Understanding the hydrate growth characteristics in systems containing kinetic hydrate inhibitors is essential for ensuring flow assurance. The poly(N-vinylcaprolactam) was employed to investigate the effects of its concentration and subcooling on hydrate growth behavior. The hydrate lateral growth rate was quantified, and the relationship among concentration, subcooling, and growth rate was established. Results indicate that higher concentration or lower subcooling yield more complex hydrate structure and slower growth rate. Typical hydrate morphologies, including smooth shell, irregular shell, and granular accumulation, were identified and associated with different growth rate. These findings provide insights for optimizing inhibitor applications in hydrate management.

As offshore oil and gas exploitation advances, the problem of hydrate pipeline blockage has become more prominent. In this work, a novel visualized loop was used to conduct a fully visualization study of hydrate formation and slurry flow. Through image visualization and data analysis, a critical hydrate volume fraction for assessing the blockage risk was proposed, which was characterized by a sudden drop in the differential pressure after reaching the first peak. When the hydrate production achieved the critical value, the hydrate would adhere to the wall and form a hydrate layer with a potential blockage risk, especially in dead-leg. Meanwhile, the effects of liquid loading and flow velocity were analyzed. The critical hydrate volume fraction increased from 0.5% to 10% as the liquid loading rose from 50 to 90 vol%, and a high flow velocity would improve the critical value, allowing for a larger safe region. Furthermore, the three stages of hydrate formation and blockage process were revealed. Its correlation with differential pressure was explored, and the corresponding mechanism was proposed. In hydrate risk management, it should be kept in the first stage, and handled in time when the second stage occurs to avoid the third stage.

With the successful application of kinetic hydrate inhibitors (KHIs) on hydrate plug prevention, there is a growing trend towards the development of affordable and environmentally friendly KHIs for solving challenges of high cost and inadequate biodegradability from their commercial KHIs. Therefore, further evaluation on the inhibitory effect of the developing inhibitors is needed under multiphase flow conditions. In this study, a high-pressure, fully visible rocking cell were used to simulate the multiphase transportation and both commercial KHIs and environmentally friendly KHIs were considered. Under the flow conditions of 60 % liquid loading and 20 % water cut, the results demonstrated that PVCap showed excellent inhibition performance with no hydrate formation at concentrations of 0.5 wt%, 0.2 wt% and 0.1 wt%. Although pectin exhibited certain inhibitory effect on hydrate growth rate and formation quantity, but it demonstrated a strong promoting effect on hydrate nucleation opportunities and rates. However, sodium alginate, which shares similar hydrate inhibition characteristics with pectin, exerted significantly promotional effects in terms of induction time, growth rate, and water conversion. Furthermore, the addition of pectin and sodium alginate led to the formation of hydrate balls in several test runs, resulting in complete pipeline blockage. Another noteworthy discovery was the relationship between the growth rate and final water conversion of hydrates, which exhibited an initial increase followed by a decrease, while the induction time exhibits an inverse trend, as the flow velocity increased.

The intricate interplay of crude oil composition and additives critically affects hydrate formation and flow behavior, which is significant for flow assurance particularly in deepwater. To investigate, we varied the hydrophilic and hydrophobic properties by proportioning two nonionic surfactants (Span 80 and Tween 80) and conducting hydrate formation experiments in a visual rocking cell. The results show that the increasing hydrophilic-lipophilic balance (HLB) value shifted emulsions from water-in-oil to multiple and oil-in-water phases and significantly affected the hydrate formation among different emulsion types, with a marked increase in the hydrate formation rate in the HLB range of 9 to 11. Slurries maintained flowability due to low hydrate conversion, yet higher HLB (>11) led to slight agglomeration and deposition. In addition, the impacts of water conversion were investigated by multiple pressurization, showing that the hydrate formation amount affected slurry flowability but was primarily dependent on the hydrophilicity and hydrophobicity of the surfactants. When the HLB was below 11, the final water conversion was about 80%, but the formed hydrate still exhibited good dispersion and flowability. In contrast, HLB exceeding 13 resulted in extensive adhesion and deposition on the cell walls. When the water conversion reached about 40%, flowability was completely lost and hydrate blockage occurred.

