Cohesive force reduced with higher AA concentration at various contact times (blue: 30min, red: 1min, black: 10s). (a) insufficient AAs are adsorbed on gas hydrate surface which leads to a water ...bridge between gas hydrate particles; (b) sufficient AAs dosed in the system and packed densely on hydrate surface, leading to a low cohesive force between gas hydrate particles. Bottom picture illustrates the salt ion interaction with AA molecules.
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Gas hydrate interparticle cohesive forces are essential to evaluate hydrate interfacial properties and determine the hydrate particle agglomeration behavior. In this study, a technique was developed in a modified high-pressure micromechanical force (HP-MMF) apparatus that enables hydrate anti-agglomerants (AAs) to be evaluated at an interfacial level under deepwater petroleum flowline conditions of high pressure and low temperature in a liquid hydrocarbon-dominated system. The hydrate cohesive force was found to decrease from the baseline (23.5 ± 2.5 mN m−1) to a non-measurable force value (<0.05 mN m−1) with increasing concentration of a high-performance AA (0.25 vol% to 2 vol% of AA1). To mimic the shut-in scenario in subsea flowlines that may be required in an emergency response, cohesive force was measured with shut-in times varying from 10 sec to 18 hr. It was observed that long contact times can lead to higher cohesive forces and eventually resulting in a system failure, i.e. irreversible interparticle interactions, if the system is under-dosed. Utilizing these results, under-dosed scenarios were identified for AA1 in the model liquid hydrocarbon. In addition, salt ions were found to promote the performance of AA1, but salt only slightly reduced the force with AA2 at the same subcooling. The results demonstrate that this modified HP-MMF method can capture the differences between high and low performance AAs. These data/tests can be used to help improve our understanding on the gas hydrate interfacial properties and determine the minimum effective dosage of AA additives for hydrate mitigation treatments in subsea flowlines.
The interfacial properties and mechanisms of gas hydrate systems play a major role in controlling their interparticle and surface interactions, which is desirable for nearly all energy applications ...of clathrate hydrates. In particular, preventing gas hydrate interparticle agglomeration and/or particle–surface deposition is critical to the prevention of gas hydrate blockages during the exploration and transportation of oil and gas subsea flow lines. These agglomeration and deposition processes are dominated by particle–particle cohesive forces and particle–surface adhesive force. In this study, we present the first direct measurements on the cohesive and adhesive forces studies of the CH4/C2H6 gas hydrate in a liquid hydrocarbon-dominated system utilizing a high-pressure micromechanical force (HP-MMF) apparatus. A CH4/C2H6 gas mixture was used as the gas hydrate former in the model liquid hydrocarbon phase. For the cohesive force baseline test, it was found that the addition of liquid hydrocarbon changed the interfacial tension and contact angle of water in the liquid hydrocarbon compared to water in the gas phase, resulting in a force of 23.5 ± 2.5 mN m–1 at 3.45 MPa and 274 K for a 2 h annealing time period in which hydrate shell growth occurs. It was observed that the cohesive force was inversely proportional to the annealing time, whereas the force increased with increasing contact time. For a longer contact time (>12 h), the force could not be measured because the two hydrate particles adhered permanently to form one large particle. The particle–surface adhesive force in the model liquid hydrocarbon was measured to be 5.3 ± 1.1 mN m–1 under the same experimental condition. Finally, with a 1 h contact time, the hydrate particle and the carbon steel (CS) surface were sintered together and the force was higher than what could be measured by the current apparatus. A possible mechanism is presented in this article to describe the effect of contact time on the particle–particle cohesive force based on the capillary liquid bridge model. A model adapted from the capillary liquid bridge equation has been used to predict the particle–particle cohesive force as a function of contact time, showing close agreement with the experimental data. By comparing the cohesive forces results from gas hydrates for both gas and liquid bulk phases, the surface free energy of a hydrate particle was calculated and found to dominate the changes in the interaction forces with different continuous bulk phases.
