A range of mathematical models and correlations is used to estimate the pressure drop for co-current two-phase flows in vertical wells in the conventional oil and gas industry. However, in the annulus between casing and tubing of a coal seam gas (CSG) well, the upward flow of gas and downward flow of water results in counter-current two-phase flows. The flow regimes developed in such a counter-current system are noticeably different to co-current flow regimes, and thus the existing models used to predict pressure profiles in co-current wells do not adequately describe two phase flows in a (pumped) CSG well.
In this study, we modified existing mechanistic models for co-current flow and counter-current flow in a pipe to predict liquid holdup and pressure profiles of counter-current flows in vertical annuli for the slug flow, which is the dominant flow regime. A model, based on the work of Taitel and Barnea (1983), was also developed to predict the transition from slug flow to annular flow in counter-current flows in annuli. Our comparison of the pressure profiles of co-current and counter-current flows in annuli for the slug flow regime indicates that the pressure loss of counter-current flows could be appreciably different to that in co-current flows under the same conditions. This highlights the need to modify the models that are currently applied in typical commercial well flow simulators to better predict the pressure drop across CSG wells.