High-pressure Gas Hydrates Research Articles

Kinetic Inhibition Effect of Valine on Methane Hydrate Nucleation Time in Oil System

Amino acids are known as novel kinetic hydrate inhibitor (KHI) for hydrate mitigation, however, the performance in oil system is not well justified up to date. In this study, the kinetics of methane hydrates in oil dominated systems using valine amino acid has been studied in a high-pressure gas hydrate reactor at 80 bar and 1 °C using the constant cooling method. Drilling oil was used in this work to mimic the oil phase in flow assurance. The results shows that valine delays hydrate formation nucleation time 4 times compared with pure water systems. This finding is contrary to most claims in literature that shows that valine promotes hydrate in gas dominated system. However, the inhibition impact of valine exhibited in this work might be due to the presence of oil with is absence in the most literature studies. The findings in the work are useful for hydrate mitigation using amino acids in flow assurance.

Dual-functional gas hydrate inhibition of tetramethylammonium chloride for carbon dioxide-methane mixed gas systems

The present work deals with evaluating the dual-functional gas hydrate impact of tetramethylammonium chloride (TMACl) in the presence of different CO2-CH4 content mixed gas hydrate systems (30%CO2 + 70%CH4, 50%CO2 + 50%CH4, and 70%CO2 + 30%CH4). A custom-made high-pressure gas hydrate reactor was used to acquire the temperature–pressure loops for the studied systems in the absence/presence of different concentrations of aqueous TMACl solutions via T-Cycle and isochoric constant cooling method for both THI and KHI investigations, respectively. The electrolyte-based thermodynamic model was also applied to validate the obtained HLwVE results for all the studied systems. The obtained results revealed that TMACl acts dual-functional (thermodynamic and kinetic) hydrate inhibitor for high CO2 content gas systems. The increased concentration of TMACl induces more shifts in HLwVE data with maximum variation attained at10 wt% concentration up to 1.46 K for a high CO2 content methane system owing to the increased hydrogen bonding ability of TMACl. Moreover, TMACl delayed the hydrate formation up to 1.4 and1.5 folds for 274.0 and 277.0 K conditions for high CO2 content mixed gas systems. Moreover, the applied electrolyte-based model could predict the HLwVE data of TMACl in the presence of a mixed gas system within the AAE value of 0.1 % for all the studied mixed gas systems. Furthermore, the KHI performance of TMACl was also compared with commercial inhibitor, i.e., polyvinyl pyrrolidone (PVP), and obtained comparable results.Therefore, the acquired dual-functional results (THI = 1.46 K, KHI = 1.5-fold delay) signpost that TMACl can efficiently work as a potential dual-functional hydrate inhibitor for CO2 enriched mixed gas systems.

Polymers of N-(Pyrrolidin-1-yl)methacrylamide as High Cloud Point Kinetic Hydrate Inhibitors

Formation of gas hydrates is a major problem in flowlines where gas and water are transported together, and can lead to blockages, downtime, economic losses, and potential accidents. One way of preventing gas hydrates from forming is by injection of kinetic hydrate inhibitors (KHIs). KHIs are typically water-soluble polymers, often containing amide pendant groups. Based on our previous work on high cloud point polymers of N,N-dimethylhydrazidoacrylamide (polyDMHAM) and N,N-dimethylhydrazidomethacrylamide (polyDMHMAM), we have now synthesized a series of N-(pyrrolidin-1-yl)methacrylamide polymers (polyNPyMA) which contains a pyrrolidine-substituted hydrazido pendant group. These polymers show improvement on all previous hydrazido polymers as they exhibit improved KHI performance while having no cloud point in deionized water or 7 wt % aqueous NaCl up to 95 °C. Their performance as KHIs have been investigated using a high-pressure gas hydrate rocker rig at a pressure of 76 bar using a structure II forming n...

