Modeling of Hydrate Dissociation in Subsea Natural Gas Production Flowlines

摘要

Gas hydrate is one of the major challenges facing production and transportation of natural gas from the remote regions of deep waters, ultra-deep waters and the arctic areas where large reserves have been discovered globally. Gas hydrate is grouped into three types; type I, II and H. Among the aforementioned types, type II is of interest to the oil and gas industry because this is the one that obstructs natural gas flow in production pipelines. In dissociating hydrate in a production pipeline, the following techniques are applied; thermal, chemical, depressurization and mechanical. However, depressurization technique is the most widely used in field of operations where absolute hydrate plugs in production pipelines are experienced. This method has been applied from time past with rule-of-thumb as there are limited models on hydrate dissociation. In this study, a mathematical model was developed based on cubic-degree polynomial using Fourier law. The developed model considered a radial dissociation from a two-sided approach on a fixed boundary which was finally programmed into software called PIPECLEAN using Visual Basic(as shown in Fig 7and 8) and validated with published experimental data. Results show that dissociation commences from a temperature of 285.7K and above 13hours to dissociate a production flowline that is plugged off by gas hydrate. The study will aid depressurization operations in the field to predict hydrate dissociation real-time,the dissociating temperature to achieve this and the hydrate penetration depth inwardly from the walls of a pipeline. More importantly, the developed model will assist in curbing the problem of guesses when dissociating hydrate in field of operations. One limitation is that the model cannot determine gas evolved during dissociation.