An Effective Method for Modelling Stagnant Liquid Columns in Gas Gathering Systems

摘要

Modeling of gas gathering systems is often complicated by the presence of localized pressure losses that are not easily explained using steady-state two-phase correlation packages. This often leads to inappropriate manipulation of base input parameters such as over use of tuning factors, or reduced pipeline diameter or even increase pipeline length to get a satisfactory pressure match. A more appropriate approach includes a review of the model inputs which typically includes rechecking the measurement of the key inputs, increasing the complexity of the model (inclusion of all fluid volumes, fluid property changes, elevation profiles) and confirmation of the appropriateness of the selected pressure loss correlation. This detailed review will often resolve the issue but a significant factor is sometimes overlooked. It has been the authors experience that these unusual pressure losses often occur at pipeline river crossings and other depressions in the terrain. In situations where superficial gas velocities are high, two-phase correlations tend to work well, but where superficial gas velocities are low, liquids can stop flowing and become trapped even though gas continues to flow. The goal of this paper will be to present case studies where liquids accumulate and where they continue to flow in a pipeline. The differences between the two situations will be highlighted and a procedure will be presented for correctly identifying when and where liquids are accumulating. These liquid accumulations, sometimes called Stagnant Liquid Columns, can significantly increase wellhead backpressures and adversely impact well productivity.