Tetrahydrofuran Hydrate Research Articles

Structural Identification of Binary Tetrahydrofuran + O2 and 3-Hydroxytetrahydrofuran + O2 Clathrate Hydrates by Rietveld Analysis with Direct Space Method

The structural determination of clathrate hydrates, nonstoichiometric crystalline host-guest materials, is challenging because of the dynamical disorder and partial cage occupancies of the guest molecules. The application of direct space methods with Rietveld analysis can determine the powder X-ray diffraction (PXRD) patterns of clathrates. Here, we conducted Rietveld analysis with the direct space method for the structural determination of binary tetrahydrofuran (THF) + O2 and 3-hydroxytetrahydrofuran (3-OH THF) + O2 clathrate hydrates in order to identify the hydroxyl substituent effect on interactions between the host framework and the cyclic ether guest molecules. The refined PXRD results reveal that the hydroxyl groups are hydrogen-bonded to host hexagonal rings of water molecules in the 51264 cage, while any evidences of hydrogen bonding between THF guests and the host framework were not observed from PXRD at 100 K. This guest-host hydrogen bonding is thought to induce slightly larger 512 cages in the 3-OH THF hydrate than those in the THF hydrate. Consequently, the disorder dynamics of the secondary guest molecules also can be affected by the hydrogen bonding of larger guest molecules. The structural information of binary clathrate hydrates reported here can improve the understanding of the host-guest interactions occurring in clathrate hydrates and the specialized methodologies for crystal structure determination of clathrate hydrates.

Permeability anisotropy and relative permeability in sediments from the National Gas Hydrate Program Expedition 02, offshore India

Gas and water permeability through hydrate-bearing sediments essentially governs the economic feasibility of gas production from gas hydrate deposits. Characterizing a reservoir's permeability can be difficult because even collocated permeability measurements can vary by 4–5 orders of magnitude, due partly to differences between how various testing methods inherently measure permeability in different directions and at different scales. This study uses a customized flow anisotropy cell to investigate geomechanical and hydrological properties of hydrate-bearing sediments focusing on permeability anisotropy (i.e., horizontal, kh, to vertical, kv, permeability ratio) and relative permeability. Two cores recovered during India's National Gas Hydrate Program Expedition 02 (NGHP-02) are tested in this study. Near in situ effective vertical stress, ∼ 2 MPa, the permeability anisotropy is approximately kh/kv = 1.86 for the “seal core” (from a fine-grained non-reservoir overburden sedimentary section) and kh/kv = 4.24 for the gas hydrate reservoir core with tetrahydrofuran (THF) hydrate saturation Sh = 0.8. Permeability anisotropy increases exponentially with effective vertical stress, as described by kh/kv = α(σv/MPa)β, with α = 1.6, β = 0.22 for seal sediment and α = 3, β = 0.5 for THF hydrate-bearing sediment. Results imply the measured permeability from permeameter tests with vertical flow may underestimate the reservoir's flow performance, which is mainly horizontal (radial) toward a vertical well. Hydrate in sediment increases the gas-entry pressure and residual water saturation, but decreases the water retention curve's shape factor (m), resulting in a steeper curve. Distributions of available pore space sizes for flow in sediment with and without THF hydrate (Sh = 0.8) follow a log-normal distribution. Hydrate formation decreases the apparent mean pore size from ∼10 μm to ∼2 μm, without evidently changing the pore size distribution's standard deviation. Gas hydrate dissociation increases effective permeability and relative permeability to gas.

Study on in-situ measurement method of THF hydrate thermal conductivity

The HotDisk thermal constants analyzer based on the transient plane source (TPS) technique can be used to measure the hydrate thermal conductivity. In order to improve the measurement accuracy, the HotDisk setting parameters, the sample preparation and measurement method are investigated in this paper. Firstly, the evaluation criteria are proposed to determine the optimal setting parameters, mainly the output power and measurement time. Four kinds of method of the sample preparation and thermal conductivity measurement in situ are proposed and discussed. The used samples include pure Tetrahydrofuran (THF) hydrate, ice and THF hydrate or ice bearing silica sand. The results show that the optimal parameters of the output power and measurement time depend on the sample. A high purity of sample can be obtained using the temperature oscillation-aging method and the accuracy of measurement can be further improved using the “increasing temperature-decreasing temperature” method.

