Quantitative Analysis of Drag Reduction in Horizontal Slug Flow
- M. Daas (Ohio U.) | C. Kang (Multiphase Interactive Systems and Technologies) | W.P. Jepson (Multiphase Interactive Systems and Technologies)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 2002
- Document Type
- Journal Paper
- 337 - 343
- 2002. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 1.10 Drilling Equipment, 4.2.3 Materials and Corrosion, 5.3.2 Multiphase Flow, 4.2 Pipelines, Flowlines and Risers
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Calculations have been made to predict the components of pressure drop in slug flow. This analysis is aimed at understanding, in a quantitative manner, the contributions of both frictional and accelerational components to total pressure drop in horizontal slug flow and the effect of drag reducing agents (DRAs) on each component. Experimental results were in good agreement with predicted values.
The DRA used in this study was effective in reducing both components of the pressure drop. The accelerational component was found to be dominant and formed more than 80% of the total pressure drop. It increased dramatically with increasing superficial gas velocity. With the addition of 20 ppm DRA, the accelerational and frictional components were noticed to decrease by a factor of 70%. At DRA concentration of 50 ppm, they decreased by a factor of 80%. Total drag reduction was found to generally decrease at higher superficial gas velocities.
In sharp contrast with expectations, the drag reduction was recovered mainly from the accelerational component, indicating that the DRA worked not only in the buffer zone but also in the mixing zone in the slug body. The accelerational drag reduction reached values as high as 89% of total drag reduction.
Since the discovery of the drag reduction phenomenon, extensive work has been carried out in horizontal and inclined pipelines to examine the effect of the addition of DRAs on pressure drop. DRAs were found to have significant influence in decreasing the frictional pressure drop in single-phase flow. Since that time, drag reducing agents were believed to work only on the frictional component of the pressure drop of multiphase flow, and not influencing the other contributions to total pressure drop (e.g., accelerational and gravitational components).
Although several theories were introduced regarding drag reduction phenomenon, a precise and exact understanding of the mechanism of drag reduction is not established yet, especially in multiphase flow. It is believed that DRAs work mostly in the region near the wall, namely the buffer zone, by reducing the friction factor of the flow through diminishing the turbulent structures and changing the velocity profile there. This study may help to recognize where drag reduction takes place in a quantitative manner. This will help in developing current theories of drag reduction mechanism.
Slug flow is attributed to relatively high pressure loss more than other flow patterns as explained below. Two components contribute to total pressure loss in horizontal slug flow. These are the frictional and accelerational components of pressure drop. The slug front moves faster than both the liquid film and the pocket of gas ahead of it. As a result, liquid is assimilated into the slug front and is accelerated to the slug velocity; the force required for pickup and acceleration manifests itself as accelerational pressure drop (see Fig. 1). Meanwhile, the gas pocket ahead of a slug penetrates the slug front forming a highly turbulent region (mixing zone) where energy is dissipated.
Frictional loss occurs in both the slug body and the stratified liquid film between each two successive slugs. Pressure loss caused by friction in the slug body is a strong function of slug characteristics; more gas content in the slug body means less dense flowing fluids and, therefore, less pressure loss. A longer slug body is always accompanied by greater pressure loss than shorter slugs. Frictional pressure loss in the slug body is proportional to the square of slug velocity. Finally, slug frequency results in more slugs and multiplied amounts of total pressure loss. Possible reduction in pressure loss is possible through significant cutdown in each component and in the slug frequency.
This work is the first of its kind to predict, in a quantitative manner, the components of pressure drop in slug flow with the presence of DRAs. Moreover, this work will provide a clear understanding, based on experimental findings and computational analysis, of the various effects of oil viscosity and pipe inclination on the performance of DRAs in reducing the different components of pressure drop in slug flow, the magnitude of these components, and their contribution to total pressure drop. Several models were examined, but few were applied in the above study to break down the pressure drop into its components in slug flow in a horizontal system.
Dukler and Hubbard1 introduced equations to calculate the contributions of both frictional and accelerational components to total slug pressure drop in an air/water system. In their model, they assumed that within the slug body the two phases are homogeneously mixed with negligible slip, and the frictional contributor could be calculated using an equation similar to ones in single-phase flow after modifying the density of the mixture and the friction factor. The accelerational contribution was calculated under the assumption that the stabilized slug can be considered as a body receiving and losing mass at equal rates. The velocity of the liquid in the film just before pickup is lower than that in the slug, and a force is therefore necessary to accelerate this liquid to slug velocity. This force manifests itself as accelerational pressure drop.
Greskovich and Shrier2 used the Dukler-Hubbard model, along with independent correlations for in-situ holdup and slug frequency, to predict pressure drops for two-phase slug flow. The holdup and frequency correlations were for the most part based on data for air/water flowing in a 1.5-in. diameter pipe. Predictions of pressure drop using this approach were compared with experimental data taken from studies utilizing various systems and pipes. Their approach was equivalent to the Dukler-Hughmark method.
Fan et al.3 introduced a new model to predict the pressure drop across a stable slug. In their model, they assumed the slug as a hydraulic jump. Furthermore, they assumed that pressure change occurs in the rear of the slug. This pressure change could be positive or negative, depending on whether the slug was decaying or growing.
Petalas and Aziz4 developed a new model for multiphase flow in pipes. According to their model, pressure drop and holdup in pipes could be predicted for all pipe geometries and fluid properties. Their model lends itself for implementation in a computer program in that a significant number of calculations were required and several of these required iterative procedures. Unfortunately, the accelerational component of the pressure drop in slug flow was not considered in their model at all, making the model questionable.
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