Flow regimes describe the nature of fluid flow. There are two basic flow regimes for flow of a single-phase fluid: laminar flow and turbulent flow. Laminar flow is characterized by little mixing of the flowing fluid and a parabolic velocity profile. Turbulent flow involves complete mixing of the fluid and a more uniform velocity profile. Laminar flow has been shown by experiment to exist at Re < 2,000 and turbulent flow at Re > 4,000. Reynolds numbers between 2,000 and 4,000 are in a transition zone, and thus the flow may be either laminar or turbulent.
Two-phase flow of liquid and gas is a very complex physical process. Even when the best existing correlations for pressure drop and liquid holdup are used, predictions may be in error as much as ±20%. Nevertheless, as gas exploration and production have moved into remote offshore, arctic, and desert areas, the number of two-phase pipelines has increased.
To determine whether two-phase flow will exist in a pipeline, the expected flowing pressure and temperature ranges in the line must be plotted on a phase
diagram for the fluid. Here, there is a picture which shows that composition B will flow as a single-phase fluid as it enters the pipeline. However, as the pressure drops it becomes a two-phase mixture through part of the pipeline. On the other hand, composition A will flow as a single-phase (dense fluid or gas) through the entire length of the line. Composition C will flow as a liquid throughout the entire length of the line.
In most production situations the fluid coming out of the well bore will be in two-phase flow. Once an initial separation is made, the gas coming off the separator can be considered to be single-phase gas flow even though it will have some entrained liquids. The liquid coming off the separator is assumed to be in single-phase liquid flow even though it will contain some gas after it has taken a pressure drop through a liquid control valve. Other than well flowlines, the most common two-phase pipelines exist in remote locations, especially offshore, where gas and oil that have been separated and metered are combined for flow in a common line to a central separation facility.
When a gas-liquid mixture enters a pipeline, the two phases tend to separate with the heavier liquid gravitating to the bottom. This figure shows typical flow patterns in horizontal two-phase pipe flow.
The type of flow pattern depends primarily on the superficial velocities as well as the system geometry and physical properties of the mixture. At very low gas-liquid ratios, the gas tends to from small bubbles that rise to the top of the pipe. As the gas-liquid ratio increases, the bubbles become larger and eventually combine to form plugs. Further increases in the gas-liquid ratio cause the plugs to become longer, until finally the gas and liquid phases flow in separate layers; this is stratified flow. As the gas flow rate is increased, the gas-liquid interface in stratified flow becomes wavy. These waves become higher with increasing gas-liquid ratios, until the crest of the waves touches the top of the pipe to form lugs of liquid which are pushed along by the gas behind them. These slugs can be several hundred feet long in some cases. Further increases in the gas-liquid ratio may impart a centrifugal motion to the liquid an result in annular flow. At extremely high gas-liquid ratios, the liquid is dispersed into the flowing gas stream. This shows how the flow regime for horizontal pipes depends primarily on the superficial gas and liquid flow rates.
The two-phase flow patterns in vertical flow are somewhat different from those occurring in horizontal or slightly inclined flow. Vertical two phase flow geometries can be classified as bubble, slug-annular, transition, and annular-mist, depending on the gas-liquid ratio.
All four flow regimes could conceivably exist in the same pipe. One example is a deep well producing light oil from a reservoir that is near its bubble point. At the bottom of the hole, with little free gas present, flow would be in the bubble regime. As the fluid moves up the well, the other regimes would be encountered because gas continually comes out of solution as the pressure continually decreases. Normally flow is in the slug regime and rarely in mist, except for condensate reservoirs or steam-stimulated wells.
Bubble Flow: The gas-liquid ratio is small. The gas is present as small bubbles, randomly distributed, whose diameters also vary randomly. The bubbles move at different velocities depending upon their respective diameters. The liquid moves up the pipe at a fairly uniform velocity, and except for its density, the gas phase has little effect on the pressure gradient.
Slug Flow: In this regime the gas phase is more pronounced. Although the liquid phase is still continuous, the gas bubbles coalesce and form stable bubbles of approximately the same size and shape, which are nearly the diameter of the pipe. They are separated by slugs of liquid. The bubble velocity is greater than that of the liquid and can be predicted in relation to the velocity of the liquid slug. There is a film of liquid around the gas bubble.
The liquid velocity is not constant; whereas the liquid slug always moves upward (in the direction of bulk flow), the liquid in the film may move upward, but possibly at a lower velocity, or it may even move downward. These varying liquid velocities not only result in varying wall friction losses, but also result in liquid holdup, which influences flowing density. At higher flow velocities, liquid can even be entrained in the gas bubbles. Both the gas and liquid phases have significant effects on pressure gradient.
Transition Flow: The change from a continuous liquid phase to a continuous gas phase occurs in this region. The liquid slug between the bubbles virtually disappears, and a significant amount of liquid becomes entrained in the gas phase. In this case, although the effects of the liquid are significant, the gas phase is predominant. Transition flow is also known as «churn flow.
Annular-Mist Flow: The gas phase is continuous. The bulk of the liquid is entrained and carried in the gas phase. A film of liquid wets the pipe wall, but its effects are secondary. The gas phase is the controlling factor.