Slug Catchers in Natural Gas Production - NTNU

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Karam – Slug Catchers in Natural Gas Production form of bubbles or droplets in the continuous liquid phase. Annular (wavy) flow (A-AW) arises when the flow rate is the highest. Hence, the liquid will form an annular film around the tube; but the film is thicker at the bottom than at the top of the tube. Some small amplitude waves disrupt the interface between the liquid film and the gas; as well, some droplets may be found in the gaseous phase (Walveribne Tube Inc, 2007, Azzopardi, 2010 and Bratland, 2010). Counter-current flow represents one of the aspects encountered in multiphase flow. Counter-current takes place normally as the flow is flowing in an upward direction. Hence, gravity plays a major role; it pulls the heavier phase of the gas-liquid mixture downwards. Each layer drags the other one oppositely to its flow direction. In such a flow type, double holdups are always expected. The bubble instability leads to a difficulty in the prediction of the flow’s velocity. Counter-current flow limitation takes place when the gas flow rate increases. This increase causes a decrease in the delivered liquid flow rate. Liquid fallback can be inhibited by a pressure difference applied on the fluids and an interfacial shear between the two phases present in the pipe. In order to inhibit this occurrence, the interfacial shear should be high. This is mainly implemented with an increase in the gas flow rate which should be able to lift the liquid existing in the form of either a film or droplets. Furthermore, the pressure differential should be high as well in order to overcome the liquid-wall stress and the gravity that pull the liquid in the other direction. To simplify, the direction of the liquid-wall shear determines whether the flow is a co-current or counter-current flow. A positive shear corresponds to a co-current flow while a negative shear corresponds to a counter-current flow. 2.2 Slug Flow Slug, which is a lump of liquid, has been one of the major concerns of the industry when it comes to transport of flow in multiphase flowlines. The slug normally forms as a result of retrograde condensation when the reservoir pressure drops below the dew point. The presence of a slug flow in the flowlines leads to an unsteady hydrodynamic behavior. The latter is the consequence of an alternating flow of liquid slugs and gas pockets. The liquid level in the inlet separator will be affected; a good separation is inhibited and in the worst case scenario, a flooding of the separator will occur. Page 4 of 56
Karam – Slug Catchers in Natural Gas Production The slug formation is a three step process that is represented in Figure 2. The first pipeline section shows a stratified flow where the gas is overlying the liquid and usually flowing at a higher velocity. The interface between these two phases is not a straight line but a wave-like boundary. As soon as the gas hits the wave, a pressure drop will take place followed by a pressure recovery. The latter will create a small force that will be sufficient to lift the wave upwards until it reaches the top of the pipe forming the slug shape. This is mainly generated by the Kelvin-Helmholtz instability. The slug shape formed consists of a nose and a tail. The first is shown on the right side of part 2 of Figure 2 extracted from the Feesa case study; as for the second, it is located to the left side. The slug is mainly pushed by the gas at a higher rate than the liquid. Hence, the presence of the tail can be explained and leads to a liquid entrance in the slug nose. Jet formation is, then, the outcome of such an incident. The result is a bubble formation which will, in turn, reduce the liquid holdup increasing thus the turbulence in the slug due to interference with the liquid ingress process. The amount of liquid to be formed in the pipelines depends upon several variables. The velocity between the liquid and gas surface is one factor that determines the amount of the slug being formed; a slip in velocity between the two phases will cause the liquid to accumulate. The length of the twophase flow pipelines through which the liquid is transported under steady-state conditions affects also the amount of liquid being deposited; the longer the distance of transport, the more liquid is deposited. The slug that comes out from the pipeline under steady state condition is changed into operating conditions when the volume flow might change. In other words, by a change of velocity which is normally up to 12 m/s in gas pipelines or by pigging, the slug will come out of the pipeline. Pigging produces the largest amount of slugs. It should be noticed that the slug flow characteristics are difficult to predict and cause some challenges due to the varying slug length and frequency, liquid holdup and pressure drop. The size of the slug and its degree of persistence in the flowline depend mainly on the flow rate, the liquid ingress and how it will affect the turbulence within the slug. The latter is also governed by several parameters such as the fluid properties in the pipeline, the pipeline inclination and the local flowing conditions as it was stated in the case study ‘Hydrodynamic Slug Size in Multiphase Pipelines’ completed by Feesa. The inclination of the pipe is one of the most sensitive parameters that affect the slug formation; an inclination of less than 1° can cause an unbalanced state in the pipe. Page 5 of 56
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Slug Catchers in Natural Gas Production - NTNU

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