Wet steam, being a two-phase fluid, has complex critical flow properties. Analytical and experimental data were used to investigate the critical steam flux, and the pressure ratio needed to achieve critical flow for a variety of popular flow-rate controlling devices. Three new parameters — critical steam flux ratio, dimensionless critical steam flux, and the Napier parameter — are introduced to correlate the critical flow data. Attention was focused on the effects of stagnation pressure, stagnation quality and design of the flow-rate control device on the critical flux. The study covered stagnation pressures from 200 to 2100 psia and stagnation qualities from 20 to 100%.
Critical flow refers to a situation where any further reduction in the downstream pressure of the flow fails to induce a corresponding increase in the mass flow rate. In other words, critical flow is a phenomenon whereby the flow rate has an upper limit for a given flow-controlling device and the fluid's upstream or stagnation condition.
In steam-enhanced oil recovery (EOR) operations, the flow rate of steam is usually controlled by a flow controlling device, such as an orifice, nozzle, venturi, choke, adjustable valve, etc. A constant flow rate of such devices can be maintained for a varying downstream pressure when the flow through the devices is under critical flow conditions.
Most steam used in EOR operations is a wet steam whose flow properties are more complex than single-phase fluids. In this study, attention was given to the effect of stagnation pressure, quality and the design of the flow controlling device on the critical flow properties. For representative field applications, this study covered steam pressures ranging from 200 to 2100 psia [1378 to 14500 kPa] and steam qualities from 20 to 100%. Flow-controlling devices discussed in the experiments include orifices, nozzles, venturi, static chokes and adjustable chokes. New parameters are proposed to correlate critical flux data. Results from this study offer a better understanding of the critical flow properties of steam so as to improve steam management for steam flooding or stimulation operations, and to explore the properties that can be used as a basis for developing new and innovative controlling and measuring devices for steam quality and steam flow rate.
Various models have been proposed to describe the critical flow behavior of steam. The homogeneous equilibrium model (HEM) is a simple model that gives a good representation of the critical flow properties of steam. It is used as the basis of this study.