ABSTRACT

Multiphase flow of gas and low loads of liquids occurs frequently in natural gas gathering and transmission pipelines for both onshore and offshore operations. Literature and experimental investigations indicate that dispersed droplet and stratified flow patterns are obtained when gas and small quantities of liquids flow concurrently in a pipe. Very few correlations exist for the prediction of holdup and pressure drop for these systems and fewer still give satisfactory results. Experimental studies for air-oil and air-water systems flowing through small diameter plastic and steel horizontal pipes ranging in size from 1-inch to 3-inches were performed. The experiments were carried out at the multiphase flow laboratories of Imperial College in London and the University of Calgary in Canada. Data from actual operating gas pipeline systems transporting small amounts of hydrocarbon liquids were also evaluated. Based on the experimental results and the operating data, two approaches for modeling these systems are proposed:

  1. A homogeneous approach for very low liquid loads (holdups up to 0.005), typical in gas transmission systems. A friction factor correlation based on the holdup has been developed for this flow regime.

  2. A mechanistic stratified two-phase approach for higher liquid loads (holdups greater than 0.005) usually found in gas gathering systems with consideration given to:

    • The reduction in the available flow area and extent of wetting of the pipe perimeter by the liquid film. The gas/liquid interface was observed to be either flat or curved.

    • The interfacial friction factor between the liquid film and the gas.

A new correlation based on the liquid and gas Reynolds numbers as well as the film thickness and hold up has been developed. This correlation has been successfully tested against both experimental and actual pipeline operating data.

1.0 INTRODUCTION

The joint flow of gas and liquids in pipes is common in the chemical process industry, particularly for oil and gas pipeline flow. Numerous theories and correlations have been proposed in the last 50 years for the prediction of pressure drop and liquid holdup in pipelines. None of them, however, gives consistently reliable results for all the identified flow patterns in multiphase gas-liquid flow. Systems transporting gas and low loads of liquids are perhaps some of the least studied in multiphase history and consequently literature and data for these systems are limited. In the petroleum industry this phenomenon occurs frequently in natural gas gathering and transmission pipelines for both onshore and offshore operations. The accompanying liquids are usually heavy hydrocarbon fractions and water and may be introduced from several sources. Liquids from compression facilities (e.g. lube oil) and treatment plants (e.g. glycol) as well as products from retrograde condensation may accompany the gas during transportation. Some water from the reservoir formation may also contribute to the liquid load. The accompanying liquids affect the transportation efficiency of the system. Most gathering pipelines (which typically have liquid loads up to 100 barrels per million cubic feet of gas (bbls/MMSCF)) transport fluids as multiphase components.

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