Summary

Annular flow is associated with production from both gas-condensate and geothermal wells. Oil wells also experience it during high-gas-to-oil-ratio (high-GOR) production. The current semimechanistic modeling approach requires estimation of film thickness before computing frictional pressure drop as gas flows past the wavy-liquid film surrounding the pipe wall. This study intends to investigate this film thickness and its impact on pressure-drop computation in wellbores producing steam-water, gas-condensate, and gas-oil mixtures.

Computational results show that this dimensionless liquid-film thickness is most likely less than 0.06 in annular flow. For such values of thin-film thickness, the computed friction factor is only slightly higher than that estimated with a smooth-channel assumption. When the homogeneous model is used to compute pressure gradient by ignoring the wavy-liquid film on frictional pressure drop, good agreement is achieved with field data and with the predictions of a semimechanistic model.

Introduction

Annular flow is dominant in gas-condensate and geothermal wells. Oil wells also experience annular flow when high-GOR production occurs after gas breakthrough or when gas lift is installed. In general, the annular-flow pattern consists of a gas core in the middle of the flow string with a thin liquid film flowing up the pipe wall. Two issues appear to dominate the modeling needs. One needs to estimate, first, the liquid entrainment in the gas core, and second, the frictional resistance that the gas core experiences when flowing past the wavy-liquid film. Note that the frictional gradient is a very large contributor to the total pressure loss in annular flow and therefore has obvious importance.

In the past, a few models treated this flow pattern assuming zero slip between the two phases in the gas core. For instance, the models of Duns and Ros (1963) and Aziz et al. (1972), who essentially adopted the Duns and Ros approach, fall into this category. Subsequently, the method of Hasan and Kabir (1988), based on the approach of Wallis (1969), estimates both the entrainment and the film-friction factors. However, the rigorous method of Ansari et al. (1994) is rooted in sound modeling of film thickness followed by accurate estimation of frictional and hydrostatic heads. The same approach was adopted by Kaya et al. (2001). At approximately the same time, Gomez et al. (2000) proposed a method based on a two-fluid approach.

The intent of this study is to present an alternative approach to modeling annular flow. We show that the liquid-film thickness is generally too small to be of any consequence when computed with the model of Ansari et al. (1994). The main objective is to demonstrate the application of a much simpler model with accuracy comparable to a semimechanistic model. In fact, the authors' recent study (Kabir and Hasan 2006) on gas-condensate wells has shed some light on the possibility of simplified modeling of annular flow.

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