Pipeline transportation has become the main way of long-distance and large-scale energy transportation because of its high efficiency, safety, economy, and environmental friendliness. During the exploitation of offshore gas fields containing CO2, the pipelines easily form hydrates under appropriate environmental conditions, which can cause great harm to the flow assurance. Therefore, adding inhibitors and surface coatings in the pipeline is used to prevent hydrate formation. To better fit the actual blockage problem of the deep-sea oil and gas pipeline transportation process, it is necessary to study the hydrate growth kinetics of mixed gases on different wettability surfaces with or without inhibitors. The crystal morphology and growth process of CH4 and CO2 mixed hydrate crystals forming from a droplet were investigated by a CCD camera and microscope. The results showed that the droplets will not form hydrates completely in a high CO2 content system, but all converted into hydrates in a low CO2 system. The villous dendrites on the surface of droplets are more abundant, and the growth rate is slower with the increase in PVCap concentration. On the superhydrophobic surface, the hydrate nucleation rate and hydrate growth rate are the lowest, which means inhibition to hydrate nucleation and growth. These research results are hoped to provide a solid theoretical basis for solving the actual flow safety problems of deep-sea oil and gas pipelines.

With the gradual advancement of oil and gas exploration into deep offshore, the hydrate blockage has emerged as a critical concern for the flow assurance. We conducted constant-velocity hydrate formation and variable-velocity rheology experiments with a novel visual-loop to analyze slurry flow and viscosity change in pipelines. Results showed staged pressure variations during hydrate formation-aggregation-deposition process, and it could be analyzed judiciously with a developed viscosity model. Initially, hydrates dispersed as small flocculent particles with minor aggregation, gradually raising differential pressure, and the critical viscosity model parameter, hydrate aggregation rate (m) was <1. Subsequently, particle aggregation and wall adhesion dominated, resulting in reduced hydrate flow volume and possible blockage of special pipelines (e.g., dead-leg), with m-values >1. Finally, as hydrate growth continued, substantial adhesion to the pipeline reduced flow diameter, significantly increasing blockage risk. However, the addition of sufficient surface-active ingredients improved hydrate dispersibility and enabled the slurry to maintain the first stage, exhibiting long-term stability with an m-value <1. Additionally, the apparent viscosity of the hydrate slurry within the pipeline was accurately determined utilizing a novel approach, accounting for its yield-pseudoplastic behavior. The calculated viscosities closely matched post-sampling rheometer measurements, and were effectively predicted by the developed viscosity model.

Crystal fouling, which refers to the accumulation of precipitates on surfaces and the associated damage, is a common problem in many industrial processes. In deepwater oil and gas transportation, hydrate blockage poses as a considerable barrier. Consequently, modifying hydrophobicity of surfaces has become an increasingly focused strategy to mitigate hydrate. However, the design of surfaces that effectively control the hydrate remains challenging. Herein, we report a superior smooth anti-hydrate material based on silanized modification. The low-adhesion silanized silicon wafer (SSW) realizes a win-win strategy of hard formation and easy removal. Through a comprehensive study combining experimental validation with theoretical analysis, the unique characteristics of SSW in the field of anti-hydrate surfaces were deeply discussed. Various hydrate crystals exhibited a completely different hydrate growth mode at the SSW surface, in which hydrate crystals spontaneously elevated and lifted themselves off from the surface. The presence of the fluorine element on the SSW surface allowed self-lifting growth of hydrate crystals after they covered the water droplet. And the loose and porous crystal structure in this self-lifting growth could reduce the contact area between the hydrate crystals and the substrate, allowing the crystals with minimal adhesion force and removal disturbance. Furthermore, the gas enrichment on the SSW surface also reduced the contact with the substrate, thereby decreasing the adhesion and allowing self-cleaning behavior. These results indicate that the silanized surface is a promising candidate for developing anti-hydrate materials for hydrocarbon production and transportation industry.