Seawater desalination using gas hydrates is a potential technique for water treatment. However, limited understanding and control in prior studies on nucleation, growth, and separation have prevented ...adequate commercialization of hydrate desalination processes. Hydrate formation and ‘memory’ experiments of C1 + C2 (74.7/25.3 mol%) gaseous mixtures were carried out using a Jerguson high pressure visual cell. The C1-C2 hydrate formation results indicate that formation onset times were three times longer in salt water compared to fresh water. Furthermore, it was observed that induction times decrease with increasing subcooling, and hydrate memory effects are reduced with increasing replenish time, i.e. the time lapse between dissociation and subsequent gas depressurization. A high-pressure desalination apparatus was also designed and constructed to produce the overflow of hydrates in the inner annulus of a gas bubble column placed inside a dissociation reactor. The working principle for this system is that the overflowing hydrates enter the outer annulus and are then dissociated. The salt removal efficiency was determined by measuring the conductivity of the recovered water. Experiments to assess the efficiency of the process were performed to determine how the salinity in the recovered water depends on different initial salt contents and subcooling for hydrate formation.
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•Nucleation and growth studies of hydrates were performed in a high pressure cell.•Hydrate formation onset times were three times longer in salt water compared to fresh water.•Clathrate formation and ‘memory’ experiments were performed•A high pressure desalination apparatus was constructed to enable the overflow of hydrates•Desalination efficiency dictated by the amount and morphology of overflowing hydrates
In recent years, there has been growing interest in gas hydrates as technological applications, such as for energy (methane and hydrogen) storage and transportation, separation (gas and ...desalination), and carbon capture. However, there are several challenges that deter large-scale applications and commercialization of these hydrate-based technologies. One of the main challenges is the long induction time and slow growth of hydrate particles, which can increase the overall operating costs of these technologies. It has been reported that the addition of additives (known as hydrate promoters) can help improve the nucleation and growth rate of hydrates. In general, there are two types of hydrate promoters: thermodynamic hydrate promoters and kinetic hydrate promoters. Thermodynamic hydrate promoters shift the hydrate equilibrium curve to milder conditions (i.e., lower pressures and higher temperatures), while kinetic hydrate promoters reduce the induction time for hydrate formation and increase the growth rate. In this review, we provide a comprehensive review of the two types of hydrate promoters (thermodynamic and kinetic) and their effects on hydrate phase equilibria, induction time, and growth rate.
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•Water-continuous systems which are common in the mature oil/gas fields are studied in a high pressure vertical loop.•Deposition and sloughing of hydrate particles in water-continuous ...slurry are verified by viscosity measurement and imaging.•The plugging mechanism of the water-continuous system includes crystal particle agglomeration and accumulation/deposition.•Gas hydrate accumulation and sloughing occur intermittently, with sloughing initiated at 0.7 m/s.
Oil-water-gas co-transport often occurs in offshore oil/gas production, in which gas hydrate formation could lead to severe flow assurance issues and subsequent safety and environmental risks. As the offshore oil/gas fields mature, water production increases and the flow conditions may convert from an oil-continuous to a water-continuous system. To understand the hydrate plugging mechanism of water-continuous hydrate systems, the flow characteristics of methane hydrate-water slurries was experimentally investigated by using a high pressure vertical loop. The slurry viscosity of different hydrate fractions was measured using an in-line viscometer. Accumulation, deposition and sloughing of the hydrate particles were investigated by video imaging and viscosity measurement. The plugging mechanism in a water-continuous hydrate system was extensively analyzed and hydrate deposition and sloughing were observed. The density difference and agglomeration of hydrate particles is considered as the major cause of hydrate accumulation and deposition, which subsequently instigates plugging in water-continuous systems.
Clathrate hydrates have steadily emerged as an important field in the areas of flow assurance, energy storage and resource, and environment. To better understand the role of hydrates in all of these ...areas, knowledge developed in laboratory experiments must be effectively transferred to address the challenges related to hydrate formation, dissociation, agglomeration, and stability. This paper highlights the recent hydrate literature focusing on the thermodynamics, kinetics, structural properties, particle properties, rheological properties, and molecular mechanisms of formation. The foundation for continued understanding and development of hydrates in engineering practice will rely on laboratory measurements utilizing traditional and innovative tools capable of probing time-dependent and time-independent properties.
Clathrate hydrate particle undergoing morphological changes due to the addition of an anti-agglomerant, but stabilized by wax.
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•Investigations of the interactions between ...hydrates, anti-agglomerants (AAs) and waxes.•Micromechanical force measurements were used to explore particle-surface interactions.•Waxes can significantly alter both the hydrate cohesive and adhesive forces.•A effectiveness when wax is present can be compromised, which could result in hydrate agglomeration and potential plug formation.
