Multiphase Flow with Gas Hydrate Particles

Abstract

“Cold Flow Technology” (CFT) is an alternative solution to deal with gas hydrate formation in subsea flowlines.

Transient multiphase flow with gas hydrate particles is an important research area of the CFT, which has not been thoroughly investigated yet. Therefore, this PhD program was initiated as a basis for further research on this aspect of the CFT.

Investigation on the phase equilibrium of gas hydrate particles was the second part of the present study. The subject was chosen as the CFT relies on the transportation of thermodynamically stable gas hydrate particles.

The first step of the PhD work was designing and building a new experimental facility at the multiphase flow laboratory of the Department of Energy and Process Engineering (NTNU-EPT). The setup consisted of an asymmetrical V-shape pipe capable of measuring flow rate, pressure and particle volume fraction. The experiments were carried out using polymeric particles representing real hydrate particles and with water as the model fluid. Finding a proper model hydrate particle and preparing a sufficient quantity of the model particle were the most challenging parts of this work completed in cooperation with the SINTEF Materials and Chemistry group. A hydrophobic model hydrate particle was initially chosen based on different criteria like density, availability and cost. In the next step, 150 Kg of the model particle were purchased, degassed and sieved. Eventually, the surface properties of the particles were modified using very high concentration chemicals. The surface treated particles could be dispersed in water without adding any chemicals.

In order to investigate the transient flow behavior of a potential hydrate plug during re-start operation of a subsea flowline facilitated with CFT, a series of experiments on flowline re-start was undertaken. The results of the experiments showed an additional frictional pressure drop due to the movement of the hydrate plug, especially in the bend. Moreover, the average particle phase velocity of the dispersed phase flowing in front of the hydrate plug was higher than the mixture velocity in the majority of the measured data. The increased particle velocity may be due to dispersion of the hydrate plug in water. Despite the particle phase velocity in front of the hydrate plug, the average particle phase velocity of the plug tail was slightly lower than the mixture velocity.

In order to model the plug flow experiments, a simple one dimensional flow model was initially developed using the continuity equation and without considering any extra terms. However, benchmarking of the flow model showed that more physical details should be included in the model. Therefore, the initial model was developed further, taking into account the axial dispersion of the particle phase, the increased particle phase velocity caused by the plug front dispersion and the additional frictional pressure drop caused by the hydrate plug. The effect of axial dispersion was taken into account by adding a diffusion term to the particle phase continuity equation. The effect of plug front dispersion on increasing the particle phase velocity was determined by applying a mass balance at the plug front. The modified model was capable of qualitatively reproducing the results of the plug flow experiments.

The thermodynamic stability of gas hydrate particles was investigated by proposing two thermodynamic models both based on the Van der Waals - Platteeuw theory. Since wellstream water is an electrolyte solution, the models were developed to precisely predict the thermodynamic equilibrium conditions of gas hydrates in presence of electrolyte solutions.

As the starting point, the rather simple Pitzer model was used to calculate the water activity in electrolyte solutions. The parameters of the Pitzer model were adjusted using the experimental data at the lowest temperature found in the literature. A mixing rule was proposed to obtain the activity of water in mixed electrolyte solutions. The results of the model were in good agreement with experimental data.

In the second thermodynamic model, a Mean Spherical Approximation (MSA) based model was employed to compute the water activity in electrolyte solutions. Initially and in order to improve the accuracy of the model, a new relation was proposed for describing the concentration dependency of the model’s adjustable parameters at 25 oC. Then, the adjustable parameters were extended with respect to temperature variations within the gas hydrate equilibrium conditions. In addition, the model was extended to calculate the solubility of CO2 in single and mixed electrolyte solutions. The results of the proposed work both on computing CO2 solubility and the gas hydrates equilibrium conditions were in good agreement with the experimental data and those presented in other works. Fewer adjustable parameters and the capability of modeling a mixture with different species can illustrate the complexity of the MSA based model. For instance, in case of a need for chemical injection, the MSA based model might be extended accordingly.