Liquid loading, which can lead to rapid gas-rate decline and can even cease gas production, is a common phenomenon found in most mature gas wells. An accurate prediction of the inception of liquid loading is of great interest to operators, for the reason that remedial measures can be applied in a timely manner to prevent such conditions from being realized, thereby extending the production life of the gas well. However, the mechanism that is responsible for liquid loading still remains controversial. In the literature, at least three different definitions of liquid loading exist. The first definition is based on the intersection of inflow and outflow curves, the second definition is based on the reversal of entrained liquid droplets, and the third definition is based on the reversal of liquid film. These definitions yield different results when predicting the inception of liquid loading. In this paper, a new definition of liquid loading is introduced. This new definition is based on the relative contributions of gravity and residual pressure drop, and it is validated by its agreement with air/water experimental data.
A new comprehensive model is developed that is based on the Barnea (1986, 1987) model. For vertical wells, the new model can better predict the inception of liquid loading than the widely used Turner et al. (1969) equation. For deviated wells, it is observed in the field and in laboratories that liquid loading starts much earlier than in vertical wells, and most liquid-loading equations are not appropriate for deviated wells. The new model takes into account the nonuniform film thickness around the circumferential position of the pipe, and, thus, it improves the prediction of liquid loading in deviated wells. The new model is validated through the use of field data in the literature and experimental data obtained at the University of Tulsa. In addition to the literature data, a new set of field data is reported and used to validate the new model, which shows a significant improvement over the droplet model as well as other film models.