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Gas hydrates phase transition is crucial for energy exploitation, natural gas transportation and CO 2 capture storage fields. This review introduces the characteristics of gas hydrate in phase transition process including the thermodynamic and kinetic characters, the dissociation and reformation characters, and the promotion methods. The phase equilibrium conditions of gas hydrate determine its thermal properties, but some accurate statistical studies are needed to obtain a variety of phase equilibrium experimental results in the future. In addition, it is necessary to quantify the factors of hydrate nucleation and provide an accurate induction time prediction model due to the randomness of the nucleation process. In hydrate exploitation applications, the depressurization and heat injection methods may be hindered due to the limitation of temperature, pressure, inhibitor and heat transfer characteristic. It will cause the hydrate reformation phenomenon and further influence the hydrate exploitation efficiency. Controlling the hydrate exploitation process requires a more systematic combination of decomposition methods, and further proving the influence factors such as memory effect and nanobubbles for the hydrate reformation. Besides, the promotion for gas hydrate phase transition also has been studied to make the hydrate rapid formation in the industrial fields come true. Stirring, spraying, bubbling method and additives are available for the promotion. However, the energy cost and efficiency improvement of hydrate formation promotion process need to be further studied. At last, several gas hydrate applications are proposed, including CO 2 capture and sequestration, natural gas storage, seawater desalination, cold storage, mixed gas separation and sewage treatment. This review presents an overall analysis of gas hydrate phase transition from characteristics to applications and contributes a reference for future development in hydrate technology. [Display omitted] • Review the characteristics and applications of gas hydrate transition and its future perspectives • Providing the scientific and technical problems of hydrate phase transitions and the research bottlenecks. • Analyzing the future research direction of hydrate decomposition and reformation. • Elaborating on specific examples of gas hydrate applications, analyzing existing problems, and proposing future prospects. • Putting forward some suggestions to improve the current technology on hydrate phase transition promotion technology. [ABSTRACT FROM AUTHOR]

More than 65% of the worldwide natural gas is transported through pipelines. Gas hydrate formation and blockage in the pipeline are commonly encountered, causing significant problems in safe and efficient transportation. Yet the instrument simulating the high-pressure multiphase flow and demonstrating the hydrate blocking behavior at flowing conditions is not available. In this work, a high-pressure visual flow loop is developed to detect and observe the hydrate nucleation, growth, and deposition during the multiphase flow. The pressure limit is designed to be 8 MPa, and the temperature can go down to −20 °C. This device allows a direct observation of the gas-liquid flow regime during transportation; investigations on the initiation and evolution of hydrate blockage in the dead-leg section, U-shaped region, and rising pipes are now possible. Sensors are distributed along the pipelines to detect the pressure and temperature change; the differential pressure is used to effectively detect the degree of hydrate blockage, which increases upon hydrate formation. The visual flow loop could also help examine the effect of shut-down and restart of the pump on the flow behavior. The system has been verified feasible demonstrating the flow behavior and predict hydrate blockage. The developed device would help understand the mechanism of hydrate blockage in the transportation pipelines and provide guidance to efficiently avoid such problems. [ABSTRACT FROM AUTHOR]

The commercial exploitation of natural gas hydrates (NGHs) has been a growing research focus due to its features of enormous reserves and clean fuel. To guarantee the safe and efficient production of NGHs, we have proposed a novel strategy of water flow erosion to promote methane hydrate (MH) decomposition based on the tremendous seawater resource and the fundamental process of water-gas flow during NGHs exploitation. In this study, the synthetic effects of traditional-thermodynamic-factors (temperature, salinity, pressure) and fluid flow on MH decomposition characteristics, which is known little about yet, are comprehensively analyzed via in-situ magnetic resonance imaging (MRI). The temporal-spatial behaviors of MH decomposition are visually investigated. The results indicate that the pressure, salinity, temperature and water flow synergistically increased MH decomposition efficiency. Additionally, the propagation of the decomposition front along the interface between MH and ambient phase shows that the water flow rate and heat transfer are two crucial factors for accelerating MH decomposition. The higher water flow rate also efficiently complements the insufficient decomposition driving force due to the heat loss during MH decomposition process. The highest average decomposition rate (1.1%/min) and the relatively less water injection volume (320 mL) can be archived in this study. Furthermore, the decomposition rate has a significant dependence on temperature under lower water flow rate. Graphical abstract: this work simulates the process of water flowing through the gas hydrate reservoir. The pores provide the flow path, differential pressure provide the flow driving force. Temperature, pressure, ions and water flow rate are the four main thermodynamic factors for hydrate decomposition during water flow erosion process. The water can be injected into the high-pressure zone, and the gas can be collected in low-pressure zone. [Display omitted] • Radial-interface dependence of MH dissociation dominated by water flow are verified. • MH dissociation rate has an obvious dependence on temperature under low flow rate. • Saturation and distribution of MH in sand sample dominate the water injection volume. • Promotion effect of multi-thermodynamic-factors on dissociation is the most obvious. [ABSTRACT FROM AUTHOR]

