Research progress on hydrate plugging in multiphase mixed rich-liquid transportation pipelines

Abstract

The plugging mechanism of multiphase mixed rich-liquid transportation in submarine pipeline is a prerequisite for maintaining the fluid flow in the pipeline and ensuring safe fluid flow. This paper introduced the common experimental devices used to study multiphase flow, and summarized the plugging progress and mechanism in the liquid-rich system. Besides, it divided the rich-liquid phase system into an oil-based system, a partially dispersed system, and a water-based system according to the different water cuts, and discussed the mechanism of hydrate plugging. Moreover, it summarized the mechanism and the use of anti-agglomerates in different systems. Furthermore, it proposed some suggestions for future research on hydrate plugging. First, in the oil-based system, the effect factors of hydrates are combined with the mechanical properties of hydrate deposit layer, and the hydrate plugging mechanism models at inclined and elbow pipes should be established. Second, the mechanism of oil-water emulsion breaking in partially dispersed system and the reason for the migration of the oil-water interface should be analyzed, and the property of the free water layer on the hydrate plugging process should be quantified. Third, a complete model of the effect of the synergy of liquid bridge force and van der Waals force in the water-based system on the hydrate particle coalescence frequency model is needed, and the coalescence frequency model should be summarized. Next, the dynamic analysis of a multiphase mixed rich-liquid transportation pipeline should be coupled with the process of hydrate coalescence, deposition, and blockage decomposition. Finally, the effects of anti-agglomerates on the morphological evolution of hydrate under different systems and pipeline plugging conditions in different media should be further explored.

Graphical abstract

Keywords

hydrate / rich-liquid phase / plugging mechanism / coalescence / deposition / anti-agglomerate

1 Introduction

Natural gas hydrates (NGHs) are a type of non-stoichiometric crystal solid formed by the reaction of light hydrocarbon compounds with water at a low temperature and high pressure. In the crystal, water molecules are connected by hydrogen bonds to form a cage structure. Each cage structure contains one or more guest molecules, in which the van der Waals force between water molecules and guest molecules maintains the stability of the cage structure [1]. As energy extraction in China gradually shifts from inland to deep-sea areas, the natural environmental conditions deep in the seabed readily induce the formation of hydrates in the mixed pipelines and cause plugging. In severe circumstances, the degree of pipeline operation is substantially affected within a few hours after hydrate nucleation, and thus, the prevention and control of hydrate formation are particularly important topics being analyzed in current research [2].

Currently, two methods for the prevention and control of hydrates in subsea multiphase mixed pipelines have been developed: traditional inhibition and hydrate dynamic management systems. Traditional inhibition mainly applies thermal insulation to pipes or adds thermodynamic inhibitors to hinder hydrate formation. However, because submarine pipelines have the characteristics of a long pipeline length, low temperature, and high pressure, the addition of thermal insulation or thermodynamic inhibitors will result in high costs, require more materials, and generate environmental pollution [3]. The hydrate dynamics management system uses kinetic hydrate inhibitors (KHIs) and anti-agglomerates (AAs), which allow the hydrate to form in the pipe and flow in the form of hydrate slurry. The two methods exhibit a substantially reduced cost and usage [3,4]. However, in multiphase mixed rich-liquid system, there are many factors that affect the form of hydrate plugging, leading to different effects on the coalescence and deposition of hydrate particles. Therefore, the hydrate has different ways of plugging the pipeline, which increases the difficulty of preventing and controlling hydrate formation in the future.

Some developed countries have analyzed hydrate in multiphase mixed pipelines, but the research on hydrate in China is still in the initial stage. Less research has been conducted on the dynamic analysis of multiphase mixed liquid-rich systems coupled with the process of hydrate coalescence, deposition, and blockage decomposition. Due to the complexity of hydrate plugging in multiphase liquid-rich pipelines, many effect factors, including water cut, pressure drop, temperature, subcooling, tube wall hydrophilicity, flow rate, and gas solubility, interact, which have different effects on the hydrate plugging mechanism. Therefore, this paper mainly summarizes the plugging process and mechanism in the liquid-phase system and divides the rich-liquid phase into an oil-based system, a partially dispersed (PD) system, and a water-based system according to the different water cuts. Besides, it summarizes the plugging mechanism of the hydrate in different systems. Moreover, it describes the mechanism of the AAs and the experiments and analyzes the effects of the anti-agglomerate on maintaining the safe flow of the fluid in the pipe. This paper provides some ideas for future studies of hydrate plugging and the safety of the pipeline under the rich-phase system of the multiphase mixed pipeline.

2 Hydrate experimental device

Currently, scholars from various countries have conducted research on hydrate plugging and hydrate dynamic management in multiphase mixed pipelines, generally pertaining to the reactor or hydrate experimental loop, in which the plugging mechanism of hydrates and the flow characteristics of hydrate slurry have been extensively studied using these devices.

2.1 Reactor

The common reactors used to study multiphase flow are stirred and rocking reactors. The former exerts interference on the system through the stirring paddle in the kettle, and the latter is regularly oscillated by a mechanical device connected to the reactor [5,6]. For the stirred reactor, the flow and shear caused by the stirring paddle have a large deviation compared with the flow and shear in the actual process, and thus a large error remains in the formation of the hydrate compared with the actual conditions. For most KHIs, including polyvinylpyrrolidone (PVP), amino acids and ionic liquids are still being used in stirred reactors [7–9]. Compared with the stirred reactor, the swing reactor is smaller in size and equipped with movable stainless steel pellets. Therefore, it is able to better mix the medium in the kettle and provide shear and disturbance to the flow [10]. Currently, the swing reactor is mainly used to analyze the AAs in the rich-liquid phase of the oil-based system. The research on the mechanism of hydrate deposition requires a supercooled section of the pipe wall and a window to facilitate the observation of experimental phenomena [11].

2.2 Experimental loop

Compared with the reactor, the loop experiment clearly shows the formation and blockage of hydrate and the hydrate slurry flow in a multiphase flow system. However, the construction of the experimental loop is relatively expensive and covers a large area. Although many institutional facilities have established experimental loops, the more famous loop experimental devices include the Archimede loop and IFP-Lyre loop of France, the Exxon Mobil loop and FAL loop of the US, the Hytra loop and PetrecoA/S loop of Australia, the GIEC hydrate flow loop of the Chinese Academy of Sciences, and the CUPB chemical loop and CUPB storage and transportation loop of China University of Petroleum [12]. The experimental loop reflects the blockage of the pipeline by measuring changes in the pressure drop and flow during the experiment. The formation of the hydrate and the blockage process of the supercooled section are observed from the set high-pressure window. The focused beam reflectance measurement (FBRM) and particle video microscope (PVM) measure the size and shape of droplets and hydrate particles in real time and monitor the coalescence and fragmentation of hydrate particles. The domestic and international experimental loops are now summarized for the statistical analyses of hydrate blockage and safe flow (Table 1) [12].

