The Turner model is widely used in industry to estimate critical gas velocity to flow a gas well and unload its liquid content under steady state conditions. The Zhou model introduced improvements to the Turner model by taking into account the influence of total Liquid (condensate and water) to Gas Ratio (LGR) on critical gas velocity. While fairly acceptable at low LGR, current models do not address the impact of liquid holdup on wellhead flowing conditions and subsequent changes in critical gas unloading rates. Multiphase modeling is used in this study to validate the applicability of current models at various wellbore conditions and LGR.
This study finds important applications in offshore and onshore gas field developments because it provides moe reliable assessment especially for gas fields in the depletion phase, or when liquid breakthrough occurs resulting in high LGR.
Dynamic simulations indicate that at low to moderate LGR existing models under predict critical gas flowrate because they under estimate critical velocity, especially at high wellhead pressures, and don't take into account the impact of increasing liquid holdup on gas flowrate. Moreover, an inversion in critical flowrate occurs at very high LGR because the film holdup is sufficient to restrict the flow of gas and offsets any increase in critical velocity at such high LGR. The onset of liquid loading (well choking) is associated with the transition from annular to churn/slug flow. This is well demonstrated from the calculated trends of entrained droplet holdup.
The significance of the current work to our understanding of critical flow in gas wells is illustrated by utilizing a multiphase simulator to better characterize the impact of entrained droplet and film holdup on critical flowrate and by predicting the inversion in critical gas flowrate at high LGR. The results of this study provide an enhanced understanding of well loading during all development phases and various production conditions to evaluate the applicability and accuracy of widely used models in a broad range of well conditions and liquid loads.