With more and more exploration of the marine environment, a series of underwater monitoring equipment such as underwater vehicles are used more widely. Reducing the operation cost of underwater equipment and improving work efficiency have become the focus of attention. Superhydrophobic coatings have excellent applications in drag reduction, antifouling and other fields due to their unique surface wettability. This paper reports a superhydrophobic polyurea/TiO2 composite coating with rapid self-healing ability by simple sprinkling or spraying of modified TiO2 nanoparticles on the brushed polyurea coating. The modified TiO2 nanoparticles were driven to migrate to the surface of the coating by the synergistic self-healing effect of the disulfide and hydrogen bonds inside the polyurea, contributing to restoring the superhydrophobicity. This superhydrophobic polyurea/TiO2 composite coating possesses-11.28 % drag reduction efficiency after being applied to underwater navigation, and it shows-11.04 times higher water loading capacity than its weight. Moreover, this coating possesses excellent antifouling properties and chemical durability. This work provides new clues for future applications of drag reduction and pollution prevention of underwater exploration equipment.

With gas production moving into the deep sea, the prediction of hydrate formation and blockage prevention are of great significance in flow assurance. In this work, hydrate formation experiments at a wide range of pressures and temperatures based on the production conditions of the Lingshui gas field were conducted by a high-pressure rheometer, and the evolution of viscosity was investigated. The experimental results showed that the slurry viscosity changed in stages during the formation of the hydrate, and the aggregation of hydrate particles was the essential cause of the significant increase in viscosity. The more severe the temperature and pressure conditions, the faster the hydrate formation and the greater the tendency of increasing viscosity. In experiments, the formation of hydrates would lead to severe blockages when the pressure exceeded 10 MPa. Compared with 100 and 300 rpm, the average hydrate formation rate was the fastest at 200 rpm. In addition, the relationship between hydrate volume fraction and viscosity was analyzed, and a critical hydrate volume fraction was proposed for a sharp increase in viscosity, which had a low value at high pressures and low temperatures and tended to cause blockages. This study can provide helpful theoretical support for hydrate formation prediction and blockage prevention in natural gas production.

Hydrate form spontaneously in submarine oil/gas pipelines. The large-scale aggregation of hydrate cause pipeline blockage and threatens safe oil/gas production. The use of anti-agglomerant agents (AAs) is a cost-effective method to prevent hydrates from clogging pipeline. However, conventional AAs such as Span 80 are not applicable in oil/gas pipelines rich in asphalt, because asphalt is easy to occupy the binding sites on the water droplet surface. Through molecular simulation, it is found that cyclodextrin (CD) shows excellent anti-agglomerant characteristics in oil-water system containing asphalt by forming stable hydrogen bonds with water molecules. CD and asphalt interact through electrostatic and dispersive interaction to form stable W/O supramolecular surfactant. Compared with Span 80, the initial contact time between water droplets and hydrate is delayed by 5 times (from 22.1 ns to 113.9 ns) after adding CD molecules. The contact angle of water droplets on the hydrate surface also increased from 48.4 +/- 2.4 degrees to 64.4 +/- 2.2 degrees. CD-asphalt interaction prevents water droplets from spreading on the hydrate surface by maintaining the stability of emulsion. The unique properties of CD provide a molecular basis for the development of more green and efficient AAs. Subsequent research will be carried out around the compounding of CD and other AAs, and it is expected to play a better anti-aggregation effect.

Deepwater oil and gas development is extremely difficult and challenging. One of the most critical challenges stems from hydrate deposition, aggregation, and the eventual blocking of the deepwater oil and gas transportation system. The low-temperature and high-pressure environment in the deepwater oil and gas field causes the combination of gas molecules and water molecules to form hydrate, thus affects the hydrocarbon transportation. In this Perspective, to discuss the commonly faced safety issues for deepwater oil, gas, and gas hydrate development, the following three critical problems are comprehensively summarized and analyzed. First, the mechanisms of phase transition, aggregation, and blockage of the hydrate in the multiphase transport system have been investigated from the microscopic perspective to macroscopic characteristics. Second, based on different theoretical models, the algorithms are discussed to introduce an online monitoring technique for hydrate blockage, which can detect the safety risks and provide early warnings. Furthermore, for hydrate blockage prevention and control, the active methods based on chemical injection and the passive methods based on the modification of physicochemical properties of pipeline surfaces are reviewed. Finally, an outlook is provided for the future development of deepwater oil and gas and for the schemes to mitigate hydrate blockage.

