Summary
A large database on critical and subcritical flow through orifice- and
cage-type chokes have been obtained in the Multiphase Flow Loop (MPFL) of Norsk
Hydro Oil and Gas R&D Center in Porsgrunn, Norway. This work is an
extension of the studies performed in 1999 (367 data points) by Schüller et al.
(2003), making a total data set of 509 data points. The downstream separator
pressure and temperature were kept at 8 bara and 50°C, respectively. The
maximum upstream pressures were approximately 40 bara, giving a maximum
pressure ratio of approximately 4. The Hydro models (Schüller et al.
2003) show a very good agreement for prediction of mass flow rate, both for
critical and subcritical flow conditions, with an average error of absolute
values of 6.2% and a standard deviation of 8.9%. Comparisons with the models of
Sachdeva et al. (1986) and Perkins (1993) show that the performances of these
models are not as good as the Hydro model. The Sachdeva model and the Perkins
model both have average errors greater than 22%, with standard deviations
greater than 25%.
The Hydro models also seem to predict reasonably correctly the transition
between subcritical and critical flow conditions.
Introduction
Oil and gas production from well clusters—where several wells flow into one
subsea flow line—often requires flow control for each well. Knowing the actual
performance of the manifold chokes is of vital importance for optimal
production. In recent years, interactive multiphase-flow models and control
systems for well allocation have been developed. As input, they use measured
data about the wellstream in the upstream tubing and combine this with data
about the choke valve opening and its flow characteristics. The Troll oil field
and several other fields in the North Sea have been successfully developed with
such automatic choke-control systems on the basis of special choke models,
which are valid for both critical and subcritical flow conditions. These
systems were based on the first versions (Selmer-Olsen 1991; Selmer-Olsen et
al. 1995) of the Hydro models.
Subcritical flow through chokes is often described by means of choke
characteristics in terms of a flow-capacity factor (CV
)parameter. The characteristics often are determined in laboratory tests using
single-phase water. To determine critical flow conditions, several approaches
are used:
- Homogeneous equilibrium flow models assuming thermodynamic
equilibrium.
- Homogeneous nonequilibrium flow models assuming no flashing of gas
affecting the flow rate through the choke (“frozen” flow) (Henry and Fauske
1971).
- Models assuming empirical or semiempirical relations describing the
kinetics of flashing of gas from the liquid (Henry and Fauske 1971).
- The main difference between multiphase flow of real reservoir fluids
through a production choke and the single-component systems in laboratory tests
can be listed as follows:
○ Reservoir hydrocarbon fluids include a large number of components from
light to heavy hydrocarbons. The kinetics caused by flashing of the light
components through a choke are expected to be different compared to
single-component systems.
○ Depending on the upstream geometry and flow rates, there exist several
different flow patterns upstream of the choke that may influence the choking
conditions, such as liquid slugs, phase-inversion phenomena, etc. (Selmer-Olsen
1991; Selmer-Olsen and Lemonnier 1995).
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
20 April 2005
- Revised manuscript received:
30 September 2005
- Manuscript approved:
1 October 2005
- Version of record:
20 August 2006
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