Hydrates and waxes are two of the biggest flow assurance challenges during subsea oil production. They are both depositing species which can cause stenosis of the pipeline, reducing the available area for flow and increasing the pressure load for production. In extreme scenarios, the pipe can become completely blocked by either of these species. However, little is known about the interaction between waxes and hydrates and less still is known about how treatments for hydrates may affect waxes or vice versa. Hydrate deposition in a waxy pipeline may follow significantly different mechanisms from a bare steel pipeline due to a shift from a typically hydrophilic steel surface to a hydrophobic wax-coated surface. This study utilized a micromechanical force measurement apparatus in order to explore some of the interactions between waxes, both surface-deposited and dissolved in the bulk phase, and hydrates which have been treated with anti-agglomerant chemicals. It was found that waxes can significantly alter both the cohesive and adhesive forces caused by hydrate particles, but that the effect when anti-agglomerants are present may vary based on the composition of the anti-agglomerant. This incompatibility is important for operators to understand better, because the ineffectiveness of an anti-agglomerant when waxes are present could result in hydrate agglomeration and potential plug formation.
Gas hydrates present a major flow assurance challenge due to the relatively fast timescales at which they can form, agglomerate, and plug a subsea flowline, resulting in loss of production and ...potential safety and environmental risks. The use of anti-agglomerants (AA) as a hydrate management strategy is of increasing interest for use in deepwater developments, maturing fields, long tiebacks, and transient operations. This review article provides a perspective of the state-of-the-art of AA screening methods including an overview of the apparatuses commonly used such as rocking cells, autoclaves, rheometers, and flowloops, as well as the key parameters used to classify AA performance, with select representative examples. This work aims to provide the reader with a summary of AA screening techniques, evaluating the advantages and disadvantages of each method and apparatus serving as a tool to facilitate equipment selection for studies that involve hydrate agglomeration.
•Hydrate anti-agglomerant screening techniques are varied and non-uniform.•Adequate AA testing is essential to select suitable chemistry for field application.•Summary of the state-of-the-art of AA screening methods and apparatuses.•Key parameters used to classify AA performance, with select representative examples.•Analysis of screening methods and apparatus to aid system selection for AA studies.
As pipeline transportation in the oil and gas industry is moving to offshore conditions, the prevalent high-pressure and low-temperature conditions in the subsea flow lines may lead to hydrate ...formation and wax precipitation occurring simultaneously. The presence of wax may alter the interfacial properties and particle interactions, resulting in the change in hydrate cohesion behavior. In this study, cyclopentane hydrate cohesive forces are measured with different wax contents using a micromechanical force (MMF) apparatus. A custom wax sample with the composition from C17 to C39 was mixed with cyclopentane and used as the bulk phase. It was found that the cohesive force decreased with increasing wax content from 0 to 5 wt % then increased with further wax contents from 5 to 8.75 wt %. Dilution MMF measurements demonstrated that two competitive mechanisms, the oleophilic effect and reduced hydrate conversion rate were synergistically responsible for the observed changes in the cohesive force. In an MMF measurement with 10 wt % wax and 6 h annealing period, the wax was found to deposit on the hydrate surface and effectively reduced the cohesive force, indicating that wax crystals have a potentially inhibiting effect on hydrate cohesion. Furthermore, the bulk phase/water interfacial tension decreased with increasing wax contents. Finally, a possible mechanism is presented to illustrate the effect of wax on the hydrate cohesive force, considering the oleophilic effect, hydrate conversion, and wax deposition. This work provides insight into the influencing mechanisms of wax on hydrate cohesion, which can be useful for flow assurance applications where both hydrates and waxes are present.
Despite the industrial implications and worldwide abundance of gas hydrates, the formation mechanism of these compounds remains poorly understood. We report direct molecular dynamics simulations of ...the spontaneous nucleation and growth of methane hydrate. The multiple-microsecond trajectories offer detailed insight into the process of hydrate nucleation. Cooperative organization is observed to lead to methane adsorption onto planar faces of water and the fluctuating formation and dissociation of early hydrate cages. The early cages are mostly face-sharing partial small cages, favoring structure II; however, larger cages subsequently appear as a result of steric constraints and thermodynamic preference for the structure I phase. The resulting structure after nucleation and growth is a combination of the two dominant types of hydrate crystals (structure I and structure II), which are linked by uncommon 5¹²6³ cages that facilitate structure coexistence without an energetically unfavorable interface.