Solid fluidization is a novel method for marine hydrate exploitation. The rheological characteristics of hydrate slurry are significant for hydrate fluidization and safe transportation. This work used a high-pressure rheometer to form hydrate slurries with marine sediments in the South China Sea. The results indicated that the higher the sediments concentration, the more significant the viscosity change after hydrate formation. Subsequently, staged heat injection was designed to simulate the dissociation process during hydrate exploitation and obtain the slurry with different hydrate volume fractions. Oscillatory measurements investigated the viscoelastic behavior of each hydrate slurry. The results suggested that both mudflows and hydrate slurries displayed viscoelastic fluid characteristics. The presence of hydrate dramatically increased the storage modulus of the slurry, and the yield stress would increase 2-700 times with hydrate increasing. Furthermore, a critical hydrate volume fraction was proposed at which the yield stress would rise sharply. A simple equation was developed to describe the relationship between this parameter and sediments concentrations. The increasing trend of yield stress changed little with hydrate increasing when the sediments concentration was less than 20 wt%, and a water conversion less than 15% was suggested to get a fluidized slurry with yield stress below 100 Pa.

As oil and gas production moves into deep-water, the effect of brine salinity in water produced with petroleum is an important factor to be considered for hydrate management in transport pipelines. In this work, methane hydrate formation was conducted in water-in-oil emulsions with various salinities in the aqueous phase by using a high-pressure rheometer. The viscosity evolution during hydrate formation was analyzed. The results suggested that the increase in salt concentration could significantly reduce the hydrate slurry viscosity, and improve the stability of the slurry. As the salinity increased from 0.3 wt% to 5 wt%, the relative viscosity of the final slurry after hydrate formation decreased from more than 300 to only 42. When the salinity was greater than 1 wt%, hydrate slurries could keep stable flow for a long time, and the relative viscosity was less than 100. Meanwhile, a shear ramp was conducted. The hydrate slurries exhibited shear-thinning, and the rheological properties were described with an established viscosity model. Good agreement was observed between the experiments and model calculations under different brine salinity, and the aggregation process of hydrate particle and viscosity change at different shear rates were analyzed. In addition, a rest and restart process was conducted, and the yield stress of time-dependent was analyzed. With increasing rest time, the yield stress increased gradually. A high brine salinity of 5 wt% in the aqueous phase could significantly decrease this yield stress value and slow its increase with the rest time. After 1280 min of rest, the yield stress increased to 83 Pa, which was only one third of the yield stress value of the hydrate slurry with 1 wt% NaCl. Particularly, with a high brine salinity, when the rest time was over 160 min, two-step yielding behaviors were first observed for the hydrate slurry in the stress sweep, and a possible mechanism for the two-step yielding in the hydrate slurry system was proposed.

The potential of hydrates formed from R141b (CH3CCl2F), trimethylolethane (TME), and tetra-n-butylammonium bromide/tetra-n-butylammonium chloride (TBAB/TBAC) to be used as working substances for cold storage was investigated to provide a solution for unbalanced energy grids. In this study, the characteristics of hydrate formation, crystal morphology of hydrates, and the stability of hydrate in cyclic formation under 0.1 MPa and at 5 °C were carried out. It found that the ice had a positive effect on the hydrate formation under same conditions. Upon the addition of the ice cube, the induction time of R141b, TME, and TBAB/TBAC hydrates decreased markedly, and significantly high formation rates were obtained. Under magnetic stirring, the rate at which TBAB/TBAC formed hydrates was significantly lower than that when ice was used. In microscopic experiments, it was observed that the TBAB/TBAC mixture formed hydrates with more nucleation sites and compact structures, which may increase the hydrate formation rate. In the multiple cycle formation of TBAB/TBAC hydrates, the induction time gradually decreased with the increasing number of formation cycles and finally stabilized, which indicated the potential of the TBAB/TBAC hydrates for application in cold storage owing to their good durability and short process time for heat absorption and release.

