document

Abstract

Production and transportation of wet gas in large diameter pipelines necessitate several operational issues to be addressed. Of particular interest is the liquid inventory management, which is essential to ensure efficient and save operation of the pipeline. This paper quantifies several options for liquid inventory management in such pipeline systems.

Often pipeline configurations include parallel pipelines. This allows for flexibility in operation of the pipelines, but at the same time introduces additional potential liquid inventory problems. At normal production rates, flow would tend to evenly distribute between two parallel pipelines with the same diameter since friction will be the dominant cause of flow resistance. At reduced production rates liquid would start to build up in the pipelines and unless proper attention is given, the in-situ liquid volumes may quickly exceed those of available separators and slug catchers.

The liquid build up can start at different rates for the two pipelines if there are differences in the pipeline profiles. Liquid build-up will also be influenced by how the liquid splits between the two pipelines at the pipeline inlets. Liquid buildup will increase the resistance of the flow in a pipeline.

The wet gas will often have an affinity to flow through the pipeline with less liquid buildup and thus less resistance. At the same time the parallel pipeline will produce less wet gas and thus gradually build up more liquid, as the production through that pipeline gets lower. Essentially the system is in an unstable condition and eventually one pipeline could end up with so much liquid that flow through it would cease. Very likely, the only remediation in such a case would be pigging.

Different scenarios are investigated to illustrate these potential problems and several suggestions to managing and operating the pipelines are presented.

Introduction

Liquids can occur in gas transportation systems, even for systems where the feed to the pipeline at inlet conditions is nominally all vapor, as in the case of feeding separator gas into a pipeline. As the pressure drops in the pipeline, the gas will cool due to Joule-Thompson cooling (auto-refrigerate) and cooler ambient temperatures, which may condense some liquid from the gas. As the pressure drop increases, the temperature drop may be significant, which again may cause a further increase in liquid production.

Both onshore and offshore pipelines may traverse hilly terrain. Multiphase flow in pipelines through hilly terrain can result in serious operating problems, relating to the inability of the gas to pull condensed liquid up the hills. This typically happens when the production drops below a certain superficial gas velocity (the superficial gas velocity is the in-situ volumetric gas flow rate divided by the pipe cross sectional area). Table 1 shows the qualitative behavior of hilly terrain pipelines transporting gas and condensed liquid at different ranges of gas superficial velocity.

Transient multiphase flow in wellbores and pipeline systems is normally analyzed with a dynamic two- or three-fluid modeling technique (1, 2, 3). Transient simulation indicates that most liquid will be swept over the hills for a gas pipeline system if the superficial gas velocity in the system is above ∼20 ft/s. When the gas velocity decreases below this value, the steepest hills will begin to fill with liquid. If the gas velocity decreases to very low values of ∼3 ft/s virtually all of the hills would fill with liquid and the total liquid content, or inventory, by volume in the pipeline may be very high.