Compressibility and particle crushing of Krishna-Godavari Basin sediments from offshore India: Implications for gas production from deep-water gas hydrate deposits

Abstract Depressurizing a gas hydrate reservoir to extract methane induces high effective stresses that act to compress the reservoir. Predicting whether a gas hydrate reservoir is viable as an energy resource requires enhanced understanding of the reservoir's compressibility and susceptibility to particle crushing in response to elevated effective stress because of their impact on the long-term permeability and geomechanical stability of the reservoir. This study investigates physical and geomechanical properties of natural sediments with and without tetrahydrofuran (THF) hydrate subjected to high effective stresses of up to 25 MPa. Experimental results show the stiffness of hydrate-free sediments is mainly governed by the stress state and history, while the stiffness of hydrate-bearing sediments reflects both the grain supporting nature of the interconnected hydrate phase and stress effects. The Poisson's ratio of hydrate-bearing sediments at low stresses is dominated by the Poisson's ratio of the interconnected pore-filling phases, and dominated at high stresses by elastic properties of both the skeleton and pore-filling phases. The stress-void ratio responses of hydrate-bearing sediments above the pre-consolidation stress yields a slightly convex-downward trend, suggesting compressibility is influenced by the stiffness of THF hydrate and sediment grains rather than only by void space reduction. The shape of the compression index (Cc) trend may be attributed to an increasing effective gas hydrate saturation as the total pore volume decreases under loading. The results also show that the presence of THF hydrate in sediments can mitigate particle crushing by suppressing particle rearrangement and supporting a portion of the load that would otherwise have to be carried by the sediment. Therefore, the loss of hydrate crystals during gas production may exacerbate sand crushing.

Amphiphilic Copolymers Containing POSS and SBMA with N-Vinylcaprolactam and N-Vinylpyrrolidone for THF Hydrate Inhibition.

Icelike gas hydrates deposited in the pipelines under low temperatures and high pressures could remarkably reduce the transport efficiency, and a low dosage of water-soluble polymers could act as kinetic hydrate inhibitors (KHIs) to prevent gas hydrate formation. It was believed that the hydrophobic moiety in the water-soluble polymers played a vital role in enhancing the KHI performance. In this work, amphiphilic copolymers containing hydrophobic polyhedral oligomeric silsesquioxane (POSS) and superhydrophilic sulfobetaine methacrylate (SBMA) as well as N-vinylcaprolactam (VCap) and N-vinylpyrrolidone (VP) were prepared, and an efficient effect of the obtained amphiphilic copolymers on tetrahydrofuran (THF) hydrate inhibition was found. When a certain amount of the amphiphilic copolymers was introduced, the THF hydrate as an analogue of structure II gas hydrates presented a prolonged induction time and gave rise to a looser state rather than a crystalline solid. Analyses of low-field nuclear magnetic resonance and differential scanning calorimetry verified that there were strong interactions between the copolymer and water molecules by incorporation of SBMA units, which could enhance the KHI properties of the prepared amphiphilic copolymers. Additionally, the hydrophobic POSS in the amphiphilic copolymers could possibly modulate the hydrophilic/hydrophobic balance, contributing to the synergistical ability of the copolymers for THF hydrate inhibition. It was suggested that the amphiphilic copolymers containing POSS and zwitterionic units with VCap or VP could have potential for the inhibition and antiaggregation of gas hydrates in the transportation pipelines.

Ultrasonic attenuation of pure tetrahydrofuran hydrate

ABSTRACTImproved estimates of the amount of subsurface gas hydrates are needed for natural resource, geohazard, and climate impact assessments. To evaluate gas hydrate saturation from seismic methods, the properties of pure gas hydrates need to be known. Whereas the properties of sediments, specifically sands, and hydrate‐bearing sediments are well studied, the properties of pure hydrates are largely unknown. Hence, we present laboratory ultrasonic P‐wave velocity and attenuation measurements on pure tetrahydrofuran hydrates as they form with reducing temperatures from 25°C to 1°C under atmospheric pressure conditions. Tetrahydrofuran hydrates, with structure II symmetry, are considered as proxies for the structure I methane hydrates because both have similar effects on elastic properties of hydrate‐bearing sediments. We find that although velocity increased, the waveform frequency content and amplitude decreased after the hydrate formation reaction was complete, indicating an increase in P‐wave attenuation after hydrate formation. When the tetrahydrofuran hydrate was cooled below the freezing point of water, velocity and quality factor increased. Nuclear Magnetic Resonance results indicate the presence of water in the “pure hydrate” samples above the water freezing point, but none below. The presence of liquid water between hydrate grains most likely causes heightened attenuation in tetrahydrofuran hydrates above the freezing point of water. In naturally occurring hydrates, a similarly high attenuation might relate to the presence of water.

