Summary
The dynamic characteristics of oil/water flow systems have not been
understood fully. The need for improved designing methods has led researchers
toward its continuous investigation. The objective of this study was to
characterize oil/water flow through experimental data. The tests were conducted
in a 2-in. horizontal test section using tap water and mineral oil
(density=0.85 g/cm3 and viscosity=15 cp), with superficial velocities ranging
from 0.025 to 1.75 m/s. Data were acquired on flow patterns, pressure drop,
phase fraction, and droplet size as a function of flow patterns and were used
in characterization of the flow and performance evaluation of an oil/water
model. A high-speed video camera was used to identify flow patterns and measure
droplets, and ten conductivity probes were used to obtain phase distributions.
This paper provides new experimental data on pressure drop, holdup, phase
distribution, and droplet-size distribution in oil/water flows that can lead to
better modeling and design of dispersed systems. Moreover, the new data provide
new information on droplet sizes that can have significant impact on separator
design. Data comparisons were performed against the data of Trallero (1996).
Three probabilistic distributions were tested for fully dispersed flows. A
Sauter mean-diameter (SMD) analysis was conducted across the pipe diameter.
Droplet-size data were used to evaluate existing models such as Hinze (1955),
Kubie and Gardner (1977), Angeli and Hewitt (2000), and Kouba (2003). An
empirical correlation to predict the SMD profile of droplets across the pipe
cross section was developed for flow pattern of dispersed oil in water (o/w)
and water. Log-normal distribution was the best probabilistic distribution for
representing the data for fully dispersed systems. The empirical correlation
gave acceptable results. More data are needed to validate the results. Model
comparisons revealed that none of the models could represent the experimental
data accurately. This paper provides significant insight into oil/water flows
in horizontal pipes. The results are significant for the design of pipelines
and separators. Moreover, the interpretation of production logs in horizontal
wells relies heavily on the flow behavior.
Introduction
Two-phase liquid/liquid flow can be defined as the simultaneous flow of two
immiscible liquids. It can be encountered in a wide range of industries,
including the oil industry, where it commonly occurs in the production and
transportation of oil and water during the later years of production.
When heterogeneous fluids are flowing together, they are characterized by
the existence of diverse flow configurations and flow patterns, or a
geometrical arrangement of the phases in the pipe. The flow patterns differ
from each other in the spatial distribution and the position of the interface,
resulting in different flow characteristics, such as velocities, holdup
profiles, and pressure gradients. These internal-flow structures depend on
variables such as flow rates of both liquids, pipe geometry, and physical
properties of the liquids involved.
The flow characteristics of oil-water mixtures are generally different from
gas/liquid systems. The differences in characteristics are caused mainly by the
large momentum-transfer capacity, small buoyancy effects, lower free energy at
the interface, and smaller dispersed-phase droplet size in liquid/liquid flows
(Trallero et al. 1997). Therefore, the characteristics of gas/liquid flow
cannot be applied directly to oil/water flow in most cases. Generally,
knowledge of the distinctive features of oil-water systems, together with those
of gas/liquid systems, can be used in the future for understanding the more
complex case of gas/oil/water mixtures, which occur daily in petroleum-industry
production systems.
From the different existing flow patterns in oil/water flows, stratified
flow in particular has received the most attention because the low flow
velocities and well-defined interface favor both experimental and theoretical
investigations. For fully dispersed systems, information is available mainly
from studies in stirred vessels. The available information is even more limited
for the intermediate flow patterns between stratified and fully dispersed flows
(Lovick et al. 2000).
The pressure drop for two-phase liquid/liquid pipe flows depends strongly on
the flow regime and, hence, on the distribution of the two liquids in the
cross-sectional area of the pipe. Turbulent mixing in the pipe can be
sufficient to disperse the initially separated phases so that dispersions are
formed, resulting in higher pressure drops. The flow behavior of dispersions of
oil and water depends on the volume fraction and the droplet distribution of
the dispersed phase (Nädler and Mewes 1997). Droplet size depends on the
competition between breakup and coalescence phenomena. Knowledge of droplet
size and distribution would improve understanding of dispersed systems and
contribute to better design and modeling of them.
Experimental data on average droplet size exist mainly for low
dispersed-phase concentrations, where a variety of measuring techniques can be
used. Few studies have looked at high concentrations, and most of them involved
surfactant-stabilized emulsions. That available data on average droplet size
and its distribution are limited, especially in unstable dispersions at high
dispersed-phase volume fractions, is partially a result of the difficulty in
performing such measurements.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
31 July 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
18 April 2008
- Manuscript approved:
22 May 2008
- Version of record:
15 December 2008
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