Prediction Of Liquid Accumulation In Pipes With Low Liquid Loading

Liquid accumulation,which commonly occurs in gas wellbore and undulating wet gas gathering pipeline,is one of the severe operating problems in natural gas fields.The liquid accumulation seriously affects the normal production of gas wells and the safety of gathering system.Accurate prediction of critical gas velocities for the continuous liquid removal from pipes helps normal production of gas wells,design and operation of gathering system,and warns operators to take the countermeasures timely.However,the mechanism of initiation of liquid loading still remains controversial.Models of liquid loading,including the minimum pressure gradient model,the liquid-droplet model and the liquid-film model,and experimental studies are reviewed,and it's believed that the liquid-film flow reversal is the controlling cause of liquid accumulation near the wall of pipe.According to the velocity profiles within the liquid film at different gas-flow-rates,the zero liquid-wall shear stress is determined as the inception of liquid loading.Annular-mist regime is the dominant flow pattern in gas wellbore and near-vertical wet gas pipeline.In order to verify the correctness of zero liquid-wall shear stress model and in consideration of the pipe diameter,liquid flow rate and liquid-droplet entrainment,a new mechanistic model to predict the critical gas velocity of liquid loading for vertical pipes is firstly developed.Predicted liquid-loading points correspond to the minimum interfacial shear stress exectly,which coincides with the liquid-film model based on the minimum interfacial shear stress.The new model and other existing models are compared through indoor experimental data of Skopich,Guner and field operating data from Turner,Coleman.The new model performs the best among them.The new model is then extended to inclined pipes because of the accuracy of the prediction results for vertical pipes.In consideration of the non-uniform circumferential film thickness in inclined pipes,the modified prediction model for entire deviated pipes with different diameters and liquid flow rates is proposed successfully.The new model and other existing models,such as Turner model,Belfroid model,Barnea model,Luo model,Shekhar model and Zabaras model,are also compared through indoor experimental data of Guner,Alsaadi and field operating data from Veeken.According to the statistical prediction errors,the new model based on zero liquid-wall shear stress predicts the critical gas velocities of liquid loading more precisely.