Flow assurance

Study Correlates Hydrate Blockage Risk and Gas/Liquid Flow Pattern in Horizontal Pipelines

This paper analyzes the risk of hydrate formation and blockage, aiming at various gas/liquid flow patterns and considering the phase distribution and interface distribution characteristics of different flow patterns.

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Current research into the risk of hydrate formation and blockage in pipelines is limited to prediction of hydrate formation. However, hydrate generation often does not lead to flow barriers. Hydrate growth and deposition rates are important to hydrate generation as a blockage risk. To help solve the problem of flow assurance in horizontal gas/liquid pipelines, this paper analyzes the risk of hydrate formation and blockage, aiming at various gas/liquid flow patterns and considering the phase distribution and interface distribution characteristics of different flow patterns.

Introduction

Hydrate blockage risk in transportation pipelines is one of the most important topics in the study of flow assurance. Large-scale natural gas development, for example, requires construction of large-scale gathering and transmission pipeline networks. Natural gas is often transported by horizontal or small inclined pipelines, and water generation and flow are often present in natural gas pipelines, which undoubtedly increases the risk of hydrate formation and blockage. When gas and water exist at the same time, there will often be a variety of gas/liquid flow patterns in pipelines, and different flow patterns will have different risk of hydrate formation and blockage. To avoid hydrate risk, it is necessary to study the correlation between hydrate generation blockage risk and gas/liquid flow pattern.

Research of Hydrate Growth and Deposition

Based on the hydrate formation-growth-deposition mechanism and combined with the hydrate experiment in the flow loop, the characteristics of hydrate ­formation-growth-deposition in pipelines under different gas/liquid flow patterns was studied. The results show that different flow patterns yield different hydrate formation and deposition characteristics as a result of different phase and interface distributions. The bubble flow, cluster flow, and slug flow have some similarities in flow patterns. The gas phase in the flow system exists in the form of bubbles, and the occurrence of thin liquid film on the tube wall under these three flow patterns is relatively rare. Accordingly, the similarity of laminar flow, wave flow, and annular-mist flow shows that thin liquid film or gas/liquid pipe wall three-phase interface will always appear in the flow process. Thus, the hydrate formation and deposition risk of the latter three flow patterns is significantly greater than that of the former three. Comprehensive analysis shows that the hydrate risk of each flow pattern is in the order of, with greatest risk first, annular-mist flow, laminar flow and wave flow, slug flow, cluster flow, and bubble flow. The annular-mist flow is the most dangerous flow pattern for hydrate formation and blockage. In this case, special attention should be paid to hydrate prevention and control.

The complete paper presents a detailed historical discussion of correct division of flow pattern and of research on hydrate growth models. Researchers have established a series of hydrate-growth kinetics models combined with crystallography, heat transfer, mass transfer, and other principles. Recent models established a hydrate growth model based on heat transfer.

The complete paper also discusses research on the sedimentation and flow characteristics of hydrate formation in pipeline transportation. For example, a gas/liquid two-phase flow system can be divided into gas-dominated and water-dominated systems. The gas-dominated system contains significant quantities of gas.

It is believed that hydrate slurry will be produced in the process of natural gas transportation. The increase of the viscosity of hydrate slurry will increase the liquid holdup of the pipeline, decrease the flow rate, and eventually stop the flow. The results of a study of a gas/liquid annular fluid system with 6% water content in a circulating pipeline show that with the increase of hydrate volume fraction, the pressure drop of the pipeline increases, and the deposition and shedding behavior of hydrate particles have a significant effect on the pressure drop.

The authors cite a previous study that argues the importance of studying the distribution of hydrate on the pipe wall and in the fluid to describe accurately the friction pressure drop of the multiphase flow system of hydrate. For water-dominated systems, another research team conducted a series of hydrate experiments using a high-pressure hydrate loop to study the process of blocking pipelines by methane hydrate with 100% water content (oil-free).

On the basis of other experimental studies, the gas/liquid flow patterns (stratified flow, wave flow, bubble flow, and slug flow) are considered, and three flow patterns of a gas-hydrate system are proposed: uniform distribution of hydrate particles under wave-flow pattern; nonuniform distribution of hydrate particles under wave-flow pattern; static bed layer at the tail of slug-flow pattern; and hydrate at other parts.