2.3 Other hydrate devices

In addition to the reactor and loop experiments, the Colorado University of Mines has designed a micromechanical force measurement device (MMF) to directly measure the adhesion between particles and the tube wall, as shown in Fig. 1 [48]. The working principle is described below. First, the hydrate particles generated are fixed at the end of the fiber cantilever, and the manipulator is controlled to move the fiber cantilever downward to allow the particles to contact each other and exert a certain force. After becoming stabilized after a certain period, the fiber cantilever is moved upward. The deformation state of the cantilever when the hydrate particles are completely separated is recorded, and the adhesion between the particles is obtained by using Hooke’s law.

3 Plugging process and mechanism of hydrate formation in rich-liquid phase

3.1 Hydrate plugging process in rich-liquid phase

The submarine mixed pipeline system is divided into a rich-liquid phase and a rich-gas phase, according to different media. Compared with the rich-gas phase, the rich-liquid phase is divided into an oil-based system, a PD system, and a water-based system, due to the difference in water cuts. At each stage, the formation and blockage of hydrates differ due to environmental conditions. In the oil-blasting stage, some surface-active substances, such as asphaltenes, are contained. When the water cut is less than 50%, a stable W/O emulsion is formed under the action of flow shearing, and the aqueous phase is sufficiently dispersed in the oil phase. This phase is considered an oil-based system. As the water cut increases, a portion of the free water separates under the emulsion system to form a PD system. No clear standards have been established for PD systems and water-based systems. Liu et al. [49], in combination with the theory reported by Sloan [50], have proposed that the oil-water system is a PD system when the water cut is below the W/O emulsion phase transition point, and the system is a water-based system when the water cut is above the W/O emulsion phase transition point. However, the plugging process of hydrates in the rich-liquid phase involves gas, liquid hydrocarbons, water, and solid hydrates, and thus the research and exploration of this process encounters difficulties.

Sum et al. [51] have postulated that in the flow of rich-liquid phase (Fig. 2), the formation of hydrates to create a blockage is mainly caused by the four stages: the pre-preparation stage in which the emulsification of the oil and water phases in the system provides a surface site for the formation of hydrates; the hydrate formation stage in which a large amount of hydrates form at the oil-water interface, and the hydrate particles generated are suspended in the slurry or deposited on the solid surface, causing the flow rate to change, since the pressure and temperature satisfy the hydrate formation conditions; the particle packing stage in which the hydrate particles continuously coalesce to form a bulk hydrate or accumulate on the surface of the deposited layer, decreasing the flow rate, due to the interactions among particles; and the plugging stage in which the hydrates continuously formed cause plugging when the fluid in the tube is not flowing. In the rich-liquid phase system, when the hydrate particles are suspended in the aqueous phase, the binding force between the hydrate particles is small and always remains dispersed. If the particles are dispersed in a continuous oil phase, the hydrate particles continue to aggregate to form a larger hydrate due to the bridging force. In addition, some scholars have proposed that the differences in the initial flow and the pressure drop also exert effect on the formation and plugging of hydrate.

Song et al. [52] have identified two types for hydrate to form a plugging in the pipeline according to the different pressure drops and the initial flow rate in the diesel+ water+ natural gas system. In the first type, the hydrate particles form a hydrate deposit on the pipe and are continuously deposited (Fig. 3). They have divided the process into four phases: the stable flow stage in which no hydrate forms, and the liquid-phase flow and pressure drop in the system remain stable (Fig. 3(a)); the initial stage of hydrate formation in which the sudden decrease in the flow rate causes hydrate formation. As the hydrates are continuously formed, the viscosity of the fluid increases, and the frictional resistance increases such that the pressure drop increases as the flow rate decreases. At this time, the formed hydrate is mainly distributed in the liquid phase, showing silt-like hydrates (Fig. 3(b)); the formation of the deposit layer stage in which, as hydrates are continuously formed, the hydrate particles begin to form a deposit layer at the bottom of the liquid phase (Fig. 3(c)); and the deposit layer growth stage in which the deposit layer reaches a steady-state, the liquid is gradually extruded, the layered structure becomes dense, and finally a pipeline safety problem occurs.

The second type is in the flow at a high water cut and high flow rate. In this type, the oil-water mixture is stratified by gravity, and the accumulation of the hydrate slurry causes a sharp increase in fluid viscosity and a drastic decrease in fluidity (Fig. 4). This type of process has also been divided into four stages: the stable flow stage in which no hydrate is detected, the liquid-phase flow rate and pressure drop are not substantially changed, and the oil-water mixture displays good fluidity; the hydrate formation stage in which hydrates are formed rapidly, but due to the high water cut in the system, slurry-like hydrates with a high viscosity are formed in the liquid and gas phase space walls; the equilibrium phase in which the hydrate formation process is basically complete. At this time, the liquid phase begins to stratify (Fig. 4(a)), the upper layer is the lower density diesel oil in the oil-water mixture, and the lower layer is the oil-water mixture containing more hydrate particles; and the plugging stage in which hydrate rapidly forms in the mixed layer, causing the viscosity of the mixed layer to increase substantially, resulting in a decrease in the flow rate and an increase in frictional resistance, and the pressure drop first increases as the friction increase and then decreases as the flow rate decreases. For the above two types of hydrate plugging processes, the flow shear forces are different due to the water cut. Therefore, the liquid-phase stratification phenomenon is not observed in the first type of hydrate plugging process. In addition, the second type of hydrate plugging process takes a short time. The mass fraction of hydrates formed is low. Therefore, no stable hydrate deposit layer forms. However, the reasons for the two hydrate shapes are not explained in detail. Meanwhile, the hydrate plugging process is often the result of a combination of multiple mechanisms, and the effect on the hydrate particle coalescence has not been considered. In addition to the two methods of hydrate plugging, studies of the mechanism of pipe plugging are divided into the oil-based system, the PD system, and the water-based system, based on different water cuts.

