Shi Bohui

Hydrate plugging mechanisms of oil-dominated, water- dominated and partially dispersed system

It is the hydrate plugging mechanisms of multiphase flow that are the key to implement hydrate risk management, which have always been the focus of flow assurance industry. This paper reviews the experiments and principle analysis conducted by many researchers, and summarizes types of experimental equipment and methods, and then concludes different mechanisms for hydrate plugging in oil-dominated system, water-dominated system and partially dispersed system, while giving out some suggestions for further researches. Because of the lack of field data and the difficulty of conducting field plugging experiments, at present, designing experimental apparatus is the common solution around the world, such as autoclave and flowloop, to undertake simulation study, with the purpose to investigate plugging mechanisms. Stirring and rocking are two main methods for autoclaves to simulate pipe flow, which results in the flow characters of autoclaves is different from the real pipe flow. So flowloops are superior to autoclaves in simulating pipe flow, while some visual autoclaves (sapphire cell and so on) are better than flowloops in direct visual observation. In micro level, FBRM and PVM could provide particle distribution and behavior information. MMF apparatus could measure cohesive force between particles and adhesive force between particles and pipe walls. These micro level apparatus would help to estimate the tensile strength and the size of aggregates, which are meaningful to predict rheological properties of the fluid and the pressure drop of the pipeline. More efforts should be done on the combination of micro measurement and macro measurement, and finding a better way for autoclaves to simulate pipe flow more accurately and a better way for flowloops to improve visual observation. Plugging mechanism will be different when it comes to different multiphase systems. For oil-dominated system, which has been researched for the longest time and has the most mature theoretical system, agglomeration, adhesion to pipe walls and bedding are the governing factors of plugging. When oil-dominated system is treated by AAs, under risk management, agglomeration will be weaken significantly. Then the plugging mechanism of this condition would be controlled by bedding and adhesion to pipe walls. More works should be done on the theoretical modeling of adhesion to pipe walls and bedding for oil dominated system, and confirm the contribution of different mechanism to the pressure drop. For water-dominated system, research of which is prospective, bedding of hydrate particles or hydrate network is the determining reason for plugging. Since the hydrate growth rate of water-dominated system is relatively low, this system has the lowest plugging tendency. The molding of hydrate growth for water-dominated system would be the next step work. Research on partially dispersed system is carried out with the increasing water cut in the offshore oil fields, which is now in the experiment exploration period. Plugging of this system is resulted from film growth of the free water and agglomeration in the bulk phase, which resulting in the most severe plugging tendency. The quantification of the free water for partially dispersed system is fundamental for the continuing study. Moreover, it is significant to take the impact of flow patterns on the plugging mechanisms into consideration.