Many model sand correlations exist to predict pressure gradient in vertical two- or three-phase flow in oilwells. Most of these are developed from large experimental databases, relying almost entirely on empiricism, thereby making extrapolation hazardous. The calculation procedure involved in these correlations are also rather complicated. Experience indicate that a sound model must be mechanistically based and be capable of accounting for the various flow patterns associated with multi phase flow. The predictive methods of Orkiszewski, Aziz et al., and Chierici et al. do take in to account the four main flow regimes - bubbly, slug, churn and annular - and propose separate correlations for pressure drop in each flow regime. Recent developments in multiphase flow, especially in predicting flow pattern transitions and entrainment estimation during annular flow, necessitate a fresh look at the problem.

In the first part of this two-paper set, we present a theoretical model for predicting pressure drop during multiphase flow. Model development consists of studying the hydrodynamic conditions that give rise to the various flow pattern transitions. The method for estimating pressure drop in each flow regime is then developed. In developing the equations for pressure gradient, the contribution of the static head, the frictional loss, and the kinetic energy loss are examined. This aspect of the study points out the importance of the static head term and the necessity of estimating the insitu gas void fraction accurately.

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