Inhibition of structure II hydrates formation by salt-tolerant N-vinyl lactam-based terpolymers

This work presents experimental results on the inhibition performance of low-dosage hydrate inhibitors (LDHIs). The studied LDHIs, namely terpolymers of N-vinylcaprolactam and alkylimidazolium bromide or benzyl ammonium chloride and hydroxyethyl methacrylate or acrylamide derivatives, were synthesized at the Mexican Petroleum Institute with the objective of producing high performance inhibitors with higher cloud points than that exhibited by the poly(N-vinylcaprolactam) homopolymer. Three experimental methods were used to evaluate the hydrate inhibition performance of the synthesized terpolymers: 1) the capacity to inhibit the formation of tetrahydrofuran (THF) hydrates (atmospheric pressure and −0.5 °C); 2) the capacity to inhibit the formation of natural gas hydrates under high pressure and low temperature conditions (∼6 MPa, ∼1 °C, a subcooling of 16 °C for the studied natural gas sample); and 3) the determination of the cloud point temperature of the terpolymers in aqueous systems with varying salinity (atmospheric pressure and temperatures ranging from 2 °C to 81 °C). The inhibition performance and the high solubility (in highly saline aqueous systems) of the synthesized terpolymers indicate that these compounds could be considered promising kinetic hydrate inhibitors or gas-well anti-agglomerants to prevent or delay the formation or agglomeration of clathrate hydrates during the production and transport of hydrocarbons.

Model test of stratum failure and pore pressure variation induced by THF hydrate dissociation

ABSTRACTA model test program for studying soil stratum failure and pore pressure variation during tetrahydrofuran (THF) hydrate dissociation considering the effects of heating and drainage conditions is presented in this paper. The temperature and pore pressures are recorded during heating. Test results show that the THF hydrate would dissociate to be liquid and then gas when heating. Once pore pressure generated by the flow of released gas/water exceeded the strength of over layer, the layered fractures and soil-gas/water mixture outburst would occur. The high heating temperature and low permeability of over layer both cause excess pore pressure generation and more serious stratum failures.

Molecular Insight into the Growth of Hydrogen and Methane Binary Hydrates

H2 is considered as the ideal fuel; however, the storage and transportation of H2 limit its usage. Clathrate hydrates are candidate materials for H2 storage and transportation. Because of the extreme conditions necessary to stabilize the pure H2 hydrate, additives are proposed to stabilize a mixed H2 hydrate. Compared to the widely studied H2 + tetrahydrofuran binary hydrates, H2 + CH4 binary hydrates contain a higher energy density. In this study, we study the growth of H2 + CH4 binary hydrates for two sets of temperature and pressure conditions by using molecular dynamics simulations with atomic models. Our results show that CH4 acts as a thermodynamic promoter for H2 + CH4 hydrate formation, while H2 acts as a kinetic promoter for H2 + CH4 hydrate growth at some of our working conditions. We find that there is a maximum growth rate of H2 + CH4 binary hydrates at 250 K when the pressure is 50 MPa, and at fixed temperature, the growth rate of H2 + CH4 binary hydrates shows a positive correlation with pre...

Characterization of free radicals in clathrate hydrates of pyrrole, thiophene, and isoxazole by muon spin spectroscopy

