Onset of Liquid Accumulation in Oil and Gas Pipelines

Yilin Fan, Research Assistant, Ph.D. Candidate


Accumulation of liquid, oil and/or water at the bottom of an inclined pipe is known to be the source of many industrial problems, such as corrosion and slugging.  The corresponding minimum superficial gas velocity that can continuously transport liquid phase upward in the upward inclined pipes is referred to as critical gas velocity.  Liquid holdup is observed to increase significantly when gas velocity is reduced below the critical gas velocity, which promotes intermittent flow and steeply increases the pressure losses.  Due to the intermittent nature of the two-phase flow, the oscillations of pressure gradient and liquid production could also cause fatigue of the pipeline itself and disturb the downstream facilities.  Therefore, accurate predictions of the critical gas velocity is of great importance for operational purposes.

For a gas well pipeline with a large diameter, the volume of water and gas condensate is very small.  Experimental studies with very low vSL are of great importance from this point of view.  However, few previous experimental studies investigated liquid accumulation in low liquid loading region, and they did not specifically study the onset of liquid accumulation.  The first objective of this study is to investigate liquid loading of two-phase flow with low liquid flow rate in an upwards inclined pipe.  Six inclination angles (1°, 2°, 5°, 10°, 15° and 20°) and three vSL values (0.001, 0.005 and 0.01 m/s) are investigated.  Two different pipe geometries have been investigated (single straight upward inclined and hilly-valley pipes).  In the transition from stratified to slug flow, a different and coherent flow structure has been observed in low liquid loading region.  This flow pattern has been classified as pseudo-slug flow, the nature of which is not well understood due to its complexity.  The secondary goal of this study is to experimentally investigate pseudo-slug characteristics, using Wire Mesh Sensor (WMS), as functions of inclination angle and gas velocity in low liquid loading region.

The modeling study of this project includes two parts: determination of critical gas velocity for the onset of liquid accumulation and pseudo-slug flow.  A new physical model for onset of liquid accumulation is proposed and evaluated with the experimental data.  Currently, there is no existing model for pseudo-slug flow due to its complex behavior.  A unified hydrodynamic pseudo-slug flow model will be developed and evaluated with experimental data.

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