ABSTRACT

Current models developed for fluid flow in pipes are structured for either singlephase or two-phase flow. In fact, fluid flowing in pipelines may traverse the fluid phase envelope such that the fluid phase changes from single-phase to two-phase or vice versa. Unfortunately, the phase of the flowing fluid in pipe is unknown a priori, and available models do not integrate the single-phase flow and two-phase flow models. The development of an integrated single/two-phase flow model is the primary objective of this study. The work focuses on the development of a model that is capable of handling phasetransition. For two-phase flow, the model incorporates flow regime predictions. As this work is intended for gas-condensate with gas as the bulk of the flowing medium, only three types of flow regimes that are most likely to occur are included. These are: smoothstratified, wavy-stratified, and dispersed liquid or mist flow regime. The model couples a phase behavior model, based on the Peng-Robinson equation of state, and a hydrodynamic model, based on the two-fluid model. Some correlations for calculating fluid properties and criteria for flow regime transition are adopted. The resulting model allows one to predict the flow behavior of a natural gas condensate system. Comparisons of the results to the limited field data demonstrate the capability of the model in describing the effects of flow parameters on condensate behavior in pipelines.

INTRODUCTION

It is possible for condensate to appear in natural gas pipelines even though conditions at the pipe inlet are such that single-phase gas flow prevails. This occurs when the pressure and the temperature changes traverse the two-phase region of the phase envelope such that the system changes from single-phase to two-phase. As the system enters the two-phase region, mass transfer takes place from the gas phase to the liquid phase. This mass transfer causes condensation and gives rise to two-phase flow in the pipeline. The presence of liquid (condensates), besides reducing deliverability, creates several operational problems such as instrumentation and compressor fouling. Such problems can be alleviated by appropriately designing and siting liquid catchers or filter separators. In order to design and site the liquid catcher or filter separator effectively, one needs a description of the relative amount of condensate and the flow regime taking place along the pipe. Unfortunately, this relative amount of condensate in pipe, the liquid holdup, is unknown beforehand. Thus, it is imperative to have a means of predicting the liquid holdup and flow regime profile in pipelines. Many attempts have been made to study liquid holdup and flow regime profiles in pipelines. However, few attempts have been reported for the use of two-fluid model for this purpose. Some of them have attempted to incorporate the flow regime prediction and others simply assume one flow regime for the entire pipe length.

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