3.2 Hydrate plugging mechanism in oil-based system

In early offshore oilfields, the water cut was low, and thus the formation and plugging of hydrate in oil-based system was also the subject of earlier research [53]. Currently, the formation of hydrate particles in oil-based systems is divided into three stages, as shown in Fig. 5 [54]. ① The combination of water droplets and oil forms a hydrate shell layer on the outer surface of the water droplet. ② The inside of the water droplet penetrates the oil phase to increase the thickness of the hydrate shell layer. ③ The inside of the shell is gradually converted into hydrate, but some water droplets are still not converted due to the effects of mass transfer and heat transfer. For the analysis of hydrate plugging in oil-based systems, researchers have proposed different hydrate plugging mechanisms to summarize the pipe plugging process. Currently, according to different factors, such as the water cut, pressure drop, temperature, flow rate, subcooling degree, conveying medium and pipe material, the plugging mechanism of the oil-based system is divided into the coalescence of hydrate particles, the adhesion of the hydrate particles to the tube wall, and the theory of hydrate particles deposition.

3.2.1 Coalescence of hydrate particles

One of the main causes of plugging is the interaction between hydrate particles. In recent years, scholars from various countries have continuously analyzed hydrate particles using micro-mechanical equipment.

Turner et al. [55] have discovered the formation and plugging model of oil-based system hydrate by performing research, and divided it into four steps (Fig. 6). ① The water phase is dispersed in the oil phase due to the shearing force and flow rate to form a W/O emulsion. ② Hydrates nucleate at the oil-water interface to form hydrate particles that encapsulate water droplets. ③ Hydrated particles continuously coalesce and form a large hydrate. ④ As the bulk hydrate continues to coalesce, the pipeline is eventually blocked. Currently, two kinds of coalescence mechanisms for hydrate particles have been proposed. In the first mechanism, the capillary bridge force causes the particles to adhere, thereby forming a hydrate deposit layer to block the pipe. Figure 7 demonstrates the capillary bridge force between the hydrate particles in the static system and considers the coalescence factor of the hydrate particles [56]. Equation (1) is used to calculate the F_cap.

(1){{\displaystyle F}}_{\mathrm{c}\mathrm{a}\mathrm{p}}=\mathrm{2}\mathrm{\pi }{{\displaystyle \rho }}_{\mathrm{2}}\sigma -\mathrm{\pi }{{\displaystyle \rho }}_{\mathrm{2}}^{\mathrm{2}}\sigma \left({\displaystyle \frac{\mathrm{1}}{{{\displaystyle \rho }}_{\mathrm{1}}}}+{\displaystyle \frac{\mathrm{1}}{{{\displaystyle \rho }}_{\mathrm{2}}}}\right),

where F_cap is capillary bridge forces, ρ₁ and ρ₂ are the radius of the curvature of the liquid bridge curved surface and the liquid bridge neck, respectively; σ is the oil-water interfacial tension; ρ₁ and ρ₂ can be calculated using {{\displaystyle \rho }}_{\mathrm{1}}={\displaystyle \frac{S+R\left(\mathrm{1}-\cos \beta \right)}{\cos \left(\beta +\theta \right)}} and {{\displaystyle \rho }}_{\mathrm{2}}=R\sin \beta +{\displaystyle \frac{\left[\sin \left(\beta +\theta \right)-\mathrm{1}\left]\right[S+R\left(\mathrm{1}-\cos \beta \right)\right]}{\cos \left(\theta +\beta \right)}}, respectively; S represents half of the surface pitch; R represents the hydrate particle radius; and θ and β are contact angles and half-fill angles, respectively.

At early stages, Austvik et al. [57] have proposed that the hydrate particles formed at the oil-water interface are initially present in a dispersed state. Then, in the mass hydrate formation stage, the free water in the system is sufficient, and the surface of the particles is covered with a layer of micro-liquid, which has strong hydrophilic properties. When the particles are in contact with each other, the micro-liquid layers fuse together to form a liquid bridge, which causes particle coalescence to form a very large hydrate layer. Then, Liu et al. [58] have established a hydrate stress balance model. The theoretical analysis and actual situation are discussed under the condition of van der Waals forces, capillary bridge forces, electrostatic forces, and separation forces. The results confirm that the capillary bridge force is the main coalescence force between hydrate particles. Regarding the effect of the degree of particle coalescence, Song et al. [59] have identified the factors affecting the degree of hydrate particle coalescence in the oil-based system by performing an orthogonal experiment. The coalescence frequency of the particles is inversely proportional to the contact angle and the oil-water viscosity, but the coalescence frequency increases exponentially as the oil-water interfacial tension increases. However, some scholars have proposed another hydrate particle coalescence process. Camargo and Palermo [60] have postulated that the wet surface of the particles during the nucleation phase of the hydrate causes a liquid bridge to form, and thus the particles adhere. However, when the system is in a stable stage after a large amount of hydrate forms, the liquid bridges between the particles eventually become hydrates. The appearance of this phenomenon raises the question of whether the liquid bridge force is the main cause of particle coalescence, and thus scholars from various countries have summarized a second explanation for the mechanism of hydrate coalescence.

The second explanation is that the interaction between hydrate particles and water causes the particles to coalesce. Palermo et al. [19] have concluded that the coalescence between hydrate particles is not caused by the action of the liquid bridge, but the interaction between the hydrate particles and water. He has described this mechanism as the contact between the hydrate particles and the water droplets to transform the water droplets into hydrate particles, or the newly crystallized nucleated hydrate particles adhere to the similarly hydrated particles to form coalescence. Then, Aman et al. [61] have measured the adhesion of hydrates in the cyclopentane system using a MMF. In addition to the action of the liquid bridge that promoted the coalescence of the particles, the hydrate particles is found to react with water to cause the particles to coalesce. He calls this phenomenon “sintering.” In addition, Fidel-Dufour et al. [14] have studied the mechanism of particle coalescence using the IFP-Lyre experimental loop and found that the water-induced mechanism is an irreversible quasi-chemical process, which can be described as

(2){{\displaystyle \mathrm{H}}}_i+\mathrm{w}\to {{\displaystyle \mathrm{H}}}_{i+\mathrm{1}},

where H_i represents hydrate particles converted from water droplets, i represents primary particles, w represents water droplets, and H_i+1 represents hydrate after coalescence, i+1 represents primary particles after coalescence. Equation (2) indicates that the growth and coalescence of hydrate particles occur at the same time, and the aggregates formed substantially withstand the shear flow force.