Gas hydrates have long been of interest to the petrochemical industry, but there has been growing interest in potential applications for carbon dioxide sequestration and hydrogen storage. This has prompted many fundamental studies of structure and host–guest interactions, but there has been relatively little investigation of chemical reactions of the guest molecules. In previous work, we have shown that it is possible to use muon spin spectroscopy to characterize H atom like muonium and muoniated free radicals formed in clathrate hydrates. Muonium (Mu) forms in clathrate hydrates of cyclopentane and tetrahydrofuran, whereas furan and its dihydro derivatives form radicals. The current work extends studies to clathrates hydrates of other five-membered heterocycles: thiophene, pyrrole, and isoxazole. All form structure II hydrates. In addition to the clathrates, pure liquid samples of the heterocycles were studied to aid in the assignment of radical signals and for comparison with the enclathrated radicals. Similar to furan, two distinct radicals are formed when Mu reacts with thiophene and pyrrole. However, only one muoniated radical was detected from isoxazole. Muon, proton, and nitrogen hyperfine constants were determined and compared with values predicted by DFT calculations to aid the structure assignments. The results show that Mu adds preferentially to the carbon adjacent to the heteroatom in thiophene and pyrrole and to the carbon adjacent to oxygen in isoxazole. The same radicals are formed in clathrates, but the spectra have broader signals, suggesting slower tumbling. Furthermore, additional signals in the avoided level-crossing spectra indicate anisotropy consistent with restricted motion of the radicals in the clathrate cages.

Effect of Eco-Friendly Cyclodextrin on the Kinetics of Mixed Methane–Tetrahydrofuran Hydrate Formation

Use of environmentally friendly additives to promote methane hydrate formation is an active area of research for gas storage via solidified natural gas (SNG) technology. The key advantages of SNG technology include a high degree of safety and a compact mode of natural gas storage in comparison to conventional methods of NG storage. In this paper, we evaluate the effect of β-cyclodextrin as a kinetic promoter for mixed methane−tetrahydrofuran hydrate formation. β-Cyclodextrin is an eco-friendly cyclic oligosaccharide containing seven glucose monomers that form a ring structure. The concentration of β-cyclodextrin was varied under different hydrate forming conditions (temperature and pressure), and the formation kinetics along with the morphology of mixed hydrate formation were observed. It is found that β-cyclodextrin served as an effective kinetic promoter for mixed methane–tetrahydrofuran hydrate formation at moderate pressure and temperatures in a simple unstirred reactor configuration. Further, no foam...

Ab initio study of formation of the clathrate cage in the tetrahydrofuran hydrate

Despite the potential applications and ubiquity of clathrate hydrates, the molecular mechanism of formation of these compounds remains poorly understood. In the present work, we performed ab initio calculations to investigate the formation of the clathrate cage of the tetrahydrofuran (THF) hydrate and its significance to the adsorption of gas molecules such as the methane, carbon dioxide, and hydrogen. We found that THF and six water molecules cooperatively organize into an initial stable structure that will allow the growth of more water faces. The formation of the clathrate cage is thermodynamically feasible, and the water-THF interactions become more significant with the increasing water molecules. However, the water-water interactions mostly dominate the formation process due to the strong hydrogen bond interactions. Further, for the adsorption of the second guests, there is little change in the structure and stability of the clathrate cage, but these second guests favor to adsorb onto the pentagonal faces rather than the hexagonal faces.

Velocities of Ultrasonic Waves in Sand Samples Containing Water, Ice, or Methane and Tetrahydrofuran Hydrates (Laboratory Measurements)

We analyze the results of investigations of acoustic properties (velocities of longitudinal and transversal waves) in sand samples containing different amounts of water, ice, and methane or tetrahydrofuran hydrates in the pores using a special laboratory setup.

Evaluation of Soy Lecithin as Eco‐Friendly Biosurfactant Clathrate Hydrate Antiagglomerant Additive

AbstractHydrate plug formation is one of the most challenging problems in upstream oilfields. The most preferred option to prevent hydrate plug formation is the use of antiagglomerants in the oil/gas flowlines and in drilling fluids. Antiagglomerants do not prevent hydrate formation but restrict the hydrate crystals from agglomerating. In this study, experiments were conducted on of soy lecithin (biosurfactant) at different concentrations to measure its effectiveness as an antiagglomerant additive. In all the experiments, tetrahydrofuran (THF) hydrate was used, and to study the antiagglomerant efficiency of the additive, visual observation was utilized. Soy lecithin was compared with cetyltrimethylammonium bromide (CTAB), and the former was found as a good environmentally friendly antiagglomerant additive. Soy lecithin at 0.1 wt% in the experimental solution increased the agglomeration time of hydrate crystals by more than 1440 min and showed agglomeration state III at 1 °C. Soy lecithin at 0.01 wt% increased the agglomeration time of hydrate crystals to 1000 min. The effect of soy lecithin on the rheological characteristics and filtration properties of drilling fluid was also studied. Even at the highest concentration of 1.0 wt% soy lecithin and at a temperature of 1 °C, the rheological parameters of the drilling fluid remained within permissible limits. Soy lecithin was thus found to effectively restrict the agglomeration of hydrate crystals.

