A new openhole multistage hydraulic fracturing system and the ball plug motion in a horizontal pipe

Gas well stimulation based on a ball-drop fracturing system is a widely adopted operation in the oil & gas industry. The flow resistance of a multistage fracturing pipe string increases as the number of stages grows, which is a major problem for the traditional graduated ball-drop multistage fracturing system, as it can increase the load of the pump and may ultimately result in a high-cost operation. A new openhole multistage hydraulic fracturing system, which has unlimited multistages, activated by ball plugs is proposed to improve the operation efficiency. In this system, all the downhole sliding sleeves are a single size, and a group of ball plugs activate them to achieve full-inner-diameter pipe access. As a key part of the fracturing system, the approximate cylinder-shaped ball plugs would suffer from a larger resistive force in the horizontal pipe than that of the sphere-shaped fracturing ball. Therefore, a simplified miniature indoor experiment setup is built to investigate the motion of the ball plug. The physical dimensions of the cylinder models and the velocities of the pumping flow vary in the experiment. In addition, the motion in the cylinder models are observed with a high-speed camera. Finally, the experimental data are analysed by a combination of dimensional analysis and the empirical equation method, and the results can help to determine the pumping rate during operations.