The coalescence of hydrate particles is a common cause of safety problems in the rich-liquid phase, particularly in oil-based systems, and studies of the hydrate particles plugging mechanisms are very important. Currently, international scholars generally assume that the force between hydrate particles is capillary bridge forces, and water droplets play an important role as a medium for the coalescence of hydrate particles. However, different opinions still exist on whether the capillary bridge force is the main cause of the coalescence between particles, and it is difficult to reach a unified conclusion. The authors of this paper believe that the adhesion process between hydrate particles is the reason for the coalescence of hydrate particles, which varies according to the water cut. The water cut in the hydrate nucleation stage is low, and thus the adhesions between most of the particles are mainly connected by capillary bridge force. On the other hand, when a large amount of hydrate forms, the water cut is high, and the particles are induced to form a large hydrate coalescence by water. At present, few experiments and models for the plugging mechanism of hydrate particles have been reported. Therefore, more experiments should be conducted and more models be established to verify these hypotheses in the future.

Currently, although the two particle plugging mechanisms are valuable for hydrate prevention and control, the change in fluid viscosity caused by the coalescence of hydrate particles does not reflect the pressure drop during hydrate formation. Therefore, some researchers have suggested that the hydrate particles adhere to the tube wall under the action of the liquid bridge due to the hydrophilicity of the tube wall, which will cause a change in the pressure drop of the tube. Sjöblom et al. [62] have postulated that when the cohesive force between hydrate particles is greater than the adhesion between the pipe walls and the flow shear force, the particles will adhere to the pipe wall, causing an increase in the pressure drop. Aman et al. [63] have proposed the need to simultaneously consider the effects of two factors on the pressure drop of the pipeline: the increasing level of hydrate coalescence in the system increases the viscosity and the adhesion of the particles to the tube wall results in a reduction in the flow area of the tube. As a result, scholars in various countries have discovered another way to block pipes in oil-based systems.

3.2.2 Adhesion of hydrate particles to the pipe wall

The adhesion of the hydrate particles to the pipe wall means that the particles formed in the pipe are in contact with and adhere to the pipe wall under the influence of the capillary bridge force, the van der Waals force, and the electrostatic force. When the amount of particles reaches a certain level, they will coalesce on the tube wall and form a large hydrate deposit layer, leading to a decrease in the flow area of the pipeline and pipeline plugging. Currently, the mechanism of hydrate adhesion to the pipe wall has mainly been studied in the oil-based system at the initial stage of mining and in PD system with a certain water cut. Domestic and foreign scholars have performed related experiments and simulations to study the mechanism of hydrate adhesion to the tube wall.

The adhesion and deposition of hydrates on the tube wall is mostly based on the particle-to-wall adhesion/removal model (Fig. 8). The model is divided into lifting, rolling, and slipping based on the analysis of the forces required for the movement of the particles to the tube wall, and the occurrence conditions are shown in Eqs. (3)–(5). However, when the actual movement modes do not satisfy the above three types, the hydrate particles are deposited on the tube wall [64]. Taylor [65] and Aspenes et al. [66] have used MMF devices to generate cyclopentane (CyC5) hydrate particles. The particles which are formed contact the tube wall to form a hydrate deposit layer, but this situation is affected by the surrounding environment. The adsorption capacity of the particles and the tube wall is proportional to the temperature, inversely proportional to the degree of subcooling, and related to the material covered by the tube wall. Since the CyC5 hydrate is the same type of hydrate as natural gas hydrate, and the nucleation process is reproducible and transient, foreign scholars typically use CyC5 in their current research on the mechanism of particle adhesion [67]. Nicholas et al. [68] have adhered the CyC5 hydrate particles to the wall of the carbon steel material and found that the adhesion is the sum of half the forces from the two hydrate particles and inversely proportional to the roughness of the carbon steel. According to Aspenes et al. [66], the presence of free water in the contact area between the wall and hydrate particles exerts a substantial effect on the adhesion in the presence of free water. The adhesion between particles and the wall of the tube is greater than the adhesion of particles to the wall of the tube in the absence of free water. Therefore, free water plays an important role in the adhesion of the particles to the tube wall and substantially affects the adhesion between particles and the tube wall.

ASPENS et al. [66] found that when there is no free water, the adhesion between the “dry” hydrate particles and the pipe wall is about 0.4 mN/m; when there is free water, the adhesion between the hydrate particles and the wet pipe wall is about 51 mN/m. These two results indicate that the adhesion between the particles and the pipe wall is strong in the presence of free water, and the hydrate deposit is also more stable.

(3)F_1>F_{\mathrm{a}},

(4){{\displaystyle M}}_{\mathrm{d}}+{{\displaystyle F}}_1\times {{\displaystyle l}}_1+{{\displaystyle F}}_{\mathrm{d}}\times {{\displaystyle l}}_2>{{\displaystyle F}}_{\mathrm{a}}\times {{\displaystyle l}}_1,

(5){{\displaystyle F}}_{\mathrm{d}}>{{\displaystyle \mu }}_f\left({{\displaystyle F}}_{\mathrm{a}}-{{\displaystyle F}}_1\right),

where F_d represents drag force, M_d is the interphase drag additional force couple, F_l represents the lift force, F_a represents the adhesion force, μrepresents liquid viscosity, R represents hydrate particle radius, and l₁ and l₂ are radial vector.

At present, relatively few studies have examined the adhesion and deposition mechanisms of hydrate particles on the tube wall. Zhao et al. [64] have analyzed the force of the particles on the pipe wall by establishing a pipe wall adhesion model. The degree of wetness of the pipe wall resulted in differences in the adhesion between the particles and the pipe wall.

In the case of a dry pipe wall, the van der Waals force is the main force underlying the adhesion between the particle and the pipe wall, but in this case, the particle adhesion is low and the particle size affects the adhesion. Equation (6) is used to calculate the van der Waals force.

(6){{\displaystyle F}}_{\mathrm{v}\mathrm{d}\mathrm{W}}={\displaystyle \frac{HR}{\mathrm{6}{h}^{\mathrm{2}}}},

where F_vdW represents the van der Waals force, H is the Hamaker constant, R represents the hydrate particle radius, and h represents the distance between the particle and tube wall.

In the case of a wet pipe wall, the capillary bridge force is the main force underlying the adhesion between the particles and the pipe wall (Fig. 9). In this case, the particle adhesion is approximately 100%, and the hydrate deposit layer formed is difficult to eliminate by the flow in the tube. Equation (7) is used to calculate the capillary bridge force.

