coiling block if only it passed the first circle of the coil could be drawn. Furthermore, the critical flow velocity for different sized ball to pass through the coiled tubing increased monotonically with the increase in the ball diameter, and exhibited an approximate quadratic relation. When the curvature radius of the coiling block was 1.268 m and the diameter of the solid steel ball was in the range of (30 mm, 50 mm), the critical flow velocity was (2.30 m/s, 2.415 m/s). This study provides a useful basis for experiment design.