Abstract: The influence of well liquid flow and coupling effect on rod string is analyzed and the mathematical model of the differential pressure force in the rod and tubing annulus is developed based on the fluid flow characteristics. And the computation of neutral point and axial force is obtained with the limit of well liquid flow. The results show that the coupling effect should be involved due to the ratio of pressure force to rod weight over 11%. The differential pressure forces on rod coupling are resulted by well liquid flow in annulus. And an enhanced product of pumping stroke and speed, an increased dynamic viscosity of well liquid and a reduced area of flow section lead to the increased pressure force. Furthermore, the great impact of pressure force on axial force increases the variation range and imbalance of polished rod load. And the axial force changes greatly with well depth and crank angle. The neutral point obtained with the limit of well liquid flow is increased by 15% and closer to actual length of partial-wear while the errors of results are just less than 3%. Then the range of partial wear for rod string increases significantly.

Key words: Axial force, Differential pressure forces, Neutral point, Sucker rod string, Well liquid flow