Critical and Sub-critical Oil/Gas/Water Mass Flow Rate Experiments and Predictions For Chokes
- Reidar B. Schuller (Agricultural U. of Norway) | Sampath J. Munaweera (Norsk Hydro ASA) | Staale Selmer-Olsen (Det Norske Veritas DNV) | Trond Solbakken (Norsk Hydro ASA)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- August 2006
- Document Type
- Journal Paper
- 372 - 380
- 2006. Society of Petroleum Engineers
- 4.6 Natural Gas, 4.1.2 Separation and Treating, 1.10 Drilling Equipment, 5.3.2 Multiphase Flow, 4.3 Flow Assurance, 4.1.5 Processing Equipment, 4.1.1 Process Simulation, 5.2 Reservoir Fluid Dynamics, 5.2.1 Phase Behavior and PVT Measurements
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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.
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).
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