(7){{\displaystyle F}}_{\mathrm{c}\mathrm{a}\mathrm{p}}=\mathrm{2}\mathrm{\pi }R\sigma \left(\cos {{\displaystyle \theta }}_{\mathrm{1}}+\cos {{\displaystyle \theta }}_{\mathrm{2}}\right),

where \sigma represents the tension of the oil-water interface, R represents the hydrate particle radius and {{\displaystyle \theta }}_{\mathrm{1}} and {{\displaystyle \theta }}_{\mathrm{2}} are contact angles.

Additionally, the presence of free water in the gas phase also causes the hydrate particles to adhere to the wall of the tube to form a hydrate deposit layer. Wang et al. [69] have conducted experiment and simulation on the hydrate formation in rich-gas phase based on this theory. In the model, they considered the formation of hydrate in the liquid phase and the hydrated particles of the free water transition entrained in the gas phase. The data simulated are consistent with the experiment data. Moreover, temperature also affects the adhesion of the particles to the tube wall. When the temperature is too high or too low, the adhesion of the particles to the tube wall is reduced. Conversely, the size of the hydrate deposit layer formed by adhesion affects the temperature variation within the pipe, too [54].

3.2.3 Hydrate particle bedding theory

The bedding deposition of hydrate particles mainly results from the sedimentation of the particles by gravity to form a hydrate deposit layer, which mainly occurs in oil-based systems or water-based systems containing hydrate inhibitors. The entire bedding process is shown in Fig. 10 [70]. When the hydrate particles are formed, the particles are small in volume and uniformly distributed in the liquid phase to maintain a similar concentration, and, in this case, a uniform suspension flow. With the continuous formation of hydrate particles, the particles begin to show an uneven distribution, and the particles at the bottom of the pipe gradually begin to settle down due to gravity, resulting in a non-uniform suspension flow. The continuously settled hydrate particles decrease the flow rate of the pipe, and the bottom of the pipe begins to form a moving deposit bed. As the height of the deposit bed continues to increase when the coalescence formed by particles are not flowing, the moving bed is transformed into a fixed bed, forming a stable hydrate deposit. Based on this analysis, the particle concentration and the flow rate of the fluid are important factors in hydrate particle bedding. Although AAs are usually added to the oil-based system to prevent hydrate particles from aggregating, when the flow rate of the fluid is less than the critical bed flow rate, the particles will still settle down to form a fixed hydrate bed, causing plugging [71].

In addition, the deposit bed formed by gravity sedimentation of hydrate particles is softer and has a higher porosity at the beginning of the formation process. Under the fluid impact force and the wall shear force, the deposit bed is readily detaches spontaneously or even completely detaches from the pipe wall [71]. Amam et al. [72] have reported the hydrate deposit layer has an annealing property. As time goes by, the deposited layer that has formed for a long time at the pipe wall gradually becomes soft and compact, and the stability is substantially improved. The accumulation of the already secured deposit layer may cause a blockage in the pipe, but the effect of annealing on the internal effects of the deposit layer has not been analyzed. Song et al. [73] have studied the mechanical properties of the deposit layer. They have divided the types of hydrate deposit layer into 6 types, and then calculated and analyzed the tensile strength and shear strength of each type. They have found that the annealing effect can significantly increase the tensile strength of the hydrate deposit layer, but has a weak effect on the shear strength. They have mainly evaluated the mechanical properties of hydrate deposit layers from the particle packing theory.

Current research on hydrate particle bedding generally uses software simulations to analyze the properties of the deposit bed. Jassim et al. [74] have studied the characteristics and locations of particle deposition using computational fluid dynamics (CFD) simulations. They have first simulated the flow field when only single-phase fluid passes through the pipeline, then established the model of particle deposition. Since the particles are not directly coupled with the flow field, the rationality remains to be analyzed. Balakin et al. [75] have simulated the coalescence of hydrated particles and the hydrate particle bedding using a combination of CFD and population balance model (PBM), and explained the formation of hydrate bedding by viscosity growth. Concerning the effects on the formation factors of deposit layer, Wei et al. [76] have simulated the deposition of CCl₃F (R11) hydrate particles in the pipeline using Fluent software. Based on the experimental results, the height of the deposit bed is found to be mainly affected by the fluid flow rate and the hydrate particle size. When the fluid flow rate is the same, the particle size and the height of the deposited layer increase. When the particle size is the same, the fluid flow rate increases and the height of the deposited layer decreases. When the flow rate reaches a certain value, the hydrate volume fraction no longer affects the deposition process. Balakin et al. [77] have used the STAR-CD software to simulate the deposition of R11 hydrate particles in pipelines and have also reached this conclusion. After this, Song et al. [78] have used Fluent to jointly solve the PBM and solid-liquid two phases flow model. The model simulates the effects of flow rate, particle size, and hydrate volume fraction on hydrate deposits. The experiment results show that the increase of flow rate will weaken the hydrated deposits. When the initial particle size and hydrate volume fraction in the pipe are larger, the initial deposition position of the hydrate particles is closer to the inlet of the pipe, and the hydrate deposition in the pipe is more serious. However, in actual flow, the self-polymerization of the particles and the action of the gas will exert effects on the formation of the hydrate deposit layer. Further simulation of hydrate deposit layer should still be conducted in the future.

According to the aforementioned studies, flow rate has a significant effect on the formation of hydrate deposits. At present, in the studies of the bedding deposition of hydrate particles in oil-based systems, domestic and foreign scholars mainly analyze the solid-liquid system. Besides, experiment simulation and mechanical properties of hydrate deposit layer stability are also conducted separately. Thus, the hydrate particle bedding should be studied by combining factors such as flow velocity, particle size with the mechanical properties of hydrate deposits. In addition, the effect of gas on hydrate injection deposition should also be considered.

The above analysis of the oil-based system indicates the hydrophilicity of the pipe wall, the water cut, the degree of subcooling between the pipe wall and the fluid, and the temperature of the system substantially affect the hydrate plugging mechanism. Among these factors, the water cut largely determines the coalescence of hydrate particles and the adhesion of hydrate particles to the pipe wall, and the flow velocity also has an important influence on the formation process of the hydrate deposit layer. Therefore, in future studies, if the influencing factors of hydrate can be combined with the mechanical properties of hydrate deposits, the mechanism of hydrate plugging can be further studied. In addition, most of the studies on the mechanism of hydrate plugging are focused on horizontal pipes. Less experiment and simulation have been conducted on inclined pipes and elbows. The establishment of hydrate plugging models on inclined pipes and elbows still needs further study.