High-pressure gas hydrate autoclave hydraulic experiments and scale-up modeling on the effect of stirring RPM effect

A synthetic quaternary gas mixture (Methane = 85.24%, Ethane = 10.03%, n-Hexane = 0.02 mol %, Carbon dioxide = 2.49% and Nitrogen = 2.22%) has been used to study the effect of different stirring rates between 100 and 1400 rotations per minute (RPM) on hydrate crystal formation in the high pressure autoclave cell (HPC). The experimental results obtained show that there exists a threshold limit above and below which hydrate formation decreases drastically. The results were used to determine the optimum-stirring rate at which maximum hydrate crystal formation is obtained in HPC. For the scale up purposes, using the experimental results, the dimensional analysis has been conducted on HPC to obtain optimum power and density ratio factors for the bench scale reactor system. So this work aims to facilitate the ongoing research for using hydrates for natural gas storage and transportation purposes.

Can cyclopentane hydrate formation be used to screen the performance of surfactants as LDHI anti-agglomerants at atmospheric pressure?

Anti-agglomerants (AAs) are a class of low dosage hydrate inhibitors (LDHIs) that have been used commercially in oilfield operations for almost two decades to prevent the plugging of flow lines with gas hydrates. AAs are surfactants and the ranking of their AA performance usually requires high pressure equipment with the ability to collect visual data to determine the plugging tendency. A simple but crude screening method for determining the AA performance of a chemical has been developed using cyclopentane hydrate at atmospheric pressure. The method requires is limited by the maximum subcooling allowable, which for our system was found to be 5.3°C from hydrate dissociation experiments if ice formation is to be avoided. The reproducibility of the screening method was found to be good although no hydrate formation occurred for some chemicals with significant kinetic inhibition activity due to the low subcooling. These chemicals were usually the commercial AAs which have good crystal growth inhibition properties. Therefore, the method was not able to distinguish good AA performance between the best AAs. In general, the AA performance results with cyclopentane hydrate correlated well with those obtained in high pressure natural gas hydrate equipment. Those surfactants with poor results with the cyclopentane hydrate method gave also poor performance using the gas hydrate method. However, some discrepancies between the results for the two test methods were found, which is sufficient to conclude that the cyclopentane hydrate test method is only a simple, crude method for first time screening of a surfactant to determine if it has any potential for further testing in high pressure gas hydrate systems.

N,N-Dimethylhydrazidoacrylamides. Part 1: Copolymers with N-Isopropylacrylamide as Novel High-Cloud-Point Kinetic Hydrate Inhibitors

Hydrophobically modified methacryl- and acrylamide polymers are well-known kinetic hydrate inhibitors (KHIs). Polymers of N-isopropylmethacrylamide (IPMAM) are now commercially available. However, both polyIPMAM and N-isopropylacrylamide homopolymer (polyIPAM) have low cloud and deposition points, making it difficult to use them in the field because of precipitation problems. Comonomers that are more hydrophilic can be copolymerized with IPMAM or IPAM to raise the cloud point. In this work, we have synthesized and investigated a series of new copolymers of IPAM and a new dimethylhydrazidoacrylamide monomer (DMHAM) as KHIs for the first time using high-pressure gas hydrate rocker rig equipment. The novel polymers have high cloud points in deionized water and also in brine solutions compared to polyIPAM. A 1:2 copolymer of DMHAM/IPAM gave the highest KHI performance for this class of copolymer with a cloud point at 58 °C in deionized water. A 1:1 copolymer gave only a small reduction in performance but has ...

Experimental Research on Hydrogen Storage Characteristics of TBAB Hydrates

The hydrogen storage characteristics of Tetra Butyl Ammonium Bromide (abbr. TBAB) hydrates were studied in a high pressure gas hydrate experimental apparatus. The effects of temperature and pressure on hydrogen storage characteristics under the condition of constant volume were discussed. The results showed that lower reaction temperature or higher reaction pressure can cause to a rapid formation of hydrogen hydrate and a larger hydrogen storage density, Comparing with tetrahydrofuran (abbr. THF) hydrates under same temperature and pressure, TBAB hydrates have faster hydrogen storage rate and larger hydrogen storage density.

