Pressure Characteristics of Slug Flow in Horizontal Pipes
Authors: AL-Dogail Ala , Gajbhiye Rahul , Alnaser Mustafa , Alnajim Abdullatif 
Published: February 2022

The slug flow in the pipes is a common type of flow observed in the oil and gas industries. It can be observed in the different parts of production systems such as tubing of the vertical or horizontal well, a network of flowlines connecting the wellhead and surface facilities as well as in the pipeline for exports & transportation of the oil and gas to the refineries. The estimation of the pressure drop in the different parts is essential for designing as well as the operation and maintenance of the production system. Among the different flow regimes in the multiphase flow slug flow is always challenging for the oil and gas industry. The wide range of variation in the gas-liquid velocities in two-phase flow makes the flow behavior more complex and difficult to predict especially in the slug flow regime. Further, the increase in viscosity of the liquid slug flow becomes the most common flow pattern in oil and gas pipelines and accurate prediction of slug flow parameters is vital for the pressure estimation.

The objective of this study is to use the pressure characteristic of slug flow for estimation of the gas fraction and liquid holdup. As an industry practice, the easily measurable data from the wellhead or pipeline is the pressure at a particular node in the production network. The pressure value alone does not reveal enough information on the phases during the multiphase flow. However, the liquid holdup is an important parameter for predicting phase distribution in multiphase flow. Through a large number of theoretical and experimental studies, prediction models for liquid holdup have been developed from early empirical models, however, these models estimations are not accurate and also predict the single value of the pressure which is difficult to obtain and correlate with the slug flow especially in the field application.

This study aims to utilize the pressure characteristic for the prediction of the gas fraction and liquid holdup which varies greatly in the slug flow although pressure magnitude differs insignificantly.

In this study, several experiments were conducted using the air/water system on the flow loop with a pipe diameter of 1 inch (2.54 cm) and a length of 25 ft (7.62m). The liquid and gas rates were varied to capture the complete range of slug flow and corresponding superficial velocities. The pressure data for the ranges of the superficial velocities were gathered with pressure transducers attached along the pipe covering the largest slug formed in the pipe. Then the generated pressure response data were analyzed to characterize the slug flow using statistical analysis and regression for correlation of the pressure characteristics with the gas fraction and liquid holdup. The pressure response data collected from the slug flow experiments showed a decrease of pressure drop with increasing gas flow rate at a fixed liquid flow rate. However, the pressure drops increased with increasing liquid flow rate at a fixed gas flow rate. Linear regression showed good agreement and accuracy between the pressure drop and gas fraction.