3.3 Hydrate plugging mechanism in PD system

The hydrate plugging mechanism in PD systems is a newly emerged research topic. Due to the high water cut in the system, the plugging mechanism is different from the oil-based system, which is also complicated problem. The water in the PD system is not only dispersed into oil droplets in the form of water droplets, but also exists in the form of a non-emulsified free water layer, Vijayamohan et al. [26] have examined a PD system containing more free water by performing a loop experiment and found that the free water layer rapidly forms hydrate on the tube wall, which is then displaced under the action of shearing forces from the flow. At the same time, the hydrate in the bulk phase also continuously forms, and finally the detached hydrate and the hydrate in the bulk phase are co-deposited on the pipe wall. The process by which a hydrate forms on the pipe wall by the action of the free water layer is called hydrate film growth. Similarly, AA-Majid et al. [79] have used a loop experiment to study the formation and plugging process of hydrate in PD systems and found that the pressure drop changes more violently and the plugging tendency becomes more obvious after hydrate formation. At the same time, in the PD system with a water cut of 90%, the formation of hydrates has undergone emulsion breaking, which resulted in the system being separated into two separate continuous phases (oil phase and water phase). Then, hydrate is formed simultaneously in the continuous oil phase and the water phase, but AA-Majid does not explain the reason for emulsion breaking. Additionally, some media, such as gas molecules, also form hydrate on the tube wall and spread into a film.

Song et al. [80] have studied the effects of temperature and pressure on the formation position, formation morphology, and morphological evolution of hydrates during hydrate growth under the condition of different water cuts. When the water cut is high, the hydrate is formed on the wall surface through hydrate film growth (Fig. 11), and the temperature difference is basically the same for the hydrate formation position, formation morphology, and evolution process. The hydrate is initially formed at the gas-liquid boundary and then covers the entire wall in the form of film growth. However, as the temperature decreases, the thickness and density of the hydrate film increase, indicating that at lower temperatures (a greater degree of subcooling), the stability of the hydrate film, and the probability of plugging increase. Aman et al. [72] have also reached the same conclusion in the gas-dominated loop. In addition, when the temperature is low, the growth rate of the hydrate film decreases, and at the same time, the gas consumption rate also decreases. Grasso [71] has found that the hydrate film formed by hydrate generation media such as gas and water droplets are also affected by temperature gradients (subcooling) at the tube wall and inside the fluid. When the temperature gradient is larger, the thickness of the hydrate film becomes thicker. Similarly, the difference in pressure exerts little effect on the hydrate formation position, formation morphology, and evolution process. During the experiment, the phenomena of hydrate particle coalescence, hydrate deposition, and hydrate film growth were observed. The hydrate formation occurs at the liquid-phase body and gas-liquid interface, where the concentrations of natural gas and water molecules are high. The liquid-phase body is the main site of hydrate formation, and the surface of the water droplet with a large particle size forms hydrate with the surface of the free water layer at the bottom of the pipe. Then, different water droplets begin to coalesce and form hydrate. After the hydrate is continuously formed, it is deposited by gravity at the bottom of the pipe and penetrates the hydrate deposit layer transformed by the free water. Subsequently, the layer deposited at the reactor bottom is gradually sintered (The internal free water is converted into hydrate.), and hydrate film begins to appear at the wall. At the same time, when the initial pressure is high, the degree of subcooling of hydrate formation is greater, the gas consumption rate is faster, and the rate of hydrate formation is faster. At present, many experiments on the factors affecting the growth of hydrate films are conducted in static reactors. However, in the actual flow process, the formation of hydrate film will still be affected by fluid impact force and the wall shear force. Therefore, the study of hydrate film growth should be carried out in flow loop in the future.

During hydrate film growth, the difference in the degree of subcooling will affect the formation of hydrate film in PD system. Additionally, Grasso [71] has found that there exists a relationship between film growth and gas solubility, as shown in Fig. 12. When the temperature difference is great, the hydrate formation is independent of the solubility of the gas. At this time, hydrate forms through the mechanism of hydrate film growth. When the temperature difference is small and the gas solubility is low, the hydrate formation is controlled by the adhesion to the tube wall and hydrate film growth, indicating that the hydrate film growth is not the only factor that affects the way hydrate is formed in the PD system. When the temperature difference is small and the gas solubility is high, film growth does not occur in the system and a large amount of hydrate is formed; finally, a hydrate deposit bed forms under the pipe. The authors of the present paper believe that when the wall temperature is much lower than the internal temperature of the fluid, the temperature gradient is the main driving force for hydrate formation. At this time, the water molecules in the fluid diffuse to the tube wall, and the hydrate is mainly generated at the tube wall. When the wall temperature is similar to the internal temperature of the fluid, the driving force for hydrate formation is mainly affected by the gas solubility. When the gas solubility is low, the tube wall adhesion caused by the diffusion of free water to the tube wall and hydrate film growth result in hydrate formation. In addition, Grasso has also divided the hydrate film growth into two categories according to the different hydrate formation media: film growth dominated by gas molecules, and film growth dominated by water diffusion (Fig. 13). Between these categories, the diffusion of gas molecules dominates the film growth in the 100% pure water system and the oil-water layered PD system, and the diffusion of water molecules dominates the film growth in the gas-dominated system and oil-dominated system in which oil and water are completely dispersed. Although the diffusion of these two categories of hydrate formation media is caused by the temperature gradient between the tube wall and the fluid, the driving forces for the diffusion of water molecules and gas molecules are different. For the diffusion caused by water molecules, the concentration gradient is the main driving force for water molecules to diffuse from the interior of the fluid to the tube wall. However, for gas molecules, the uphill diffusion is the main driving force for the diffusion of gas molecules from the interior of the fluid to the tube wall [81].

In addition to hydrate film growth, other forms of hydrate plugging have been identified in PD systems. Akhfash et al. [5] have conducted an experiment with 70% water cut in a reaction kettle using paraffin oil and deionized water. The formation of hydrate particles in the initial stage cause the oil-water interface to migrate, and the system will change from a PD system to a fully dispersed one. At this time, the free water layer disappears. When the pressure and temperature satisfy the conditions for hydrate formation, hydrate begins to form at the bottom of the pipeline. The initially formed hydrate particles destroy the oil-water interface, causing the water phase to be completely dispersed in the oil phase, and accelerating the hydrate particles to coalesce and deposit. The blockage of the pipe is attributed to the formation of film growth on the pipe wall, accompanied by the adhesion and deposition of hydrate particles. Based on the above phenomena, the mechanism model of the PD system is established (Fig. 14). However, this statement fails to explain the reason for the migration of the oil-water interface, and the transformation from a PD system to a fully dispersed one is determined using the naked eye. This hypothesis does not have a certain level of rationality and requires a specific analysis.