High-Pressure Gas Hydrates of Argon: Compositions and Equations of State

Volume changes corresponding to transitions between different phases of high-pressure argon gas hydrates were studied with a piston-cylinder apparatus at room temperature. Combination of these data with the data taken from the literature allowed us to obtain self-consistent set of data concerning the equations of state and compositions of the high-pressure hydrates of argon.

Proton disorder and energetics of gas hydrate frameworks

A method of combinatorial optimization of gas hydrate frameworks based on the discrete model of intermolecular interaction is proposed. The stability of a large number of expected frameworks of high-pressure gas hydrates is analyzed.

High-pressure gas hydrates

It has long been known that crystalline hydrates are formed by many simple gases that do not interact strongly with water, and in most cases the gas molecules or atoms occupy 'cages' formed by a framework of water molecules. The majority of these gas hydrates adopt one of two cubic cage structures and are called clathrate hydrates. Notable exceptions are hydrogen and helium which form 'exotic' hydrates with structures based on ice structures, rather than clathrate hydrates, even at low pressures. Clathrate hydrates have been extensively studied because they occur widely in nature, have important industrial applications, and provide insight into water-guest hydrophobic interactions. Until recently, the expectation-based on calculations-had been that all clathrate hydrates were dissociated into ice and gas by the application of pressures of 1 GPa or so. However, over the past five years, studies have shown that this view is incorrect. Instead, all the systems so far studied undergo structural rearrangement to other, new types of hydrate structure that remain stable to much higher pressures than had been thought possible. In this paper we review work on gas hydrates at pressures above 0.5 GPa, identify common trends in transformations and structures, and note areas of uncertainty where further work is needed.

Phase formation and structure of high-pressure gas hydrates and modeling of tetrahedral frameworks with uniform polyhedral cavities

During the last decade, a number of high-pressure gas hydrates have been prepared and structurally characterized. One of the most interesting results obtained is the formation of compounds having tetrahedral water frameworks with uniform polyhedral cavities (e.g., clathrates THF·6H2O and 2Ar·6H2O), unprecedented among ambient pressure hydrates. In order to predict, reveal and perform effective investigations of such gas hydrates at high pressures it is plausible to know all possible tetrahedral frameworks of this kind. The solution of this problem demands to elaborate methods of a topological design of the tetrahedral frameworks represented as space-filling packings of uniform polyhedra with trivalent vertices (simple polyhedra). There are two related approaches to solve this problem: tiling of 3D space into symmetrically equal polyhedra (stereohedra) and generation of periodic four-connected nets. At present, the complete set of tetrahedral frameworks built of uniform simple 14-hedra (23 packings) was obtained and more than 800 frameworks were constructed from larger simple stereohedra. This article gives a discussion of structural and energetic characteristics of the frameworks generated, as well as the possibilities of using these results for interpretation of experimental structural data and a deliberate synthesis of clathrate compounds possessing new structures. Experimental and theoretical data show a high probability of finding a whole series of novel high-pressure gas hydrates with the tetrahedral water frameworks built of simple stereohedra. Of these, the most probable structures are those with 14-hedral cavities, these structures having the stoichiometry of six water molecules per cavity. In our opinion, the formation of such hydrates could be expected for guest molecules with van der Waals diameters from 5.8 to 7.2 Å. However, these hydrates cannot be excluded for substantially smaller guests as well, provided that the water framework cavities include two guest molecules. Packing polymorphism (different space filling arrangements of the same stereohedron) revealed in the search for tilings of 3D space into stereohedra offers experimental discovery of this phenomenon, which can be promoted by such delicate effects as the guest-guest interaction. The set of the derived water frameworks as stereohedra space-fillings gives opportunities to select starting structure models in the course of a diffraction study of polycrystalline high-pressure gas hydrate samples.

