Bohui Shi

Natural gas hydrate shell model in gas-slurry pipeline flow

Abstract A hydrate shell model coupled with one-dimensional two-fluid pipe flow model was established to study the flow characteristics of gas-hydrate slurry flow system. The hydrate shell model was developed with kinetic limitations and mass transfer limitations, and it was solved by Euler method. The analysis of influence factors was performed. It was found that the diffusion coefficient was a key parameter in hydrate forming process. Considering the hydrate kinetics model and the contacting area between gas and water, the hydrate shell model was more close to its practical situations.

Hydrate slurry flow property in W/O emulsion systems

Hydrate risk management strategy has become a promising way of dealing with hydrates in subsea transportation pipelines in recent years. In this way, hydrates are allowed to form in the pipeline and are treated as a slurry flow with the help of anti-agglomerants. This work investigated the effect of hydrate formation on the flow friction factor in water in oil (W/O) emulsion systems. A series of hydrate formation and slurry flow experiments were conducted using a high pressure flow loop. Results show that the friction factor is in direct proportion to the volume fraction of hydrates formed, as it increases significantly after hydrate formation onset and then increases gradually with hydrate growing. A novel method is proposed in this work to amend the effective hydrate volume fraction and take into account the effect of hydrate agglomeration and water occlusion. In addition, it is found that the slurry flow velocity has a significant effect on the friction factor variation. As a larger flow velocity can lift the particles suspension height and cause the particles to be away from the pipe wall surface, so it gives a smaller friction factor by reducing the collisions between hydrate particles and the pipe wall surface. With the modified effective hydrate volume fraction and particle chord length distribution data, a model is proposed to estimate the hydrate caused friction factor in W/O emulsion systems, which shows a good prediction accuracy in 10% and 20% water cut conditions.

Study of hydrate formation in gas-emulsion multiphase flow systems

As the oil & gas industry moves into deep water, hydrate has been a major hazard to the deep sea flow assurance. The objective of this work is to study the hydrate formation kinetics in a gas-emulsion multiphase flow system. A series of experiments were carried out with different gas/liquid flow rates using a high pressure flow loop. Results showed that the experimental data were remarkably reproducible in the flow loop system. It was found that as the gas flow rate and liquid flow rate increased, the hydrate formation induction time increased and the critical supercooling degree decreased. The gas/liquid flow rates exhibited little effect on the hydrate formation amount. As the liquid hold-up increased, both the induction time and the critical supercooling degree increased at first and then decreased. In addition, the hydrate formation amount remained almost constant when the liquid hold-up was higher than 20%.

Experimental Study on Blockage of Gas Hydrate Slurry in a Flow Loop

Hydrate formation and blockage in long deepwater pipelines has long been a trouble for offshore petroleum production. Consequently, understandings of the procedures as well as influencing factors of hydrate blockage are key points to make reasonable flow assurance strategies. Thus two series of experiments have been conducted in a high-pressure hydrate flow loop newly constructed by multi-phase flow research group in China University of Petroleum (Beijing). One of the systems consists of water and CO2, while the other one includes water, diesel oil and natural gas. The relative time of hydrate blockage has been studied by varying pressure and flow rate for both two systems. The dimensions of hydrate particles in fluid during plugging are also investigated. The results indicate that the influencing factor exerts a similar effect on the relative time for the different systems. Besides, the sizes of particles in the fluid would change significantly due to hydrate formation.Copyright

Investigation on the Transition Criterion of Smooth Stratified Flow to Other Flow Patterns for Gas-Hydrate Slurry Flow

A stability criterion for gas-hydrate slurry stratified flow was developed. The model was based on one-dimensional gas-liquid two-fluid model and perturbation method, considering unstable factors including shear stress, gravity, and surface tension. In addition, mass transfer between gas and liquid phase caused by hydrate formation was taken into account by implementing an inward and outward natural gas hydrates growth shell model for water-in-oil emulsion. A series of gas-hydrate slurry flow experiments were carried out in a high-pressure (>10 MPa) horizontal flow loop. The transition criterion of smooth stratified flow to other flow patterns for gas-hydrate slurry flow was established and validated and combined with experimental data at different water cuts. Meanwhile, parameters of this stability criterion were defined. This stability criterion was proved to be efficient for predicting the transition from smooth to nonsmooth stratified flow for gas-hydrate slurry.

Advances in coupling thermodynamics and kinetics studies of wax precipitation-deposition and hydrate nucleation-formation

During the development of deep-sea petroleum industry, wax and hydrate are prone to form at the same time in the transportation system from reservoir to downstream equipment under high pressure and low temperature condition for the petroleum reservoir with high wax content, high pour point and high viscosity, which will increase the risk of plug or blockage and enhance the safety hazards of the system. Therefore, it is significant to investigate the characteristics of the coexistence system with wax and hydrate, which is also one of the important issues in flow assurance problems. The studies on wax and hydrate separately as well as the coupling studies in the coexistence system from aspects of thermodynamic and kinetics were reviewed and commented, combined with some initial experiments of this coexistence system carried out in high-pressure reactor and flow loop. Several thermodynamic models, like Wilson model and UNIQUAC model, are established or modified to describe wax solid phase more accurately. The classical thermodynamic models for hydrate formation prediction include vdW-P model, which is the most extensively used one, and Chen-Guo model, which is more simple and convenient to use based on a two-step mechanism. Based on the above models, the coupling study of wax and hydrate in thermodynamic aspect thus needs a roubust flash algorithm to characterize complex systems where several phases, i.e. vapor, water, hydrate, wax, liquid hydrocarbon may coexist. Mutual effects of wax and hydrate are explored by using the integrated thermodynamic model and algorithm. A lot of researches have been done on kinetics of wax deposition and hydrate formation separately. Single-phase pipe flow and multiphase pipe flow experiments have been carried out to explore the effects of such factors as molecular diffusion, shear dispersion, adhesion, flow pattern on wax deposition. The non-stoichiometry hydrate formation includes nucleation and growth. Based on experimental studies, physical modeling and mathematical modeling have been done to describe the kinetics of hydrate formation. At present, only a few studies of coupling kinetics of wax deposition and hydrate formation are reported, which are in an early stage of exploration and research. There is no systematic research achievement and no definite conclusion about the dynamic interaction between wax and hydrate. Some advanced experimental means, such as FBRM, PVM, MMF, can provide information in micro level. The main issues to be researched in the future were proposed, which should be based on the independent researches of thermodynamics and kinetics of wax and hydrate. For the study on coupling thermodynamics, a robust and valid coupling thermodynamic model should be established to understand the interaction mechanism of phase behaviors between wax and hydrate, by using the improved model for describing the non-ideality of the liquid and solid phases and focusing on the component changing in each phase. For the study on coupling kinetics, the microscopic distribution mechanism of the solid particles and emulsion phase in the coexistence system should be investigated, and the diffusion mechanism of wax, the adsorption and growth characteristics of hydrate interface should be explored, with the help of visual testing and equipment, from the two aspects of the influence of wax on hydrates formation and hydrate on wax precipitation and deposition, to obtain the interaction mechanism of kinetics behaviors between wax and hydrate.