To summarize the above findings, the plugging in PD systems is mainly caused by hydrate film growth. The existence of the free water layer will have effects on the hydrate plugging mechanism. In the future, the mechanism of emulsion breaking in PD systems and the reason for migration of the oil-water interface should be further studied. Meanwhile, the quantitative property of the free water layer is also the key to preventing hydrate plugging. For the study on the mechanism of hydrate plugging in PD systems, the difference between the pressure and temperature mainly causes a change in subcooling, which leads to the differences in the thickness and density of the hydrate film. In addition to the effect of subcooling on film growth, gas solubility and the differences in hydrate formation media can also lead to different types of hydrate film growth modes. Therefore, the hydrate plugging mechanism in a PD system is more complicated than that in other systems. In fact, when studying the plugging mechanism in PD systems, domestic and foreign scholars have identified different forms of hydrate formation methods that affect the blockage of the pipeline in addition to the hydrate film growth, increasing the difficulty of preventing and controlling hydrate formation in PD systems. In the future, research examining the hydrate plugging mechanism in PD systems requires further exploration.

3.4 Hydrate plugging mechanism in water-based system

The water-based hydrate research started later than the oil-based systems, and research on this topic is still in its infancy. Currently, the research on water-based systems is generally the law of pipeline plugging studied at a high water cut in the later stage of oil exploitation. According to the conclusion of Liu et al. [49], when the water cut is above the inversion point of the oil-water emulsion, namely, when the W/O emulsion is formed, it is considered a water-based system.

Research conducted on water-based systems at home and abroad mainly examines the hydrate plugging mechanism in a pure water system, and the hydrate plugging mechanism in a water-based system with a high water cut and the presence of an oil phase in the form of emulsified droplets.

Regarding the research on the plugging of the two water-based systems described above, scholars generally believe that the blockage of the pipeline is caused by the deposition of hydrate particles. Joshi et al. [24] have studied the flow of pipes at high water cuts in a hydrate loop pipeline. Hydrate grows into hydrate particles at the gas-liquid interface, and particle distribution is determined by hydrate concentration. When particles are distributed in a homogeneous flow, hydrate particles are evenly distributed and the number and volume of particles are small. When transitioning from a homogeneous flow to a heterogeneous flow, the volume and quantity hydrate particles begin to increase substantially, and the flow rate of the slurry in the system begins to decrease. Then the coalescence of hydrate particles creates a hydrate deposit layer that settles down to the bottom of the tube.

Based on the studies described above, the continuous sedimentation of the hydrate deposit formed by hydrated particles is the main cause of the plugging of the water-based system. The coalescence frequency has been used to analyze the process of hydrate particle coalescence in the water-based system [82]. Song et al. [59] have performed orthogonal experiments and found that the coalescence frequency γ of hydrate particles can be replaced by collision frequency β and aggregation efficiency α in the pure water system. The specific relationship is as expressed in Eq. (8).

(8)\mathit{\gamma }=\beta \times \alpha .

The collision frequency is calculated using the flow shear collision frequency equation, as shown in Eq. (9).

(9)\beta =\sqrt{{\displaystyle \frac{\mathrm{8}\mathrm{\pi }}{\mathrm{15}}}}G\left({\displaystyle \frac{{\left({{\displaystyle L}}_i+{{\displaystyle L}}_j\right)}^{\mathrm{3}}}{\mathrm{8}}}\right),

where G is the local shear rate of the flow field and L represents the size of hydrate particles, i and j represent primary particles that make up two agglomerates.

The coalescence efficiency α is usually calculated from the ratio ε of the coalescence force to the shear force of the particles in the flow field, by using

(10)\alpha =\mathrm{0.87}{ϵ}^{\mathrm{0.18}},

where ε is calculated using different calculation methods in different flow field systems. In the pure water system, the coalescence force of particles is mainly attributed to van der Waals forces, which can be calculated using

(11)ϵ={\displaystyle \frac{H}{\mathrm{36}\mathrm{\pi }\mu {GR}^{\mathrm{3}}}},

where H is Hamaker constant, m is liquid viscosity and R is the radius of the two particles that collide.

The coalescence frequency of hydrate particles in a pure water system is obtained using Eqs. (8)–(11). Based on the results of the orthogonal test, the degree to which each factor affects the particle coalescence frequency in the pure water system is listed in the following order: L>μ>G>H. Although this formula effectively analyzes the coalescence of hydrated particles in water-based systems, it is only suitable for pure water systems. For water-based systems in which the water cut is high and the oil phase exists in the form of emulsified droplets, the coalescence force of the hydrate particles should include both liquid bridge forces and van der Waals forces. Therefore, the coalescence of hydrate particles in water-based systems remains to be explored.

In the water-based system with a high water cut, the blockage of the pipeline is mainly caused by the deposition of hydrate particles. As the water cut increases, the rate of formation and production of hydrate particles decreases, and the risk of plugging is substantially reduced. However, a unified conclusion about the coalescence model of hydrates in water-based systems has yet to be achieved. The authors of the present paper propose that as the water cut increases in the water-based system, the force between hydrate particles gradually changes from the combined action of liquid bridge forces and van der Waals forces to the van der Waals force as the main coalescence force. Therefore, in the water-based system with a high water cut, the synergy between liquid bridge forces and van der Waals forces is also a direction for establishing a hydrate particle coalescence frequency model in the future.

According to the research described above on the plugging process and mechanism of the oil-based system, the PD system, and the water-based system, the experiment process and conclusions are mainly focused on a single system or a static system. However, the rich-liquid system in a multiphase pipeline is subjected to dynamic conditions in the actual flow. Therefore, future experiments should be conducted to more closely analyze the flow dynamics of multiphase pipeline rich-liquid systems by investigating the coalescence, the deposition, the plugging, and the decomposition processes of hydrate.