High-pressure gas hydrates

High-pressure gas hydrates

New Type of Phase Transformation in Gas Hydrate Forming System at High Pressures. Some Experimental and Computational Investigations of Clathrate Hydrates Formed in the SF6−H2O System

In this work, we present a new, previously unknown type of structure transformation in the high-pressure gas hydrates, which is related to the existence of two different isostructural phases of the sulfur hexafluoride clathrate hydrates. Each of these phases has its own stability field on the phase diagram. The difference between these hydrates consists of partial filling of small D cages by SF(6) molecules in the high-pressure phase; at 900 MPa, about half of small cages are occupied. Our calculations indicate that the increase of population of small cavities is improbable, therefore, at any pressure value, a part of the cavities remains vacant and the packing density is relatively low. This fact allowed us to suppose the existence of the upper pressure limit of hydrate formation in this system; the experimental results obtained confirm this assumption.

Gas Hydrates of Argon and Methane Synthesized at High Pressures: Composition, Thermal Expansion, and Self-Preservation

For the first time, the compositions of argon and methane high-pressure gas hydrates have been directly determined. The studied samples of the gas hydrates were prepared under high-pressure conditions and quenched at 77 K. The composition of the argon hydrate (structure H, stable at 460-770 MPa) was found to be Ar.(3.27 +/- 0.17)H(2)O. This result shows a good agreement with the refinement of the argon hydrate structure using neutron powder diffraction data and helps to rationalize the evolution of hydrate structures in the Ar-H(2)O system at high pressures. The quenched argon hydrate was found to dissociate in two steps. The first step (170-190 K) corresponds to a partial dissociation of the hydrate and the self-preservation of a residual part of the hydrate with an ice cover. Presumably, significant amounts of ice Ic form at this stage. The second step (210-230 K) corresponds to the dissociation of the residual part of the hydrate. The composition of the methane hydrate (cubic structure I, stable up to 620 MPa) was found to be CH(4).5.76H(2)O. Temperature dependence of the unit cell parameters for both hydrates has been also studied. Calculated from these results, the thermal expansivities for the structure H argon hydrate are alpha(a) = 76.6 K(-1) and alpha(c) = 77.4 K(-1) (in the 100-250 K temperature range) and for the cubic structure I methane hydrate are alpha(a) = 32.2 K(-1), alpha(a) = 53.0 K(-1), and alpha(a) = 73.5 K(-1) at 100, 150, and 200 K, respectively.

Direct measurement of stoichiometry of the structure H argon gas hydrate synthesized at high pressure

For the first time the composition of a high-pressure gas hydrate has been studied analytically. The experimentally determined composition of the argon hydrate of hexagonal structure III (structure H) is Ar· (3.27±0.17)H2O, with the hydrate being stable in the range of pressures 460–770 MPa. The new results confirm the reliability of previous results based on the refinement of argon hydrate structures using neutron powder diffraction data and help to elucidate the evolution of hydrate structures in Ar-H2O system under high pressures.

Design of tetrahedral frameworks as layered packings of uniform polyhedral cavities

A topological geometrical approach to the generation of polyhedral frameworks is proposed. The polyhedral frameworks are reduced to layered packings of symmetrically equivalent polyhedral cavities (stereohedra) in form of convex 14-hedra with quadrilateral, pentagonal and hexagonal faces and trivalent vertices. The consideration of various mutual arrangements of the polyhedra within a layer and the stacking of the layers themselves makes it possible to deduce 16 types of frameworks; ten of them were found for the first time. Some of the stereohedra found (for example, 435665 and 445466 types) may form several types of the packings. The application of the obtained results to the crystal chemistry of high-pressure gas hydrates and related clathrate compounds is discussed.

The piston-cylinder apparatus for in-situ structural investigations of high-pressure phases of gas hydrates with the use of synchrotron radiation

The piston-cylinder apparatus for the investigation of high-pressure gas hydrate phases by the powder diffraction method is presented. The first results concerning the nature of the high-pressure gas hydrate phase in the sulfur hexafluoride–water system are reported.

A new high-pressure gas hydrate phase in the sulfur hexafluoride–water system

A gas hydrate of the cubic structure I (CS-I) was detected in the title system at pressures higher than 50 MPa with the use of in situ X-ray diffraction analysis.