• Hydrate blockage formation and removal were studied in a fully visual flow loop. • Hydrate blockage first occurred in the deadleg. • Shut-in operation caused hydrate blockage due to the temperature decrease and hydrate annealing. • Lower liquid loading led to a faster hydrate blockage process and more efficient blockage removal using depressurization. Hydrate blockages impose a serious flow assurance problem on gas transportation pipelines. Regular inspections for the formation of hydrate blockages are imperative along with the use of an effective blockage removal method. In this study, the processes of hydrate blockage formation and removal were investigated using a fully visual flow loop. Comprehensive analysis of the pressure drop was performed along with a discussion of the acquired visual images of the hydrates to describe the phenomena. Hydrate blockage first occurred in no through-flow zone of the pipeline, specifically in deadleg. The results showed that higher LL (liquid loading: the initial liquid volume fraction) caused a blockage in the inlet, while lower LL caused a faster and more widespread blockage. Moreover, shut-in and restart operations also caused blockages due to the temperature decrease and annealing of hydrate. The experiments showed that blockages formed more quickly as a result of lower LL during the shut-in operation. This necessitated the reduction of shut-in duration and maintenance of the temperature at phase equilibrium condition. In addition, the depressurization method was used to decompose bulk hydrate in order to remove the blockage. The results confirmed that stepwise depressurization could be applied to prevent hydrate reformation at a higher LL and water conversion rate, while a lower backpressure could be used at lower LL to efficiently remove blockages. [ABSTRACT FROM AUTHOR]

• Performed complete hydrate slurry flow characteristics in fully visual flow loop. • Providing the alarm points for hydrate agglomeration, deposition and bedding. • Analyzing the water conversion rate and the hydrate effective volume fraction. • Finding the hydrate accumulation position in the slope pipes. Gas hydrates pose impeded flow risks and serious safety hazards in oil and gas transportation pipelines. This makes it imperative to look for appropriate hydrate control strategies. In this study, the hydrate slurry flow characteristics were investigated under multiphase flow conditions using a high-pressure fully visual flow loop. At different liquid loadings and mixture velocities, pressure drop variations were monitored from the initial hydrate formation to deposition and bedding, and the flow patterns were observed throughout the experiments. It was found that the hydrate slurry flow may involve four stages of hydrate formation, agglomeration, deposition and bedding prior to pipeline blockage. The effective volume and water conversion fractions were also obtained at different stages. The results showed that the lower liquid loading causes quicker and more severe hydrate blockage problem mainly due to more mass transfer of gas into water hence higher water conversion rate. Moreover, lower mixture velocity resulted in a higher hydrate bedding tendency. The results also confirmed further formation and deposition of gas hydrates due to a drastic temperature drop immediately after the liquid flow stopped in the system. In addition, the flow characteristics in sloped pipe sections (both upslope and downslope) were studied to investigate the effect of driving forces on the hydrate deposition. Slug flow was observed in the upslope while stratified flow in the downslope. Furthermore, hydrate accumulation tends to occur in the transition position between the horizontal pipes and the sloped pipes due to the effect of gravity. [ABSTRACT FROM AUTHOR]

• The variation of induction process was studied in hydrate formation in a visual flow loop. • The initial pressure is inversely proportional to the hydrate formation induction time and reaction time. • With the increase of the flow rate, the hydrate formation induction time and reaction time are shortened, but there is a flow rate threshold. • With the increase of inclination angle, the induction time tends to shorten and the reaction time generally prolongs. One of the primary issues with natural gas transportation assurance is pipeline obstruction due to the formation and deposition of gas hydrates. Thus it is crucial to study the prevention and control methods of natural gas hydrate formation. A high-pressure visual flow loop was established in this study to investigate the effects of pressure, liquid velocity, and loop inclination angle on the formation of natural gas hydrates, hydrate slurry flow and morphology. For the formation of hydrates, it was demonstrated that the pressure was inversely related to the induction time and formation time. As the initial pressure increases, the conversion rate of water increases, leading to poor fluidity. With the increase of liquid velocity, the conversion rate of water increases and there is a threshold liquid velocity. Compared with horizontal loop, the hydrate particles in the slope were repeatedly deposited and removed off the surface of the sedimentary layer by gravity, and some large hydrate particles in the slope section gathered near the bottom of the pipeline. The induction time shortens and the formation time prolongs as the inclination angle increases. The inclination angle shows little effect on the final conversion rate of water, but it can delay the growth of hydrates obviously. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrate formation induced by cold seeps is first investigated by MRI. • Local heat release by hydrates formation leads to the dissociation of nearby hydrates. • Cold seeps with high gas flow rate and pressure are more valuable for hydrate resource surveys. Gas hydrates could occur as outcrop resources overspreading on the seafloor, coexisting with bubbling gas in activated cold seeps. This involves a complicated process of hydrate nucleation and accumulation in the sedimentary matrices as well as in the bulk water with an underlying gas plume. In addition, the escaping methane gas could also be a major concern in terms of its greenhouse effect. Consequently, the behaviors of hydrate formation in the sediments with a successive gas flow to mimic the cold seeps were monitored through magnetic resonance imaging technique in this work. It was found that the initial hydrate nucleation could occur universally throughout the sedimentary matrices resulted from an enhanced gas–water contact upon the gas flow. Specially, the substantial heat released upon hydrate formation could even locally trigger dissociation of adjacent hydrates. A faster hydrate formation was observed under higher gas flow rates and formation pressures, consolidating the dominant role of a sufficient gas–water contact in the hydrate formation. Nevertheless, large amount of residual water could still remain till the very end of the formation process; the subsequent reaction would be significantly limited by the gas diffusion barrier effect of the existing hydrate shell. The findings could provide insights into the kinetic process of hydrate formation in the cold seeps and expand our knowledge on the diverse occurrence of gas hydrate under the sea. [ABSTRACT FROM AUTHOR]