4 Effect of AAs on hydrate plugging in a pipeline

AAs are currently widely used as a surface active material in a multiphase mixed pipeline. They do not change the thermodynamic conditions of hydrate formation, nor do they allow the formation of hydrate particles. Instead, they prevent the hydrate particles from coalescing. The particles are uniformly dispersed in the liquid phase to maintain a stable flow of the fluid in the tube [4]. The mechanism of the action of the anti-agglomerate is generally believed to be attributed to the presence of a hydrophilic group and a long-chain lipophilic group in the molecule that divides it into two parts: an emulsified molecule and an inhibitory molecule. After the addition of the mixed solution, the hydrophilic group will adsorb on the surface of the water droplet due to the action of the emulsified molecule. Besides, the long-chain lipophilic group will protrude into the oil phase. Following the formation of the hydrate particle shell, the emulsified molecules gradually fail, but under the action of the inhibitory molecules, the hydrophilic groups are adsorbed on the surface of the shell, and the long-chain lipophilic group extends into the oil phase to control the size of the hydrate particles, preventing the hydrate particles from coalescing [83].

Although anti-agglomerate plays an important role in preventing the hydrate accumulation and the maintaining slurry flow in a multi-phase pipeline, it exerts different effects on the fluidity of the hydrate slurry due to factors such as the water cut. Melchuna et al. [17] have studied the effect of the anti-agglomerate AA-LDHI on the flow characteristics of hydrate slurry in a water-based system with a water cut of 60%–90% and found that the addition of the AA-LDHI exerts a substantial effect on maintaining the stability of the slurry flow. It is manifested by increasing the uniformity and stability of the emulsion, and to some extent, inhibiting the coalescence of the hydrate particles and reducing the water conversion rate of the hydrate formation. However, different AAs have different effects on the fluidity of hydrate slurry at a high water cut. According to Gao [84], AAs do not have anti-blocking effect when the water cut is higher than 80%. Li et al. [85] have studied the stability of the hydrate slurry in the three phase oil-gas-water system in the presence of Span20 and Zjj2 AAs and found that a higher water cut in the system results in a more obvious decrease in the hydrate slurry flow rate, and an increase in the apparent viscosity of the slurry. At the same time, they have compared the stability of the slurry in the presence of the two AAs, and found the highest fluidity in the slurry containing Zjj2.

Although AAs hinder the coalescence of hydrate particles in a water-based system with a high water cut, the addition of AAs will reduce the induction period of hydrate nucleation and accelerate the blockage of the pipeline in the oil-water system with a lower water cut. Wu et al. [45] have studied the effects of initial pressure and anti-agglomerate concentration on the rate of hydrate formation in an oil-based system with a water cut of 15%, and found that an increase in the anti-agglomerate concentration shortens the induction period of hydrate formation and accelerates the rate of hydrate formation. However, the effect of AAs on the rate of hydrate formation is studied under a certain condition, and the rationality under other conditions has yet to be analyzed. Regarding the analysis of the effects of AAs on promoting hydrate formation, Lv et al. [39] have studied the formation of hydrate and the flow of slurry in multiphase transportation system in the presence of AAs, and found that when the initial flow rate of the oil-water emulsion is lower than a certain critical flow rate, the addition of the anti-agglomerate promotes hydrate formation and accelerates plugging. When the flow rate is higher than the critical flow rate, the high-speed shear of the fluid maintains the stability of the slurry. They have also found that AAs have a minimum critical flow rate for maintaining the safe flow of hydrate slurry. For different AAs, the minimum critical flow rate under safe flow conditions will also be different. In addition, Li et al. [35] have assessed the fluidity of horizontal pipe hydrate slurry using diesel oil and an on-site collected condensate in an oil-water system with a low water cut. By analyzing the changes in the gas consumption, the slurry flow rate, and the pressure drop, they have found that the anti-agglomerate exerts less effect on the system in the case of lower water cut, indicating that the working condition of the anti-agglomerate has an optimal water cut range. However, they failed to determine the optimal water cut range of the anti-agglomerate. Therefore, further study on the flow properties of hydrate slurry is still needed in the future.

To summarize the above findings, AAs not only play an important role in maintaining the fluidity of hydrate slurry, but also affect the delivery tube due to factors, such as the water cut, anti-agglomerate material, fluid flow rate, and concentration of the anti-agglomerate. Because the actual pipeline flow environmental conditions are more complicated, the addition of AAs will exert different effects on different systems. Therefore, in the future, the anti-blocking effect of the anti-agglomerate on the pipeline should be further analyzed experimentally, and a clear standard should be made on the use of anti-agglomerate.

5 Conclusions

In recent years, domestic and foreign scholars have studied the hydrate plugging process in rich-liquid systems, including the coalescence of hydrate particles, adhesion of hydrate particles to the pipe wall, hydrate particle bedding, and hydrate film growth. However, in the flow of rich-liquid phase under actual conditions, hydrates block the pipeline through different mechanisms, due to differences in the water cut, the flow rate, the degree of subcooling, and the gas solubility, and thus it is difficult to study hydrate inhibition. Therefore, the following suggestions are proposed for the study of the hydrate plugging mechanism in the rich-liquid phase system in the future:

1)In oil-based systems, the effect factors of hydrate are combined with the mechanical properties of hydrate deposit layer. In addition, most of the studies on the mechanism of hydrate plugging mechanisms are focused on horizontal pipes. Less experiment and simulation have been conducted on inclined pipes and elbows. The research on establishing hydrate plugging models in inclined pipes and elbows will also be the topic of the future.

2)The mechanism of emulsion breaking in the PD system and the reason for migration of the oil-water interface should be further studied, and the property of the free water layer on the hydrate plugging process should be quantified. Meanwhile, the factors affecting the plugging mode should be systematically identified and considered, and a complete PD system hydrate growth plugging model should be established.

3)In the water-based system with a high water cut and an oil phase in the form of emulsified droplets, the synergistic effects of liquid bridge forces and van der Waals forces on the particle aggregation frequency model should be further studied, and the aggregation frequency calculation model should also be established.

4)Currently, the experimental process used to study hydrate plugging and conclusions regarding hydrate plugging are mainly based on a single system or a stationary system, but the multiphase mixed rich-liquid transportation system is subjected to dynamic conditions in the actual flow. Therefore, future experiments should be conducted to more closely analyze the flow dynamics of multiphase pipelines with a rich-liquid system by examining the coalescence, deposition, plugging and decomposition processes of hydrates.

5)Further explorations should be made of the slurry flow of the anti-agglomerate in the presence of different environmental factors and the effects of AAs on the morphological evolution of hydrate in different systems. In addition, the anti-blocking effect of the pipeline by compound anti-agglomerate should be analyzed and pipeline plugging conditions in different media should be clarified in the future.

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