Experimental Study of Gas-Liquid Two-Phase Flow in a Vertical Wellbore with Reciprocating Pumping Rod

Abstract

Abstract For the rod lift system, the up-and-down reciprocating motion of the sucker rod continuously exerts external forces on the fluid in the wellbore, which intensifies the instability of the gas-liquid two-phase flow. At the same time, due to the presence of the sucker rod, the cross-section of the flow channel in the wellbore is changed from a circular to a rod-pipe annulus, making the fluid more susceptible to the sucker rod motion. In order to study the effect of the reciprocating motion of the sucker rod on multiphase flow, an experimental platform was built to study the pressure and flow pattern changes in the wellbore under various gas-to-liquid ratio and lifting parameters, e.g., number of strokes, gas and liquid injection rate. Observed from the experiments, four flow patterns under the reciprocating motion of the sucker rod were identified: bubbly, slug, churn, and annulus flow. It is the relative motion between the rod and the fluid that intensifies the gas-liquid slip effect, and the collision fusion between bubbles is more frequent. However, when the gas injection rate exceeds a certain limit, the influence of sucker rod motion gradually decreases and the flow pattern is not changing. This paper further investigated the pressure distribution in the wellbore with different number of stokes and gas/liquid injection rates. The analyses indicated that the pressure at different test points showed periodic changes with time. Meanwhile, the higher the number of stoke, the more pronounced the periodicity of pressure, and the pressure fluctuation is highly associated with the up-and-down motion of the sucker rod, i.e., the number of strokes is proportional to the frequency and amplitude of pressure fluctuation. At the position of rod coupling, the chance of collision and fusion between bubbles increases and air slug are formed. Based on the experiment data, the drift-flux model (DFM) is modified for multiphase flow in the rod lift wellbore. Because the relative motion between the rod/coupling and the fluid, the presented DFM in this paper has higher prediction accuracy at low and high-volume fraction of gas compared with the literature DFM for a vertical wellbore. This mechanism study not only helps to optimize lift parameters to maximize pumping efficiency, but also provides experimental data in developing or verifying mathematical gas-liquid two-phase flow model for the rod lift system.