Other researchers cited in the complete paper used a transparent loop to study hydrate formation and flow in a gas/water system under high water content. The results show that flow pattern affects bubble size and hydrate growth rate. Later, a population equilibrium model was established describing the nucleation, growth, aggregation, and fragmentation of hydrate under turbulent conditions. The simulation results show that the aggregation and fragmentation of hydrate particles have the same effect on particle size distribution as nucleation and growth, and the size distribution of hydrate particles is expressed as a normal distribution law.

This study focuses on the horizontal or small inclined pipelines commonly used in natural gas pipeline transportation. Different gas/liquid flow patterns in horizontal pipeline are considered. In combination with the existing hydrate experiment in the flow loop, the characteristics of hydrate formation and blockage under different gas/liquid flow patterns are analyzed, and the risk of hydrate formation and pipelines blockage under different gas/liquid ratio conditions can be evaluated. This study can provide theoretical support for hydrate risk assessment in the process of pipeline transportation, help ensure the safety of pipeline transportation, and improve the economic benefits in the process of pipeline transportation.

Gas/Liquid Two-Phase Flow Pattern

In the process of gas/liquid two-phase flow, the determination and identification of two-phase flow pattern is an important link, which can provide reliable technical support for the safe operation of industrial production processes. The complex change of flow pattern is caused by the continuous change of gas/liquid interface and gas/liquid two-phase distribution in the process of gas/liquid flow. In addition, the influence of gravity makes the gas/liquid two-phase flow state different from the vertical pipe flow. Generally speaking, the gas/liquid two-phase flow in horizontal pipelines can be divided into three types: dispersed flow, intermittent flow, and separated flow. The complete paper discusses several main flow patterns.

Hydrate Formation and Blockage Process in Pipelines Under Different Gas/Liquid Flow Patterns

The complete paper discusses and illustrates hydrate formation and blockage mechanisms under bubble flow, cluster flow and slug flow (Fig. 1); the mechanism of hydrate formation and blockage in stratified and wave flows; and the mechanism of hydrate formation and blockage in circulating mist. The paper then presents a detailed, illustrated discussion, augmented with equations, of risk assessment of hydrate formation and blockage under different gas/liquid flow patterns, including bubble and cluster flow, stratified and wave flow, slug flow, and annular-mist flow.

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Fig. 1—The diagram of hydrate formation and deposition in a pipeline under bubble-flow or cluster-flow pattern.

Conclusions

  • The problem of hydrate in pipelines is an important subject in the study of flow assurance. In fact, hydrate formation is only the first step to forming flow barriers in pipelines. It is only when hydrate can deposit stably and form blockage in a pipeline after hydrate formation that a flow barrier is formed. However, at present, many studies focus on the formation of hydrates exclusively; the formation and deposition of hydrates should be considered simultaneously.
  • Hydrate formation requires gas/liquid coexistence and appropriate temperature and pressure conditions, and hydrates are often formed at the gas/liquid cross section. Because of the different gas/liquid ratio, there are many flow patterns in the gas/liquid pipeline transportation system. According to the gas/liquid ratio, bubble flow, cluster flow, stratified flow, wave flow, slug flow, and annular-mist flow will appear in turn. Different flow patterns show different hydrate formation and deposition characteristics owing to different phase and interface distributions.
  • Hydrate is easier to form and deposit when thin liquid film is attached to the pipe wall. Bubble flow, mass flow, and slug flow have some similarities in flow patterns. The gas phase in the flow system exists in the form of bubbles, and the occurrence of thin liquid film on the tube wall under these three flow patterns is relatively rare. Accordingly, the similarity of stratified flow, wave flow, and annular-mist flow shows that a thin liquid film or gas/liquid-pipe wall three-phase interface will always appear in the flow process, which will undoubtedly make the formation and deposition risk of the latter three hydrates significantly greater than that of the former three flow patterns.
  • Annular-mist flow is the most dangerous flow pattern for hydrate formation and blockage. In this case, special attention should be paid to hydrate prevention and control.

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper Paper SPE 197534, “Study on the Correlation Between Hydrate Formation-Blockage Risk and Gas/Liquid Flow Pattern in Horizontal Pipelines,” by Wenyuan Liu, Jinqiu Hu, and Fengrui Sun, China University of Petroleum-Beijing, et al., prepared for the 2019 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 9–12 November. The paper has not been peer reviewed.