Developing new mechanistic models for predicting pressure gradient in coal bed methane wells

•Developed new mechanistic models to predict pressure gradient of bubble and slug regimes in counter-current flows in annuli.•Proposed a new model to predict the transition from slug-annular regimes following the approach of Taitel and Barnea (1983).•Showed the predicted dp/dz of counter-current flow is appreciably different to that in co-current flow at high flow rates.•Showed that the available models in industry simulators overestimate the dp/dz of counter-current flows in CBM wells.

A range of mathematical models and correlations is used to estimate the pressure drop of coal bed methane (CBM) or coal seam gas (CSG) wells. These correlations were originally developed for co-current two-phase flows in conventional wells in the oil and gas industry. However, the upward flow of gas and downward flow of water in the annulus between casing and tubing of a CSG well results in counter-current two-phase flows. The flow regimes developed in counter-current two-phase flows in annuli are noticeably different to co-current two-phase flow regimes in pipes, and thus the existing models used to predict pressure profiles in co-current wells do not adequately describe two phase flows in a CSG well.In this study, we develop new mechanistic models for predicting holdups and pressure gradients of counter-current bubble and slug flows in vertical annuli following the existing models of co-current and counter-current flows in annuli and pipes. A model based on the work of Taitel and Barnea (Taitel and Barnea, 1983) was also developed to predict the transition from slug to annular flow regime in counter-current flows in annuli. Our comparison of the pressure gradients of co-current and counter-current flows in annuli shows that the pressure gradients of counter-current flows are appreciably different to those in co-current flows under the same conditions at high liquid flow rates. This indicates that the models currently employed in typical commercial well flow simulators may considerably overestimate the pressure gradient across a CSG well.