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.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
31 May 2006
- Revised manuscript received:
27 August 2006
- Manuscript approved:
29 August 2006
- Version of record:
20 May 2007
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