Prediction of Slug Frequency for High Viscosity Oils in Horizontal Pipes

Abstract

Slug frequency is defined as the number of slugs passing at a specific point along a pipeline over a certain period of time. Most experimental studies related to slug frequency in the literature were conducted using air and water. Data with a viscous liquid phase are scarce. Knowledge of the effect of liquid viscosity on slug flow is crucial to size pipelines and design pre-process equipment. In this study, the effects of high oil viscosity on slug frequency for horizontal pipes are experimentally investigated. The experiments are performed at oil viscosities between 0.181 and 0.589 Pa•s in a horizontal pipe. Experimental results are compared with the existing slug frequency correlations. Experimental observations reveal that slug frequency appears to be a strong function of liquid viscosity. However, existing slug frequency closure models do not show any explicit dependency on liquid viscosity. A closure model taking into account viscosity effects for horizontal pipes on slug frequency is proposed. The proposed slug frequency model is compared against published data. The comparison of proposed closure model against limited published data shows that it is a better alternative than existing correlations for high viscosity oils. The proposed slug frequency closure model can improve the performance of the existing mechanistic models for high viscosity oil applications. Introduction Slug flow is one of the most observed flow patterns that characterize the gas-liquid flow in multiphase transportation pipelines. The most distinctive characteristic of slug flow is its intermittent nature due to a unique phase distribution. It occurs over a wide range of gas and liquid flow rates. Extensive experimental studies have been conducted to understand the mechanism of slug formation. However, very few studies have addressed effect of high viscosity behavior in slug characteristics. Gokcal et al. (2006) observed slug flow to be the dominant flow pattern for the high viscosity oil and gas flows. They found that slug frequency increased while the slug length decreased as the liquid viscosity increased. Prediction of slug frequency is important for design of transportation pipelines and gas-liquid receiving facilities. Slug frequency is required as an input in mechanistic models to predict slug flow characteristics such as pressure gradient and liquid holdup accurately.

Application of the gamma radiation method in analysing the effect of liquid viscosity and flow variables on slug frequency in high viscosity oil-gas horizontal flow

“…However, after a certain superficial gas velocity is reached, the gas phase suppresses liquid hold-up causing the decline of slug frequency thereafter. This is consistent with findings of a number of researchers such as Gokcal et al [5] for high viscosity liquid-gas flows and Hernandez-Perez [6] for low viscosity liquid-gas flows.…”

“…An increase in liquid level increases the probability for the occurrence of the critical liquid height required for the formation of slug bodies in accordance with the KelvinHelmholtz stability phenomenon. The findings reported above are consistent with experimental observations made by Colmenares et al [1] and Gokcal et al [5]. It is also clear from Figures 7-9 that slug frequency is dependent on liquid viscosity.…”

Experimental investigation of the effect of liquid viscosity on slug flow in small diameter bubble column

“…This constitutes the viscous effect which leads to an increase in drag force and a subsequent decrease in the bubble rise velocity, with the length of slug unit increasing due to coalescence, hence a decrease in bubble frequency as liquid viscosity increases. This is contrary to what was proposed by [15]. This is probably due to the use of horizontal pipe.…”

“…From equation (15), the drag force is directly proportional to viscosity, ୈ Ƚ Ɋ which implies that as viscosity increases, drag force otherwise called fluid resistance increases. This is confirmed by the plot shown below:…”

“…The film liquid holdup was said to be directly proportional to the liquid viscosity due to an increase in the interfacial and wall shear forces on the liquid film. [2] from the empirical correlation developed for slug liquid holdup as a function of liquid viscosity explained that the slug liquid holdup is directly proportional to the liquid viscosity [27] and [15] reported an increase in slug frequency with increasing liquid viscosity for which they developed an empirical slug flow correlation.…”