To enhance the separation efficiency of downhole oil–water hydrocyclones in co-produced wells, an axial center-piercing hydrocyclone structure is proposed. A mathematical relationship model between the structural parameters and separation efficiency of the axial center-piercing hydrocyclone is constructed based on the response surface methodology. The numerical simulation method is employed to analyze both the unoptimized and optimized hydrocyclone structures, and their separation performances are simulated under identical operational conditions. The results indicate that the optimal structural parameter values are as follows: main diameter D = 70.4 mm, large cone angle α₁ = 32.4°, small cone angle α₂ = 3.9°, bottom flow tube length L₃ = 311.7 mm, and inverted cone length L₄ = 166.0 mm. The optimal operation parameters of the optimized structure are also obtained. Under the same operating parameter conditions, the separation efficiency of the optimized structure is consistently higher than that of the unoptimized structure. The highest efficiency achieved by the optimized structure is 98.6%, which is a 2% improvement over the unoptimized state. Finally, experiments are conducted with the optimized hydrocyclone separator structure under different split ratios. This study significantly contributes to the field of injection and production in a single well, particularly in promoting the application of hydrocyclones.

Investigating the motion of discrete oil droplets in a rotating flow field can provide a theoretical basis for optimizing the flow field and structural parameters of hydrocyclones and centrifugal separation equipment. In the present work, the particle image velocimetry (PIV) method was applied to study the velocity distribution of a three-dimensional axial-rotor-driven rotating flow field and the influence of the velocity distribution of different rotor speeds on the flow field. The radial migration of oil droplets with different particle sizes in the rotating flow field was visually analyzed using high-speed video (HSV). The results showed that the oil droplets with the same radial position had diameters of 2.677 and 4.391 mm, whereas the movement times to the axis were 0.902 and 0.752 s. The larger the oil droplet size, the shorter the time to move to the axial center of the rotating flow field. The radial velocities of oil droplets with diameters of 2.677 and 2.714 mm were 0.0221 and 0.02 m·s⁻¹, respectively. In addition, a mathematical expression was established between the radial migration time and the oil droplet size in the rotating flow field. The accuracy of the proposed expression was verified using experiments.