Experimental and theoretical study of the flow, aggregation and deposition of gas hydrate particles

Abstract

Gas hydrate plugging is considered to be a very problematic topic during petroleum production and transportation. The phenomenon of hydrate plug formation involves inter-related effects related to different disciplines, namely multiphase flow, thermodynamics, surface chemistry and solid mechanics. At present the problem is not fully understood, although much information is becoming available about hydrates in general. One of the challenging problem in gas hydrate research is the difficulty of reproducing industrial conditions on a laboratory scale as natural gas hydrates require high pressure to form, limiting the possibilities for gaining insight into the process by direct observation due to safety considerations. The scale of the process in combination with industrial flow conditions is not also absolutely repeated on a lab-scale. The problem of limited direct information about the process can be alleviated by simplification of the experimental conditions such as the use of low-pressure models for the hydrates and shifting from pipeline systems to agitated vessels. In addition, computational fluid dynamics (CFD) models of the evolution of gas hydrates in pipelines can give valuable information. The present state-of-the-art of CFDresearch is such that the models need to be validated experimentally. This can be done with the macroscopic parameters of the process (e.g. pressure, temperature and velocity), which are relatively simple to monitor even in a high-pressure system. A CFD-model can predict the detailed behavior of hydrate particles, including their interactions with the continuous phase and with each other. This will help to understand, for instance, the mechanism of hydrate deposition in turbulent flow; or the agglomeration of particles in a pipeline during transportation. This work includes both an experimental study of water-hydrate slurry behaviour in a lab-scale, low-pressure flow loop and a CFD model that mimics the experiment. The experimental part of this dissertation is focused on the rheological behaviour of freon hydrate slurries: their apparent viscosity and yield stress were empirically examined in the low-pressure flow loop. Sampling of hydrate particles was carried out for determination of their size distribution and maximum hydrate volume fraction (i.e. the packing limit). The numerical modelling part involves a step-by-step development of models for hydrate deposition and aggregation. Initially, a model built using the commercial CFD-package STAR-CD was validated using experimental data from the literature in terms of its ability to correctly predict deposition of particles in a quiescent fluid. In parallel with this a population balance model (PBM) was developed and validated for prediction of hydrate particle nucleation, growth, aggregation and breakage in the pipeline. Based on this modelling approach tested in the way described above, a CFD-model of the experimental rig was developed and tested in the homogeneous flow regime, where the rheology of the hydrate slurry was the factor determining the system behaviour. After this, the model was updated with the PBM expressions for hydrate particle size development and the process of deposition in a turbulent pipeline flow was studied.