The Study On The Stability Of Continuous Gas-Lift Wells

Gas lift is one of artificial lift methods, which has been used for the longest time in oil industry. Due to the characteristics of multiphase flow, the production of gas lift wells is often unstable, i.e. the pressure at any point in the tubing cyclically changes. Unstable production can result in poor separation of gas and liquid, wells shutdown and other problems, thus affecting the normal production of gas lift wells. In addition, once the gas lift wells with unstable production, it is inevitably accompanied with the loss of productivity. Oil-fields usually take measures to achieve the expected recovery ratio by extending the final production work, which would increase the cost of the work and reduce the economic benefits. In order to improve the economic efficiency of the oil-fields and make gas lift production safer and more efficient, it is necessary to analyze the stability of the gas lift and find the solution of the instability problem.Firstly, the instability of gas lift is theoretically analyzed, finding the cause of instability of gas lift, and understanding the current types of instability(casing heading and density wave).Then, selecting a suitable criterion which can judge stability on both casing heading and density wave, applying MATLAB to design its programs, and analyzing these two types of unstable gas lift wells with practical simulation. Finally, practical analysis results using MATLAB are verified by multiphase flow dynamic simulator OLGA7.0, and the solution of instability problem can be obtained from them.The main results of this study are:(1) Similar sensitivity curves of the casing heading instability can be obtained by simulation analyses using MATLAB and dynamic simulatorOLGA7.0: increasing gas injection and production index, reducing the choking, the tubing diameter, casing diameter and orifice diameter can improve the stability of the gas lift wells, but increasing the separator pressure have no stabilizing effect. This result is consistent with the previous studies, indicating that the two methods are reliable and can be used as a tool to study the instability of density-wave.(2) Increasing the production index, tubing diameter, separator pressure and depth will increase the instability of density-wave, but reducing choking has no big effect on stabilizing. It is worth noting that the results of increasing the production index and adjusting choke to the two stability types are inconsistent. Before taking measures, we need to classify the type of instability, in order to avoid having no stabilizing effect.(3) According to the dynamic simulation results, it is proved that the linear analysis method can be used as new judgment of the instability of gas lift wells, but by contrasting the data, we find that the linear method is relatively conservative, stable operating can be judged unstable, so when it is applied to field cases, we need to fit a correction coefficient to increase its accuracy. The instability problem of gas lift can be basically diagnosed and soluted by the results of this study, which has a certain significance to the on-site operation of oil-field.