Understanding the critical velocity at which hydrate particles can be removed from a solid surface is crucial for flow assurance issues. An improved model is proposed to predict the critical velocity in hydrate particle removal process by considering a deformed liquid bridge. In situ experiments were performed for validation, and it was observed that hydrate particle decomposition occurred during the removal process. The particle diameter was determined to be a more dominant factor influencing the critical velocity than the initial liquid bridge volume. The resulting confidence level was 95% within a 30% error, showing its robust capability for engineering applications.

Solid fluidization is a current promising method to explore hydrate resources. The rheological properties of hydrate slurry are significant to solid fluidization and prevent hydrate blockage during pipeline transportation. To determine the impact of hydrate on the rheological properties of mudflow, a rheological experiment was conducted on methane hydrate slurry formed from a mudflow, which was prepared with marine sediment sampled from South China Sea. It was found that both mudflow and hydrate slurry exhibited viscoelastic fluid properties. The existence of hydrate substantially increased the apparent viscosity of the mudflow, and when the hydrate formed at a rapid rate, the apparent viscosity rose rapidly. With different initial pressures, the amount of hydrate formed in mudflow was different. A larger water conversion rate would increase the viscosity of the hydrate slurry significantly. Meanwhile, a yield stress measurement was conducted by stress sweep after hydrate formation. When the water conversion fractions were 17.8%, 20.3%, and 21.2%, the yield stresses of the hydrate slurry were 57.8, 465.2, and over 10,000 Pa, respectively. In addition, a frequency test sweep was conducted, and the results showed that the existence of hydrate would significantly increase the elasticity of the slurry.

Predicting the viscosity of hydrate slurries is essential in evaluating the transport performance of oil and gas in deep-water pipelines and the risk of blockage. In this work, based on the Camargo-Palermo model and the bimodal suspension model, a new viscosity model was developed for hydrate slurries. In this model, the viscosity of slurry is determined jointly by small hydrate particles and larger hydrate aggregates, in which the maximum critical diameter of the aggregate is controlled by shear force and cohesion force, while amount of hydrate particles and aggregates can be described by a dimensionless parameter proposed. In addition, rheological measurements of methane hydrate slurry formed from water-in-oil emulsion were conducted with a high-pressure rheometer to validate the developed model. The predictions of the model agree well with the experimental data for metastable hydrate suspension in oil dominated systems with either different initial water cuts or different water conversion rates. (C)2021 Elsevier Ltd. All rights reserved.

Technologies for preventing hydrate blockage play an important role in oil and gas production, transportation and processing. In this paper, a fully visual rocking cell was used to study the effects of water cut, surface wettability and kinetic inhibitors (PVCap, apple pectin and three ionic liquids) on gas hydrate nucleation, growth and blockage. The amount of water converted to hydrates reached the highest when the water cut was about 60%. The evolution process of hydrate was observed by CCD camera, including initial crystallization, water film condensation, forming the hydrate bed, and the top blockage. The hydrophilic surface can reduce the nucleation rate of hydrate to some extent, but it can not reduce the final hydrate production. The hydrophobic surface can increase the formation rate of hydrate and the amount of water converted to hydrates, but the pipe blockage did not occur because it is difficult for hydrate to adhere to the pipe wall. In terms of induction time, the inhibition to structure II hydrate of 0.2 wt% apple pectin was similar to that of PVCap, and the inhibition of high concentration apple pectin decreased because of its low solubility. Apple pectin can delay the formation rate of hydrate, but will not change the final production of hydrate. [HMIM]BF4 and [EMIM]Cl can make hydrate disperse evenly in water and prevent pipeline blockage, which play a role like anti-agglomerates. From the inhibition effects of cations and anions on hydrates, the order was as follows: Cl- > BF4- > [HMIM](+) > [EMIM](+). This study might provide a basis for hydrate plugging prevention.