Sieving of Hydrogen-Containing Gas Mixtures with Tetrahydrofuran Hydrate

Using the different diffusion rates of gas mixture components in hydrate crystals is a promising new method for the separation of gas mixtures. The diffusion behavior of hydrogen (H2), methane (CH4), and carbon dioxide (CO2) in tetrahydrofuran (THF) hydrate was investigated experimentally by exposing a THF hydrate layer to H2, H2 + CH4, or H2 + CO2 gas or gas mixtures and monitoring the penetration of the molecules into the hydrate by Raman spectra. The experimental results demonstrate that only H2 molecules can penetrate the entire 5 mm thick hydrate layer, while the THF hydrate is resistant to CH4 and CO2 molecules on the experimental time scale employed. The minimum partial pressure for H2 to diffuse through the THF layer is 2.9 MPa, and the diffusion coefficient of H2 in the THF hydrate layer was determined to be 6.1 × 10–12 m2/s via time-resolved Raman collection. This is the first observation of the sieving behavior of a massive hydrate layer toward different gas molecules. This work also suggests t...

Morphological Studies of Mixed Methane Tetrahydrofuran Hydrates in Saline Water for Energy Storage Application

Solidified natural gas (SNG) in the form of clathrate hydrates has been considered one of the most promising technologies for natural gas storage and transportation because it is safe and economically feasible compared to liquefied natural gas (LNG) and compressed natural gas (CNG). In this direction, we recently observed that mixed methane tetrahydrofuran hydrates had the ability to store a significant amount of methane gas close to ambient temperature conditions. To further improve the economic feasibility of methane gas storage and transportation process, utilization of saline water was recommended. In the present work, we investigate the morphology of mixed CH4/THF hydrate formation and dissociation in presence of NaCl for the first time. Morphology changes observed during the nucleation, hydrate growth and dissociation of mixed hydrates at two different operating conditions are presented. All the experiments were performed with a stoichiometric concentration of THF (5.56 mol% THF). More over, the gas uptake was also measured and compared with non-salt experiments.

Rheology of Tetrahydrofuran Hydrate Slurries

In this work, we study the rheology of hydrate slurries in a mixture of water and THF (tetrahydrofuran, C4H8O). This hydrocarbon is miscible in water, and forms hydrates at ambient pressure and temperatures above 0 °C. Rheological tests, constant shear rate, flow curve, creep, and oscillatory, are carried out for different concentrations. Transient and steady-state results are obtained, suggesting that the rheology is strongly affected by agglomeration and breakage of hydrate crystals that seem to happen simultaneously, when the slurry is sheared during hydrate formation.

Accurate thermodynamic description of vapor–liquid and solid–liquid equilibria of THF, water and gas hydrates with a unique set of parameters

In the context of gas hydrates technology for gas storage and separation processes, tetrahydrofuran (THF) has a recognized performance as a thermodynamic promoter. Literature presents a large amount of phase equilibrium experimental data for the THF hydrates systems, however, a consistent thermodynamic modeling capable of describing vapor–liquid, liquid-hydrate and liquid-ice equilibria of THF and water is still needed.With the goal of obtaining this needed modeling, we propose a strategy for the estimation of a unique set of parameters for suitable thermodynamic models for THF and water mixtures in the conditions of vapor, liquid and hydrate. To perform phase equilibrium calculations, we selected the NRTL GE model, the Peng-Robinson equation of state and the van der Waals and Platteeuw hydrate model with the Kihara potential to correlate independent properties. We regressed parameters for the NRTL and Kihara models in two sequential steps using experimental data at conditions with pressure from 40kPa to 101.33kPa and temperature from 272.00K to 343.15K. The first steps determines optimal parameters for NRTL with, simultaneously, vapor–liquid equilibria and infinite dilution activity coefficient data. The second determines optimal Kihara parameters with liquid-hydrate equilibria data using a data reconciliation methodology. We verified the potential of the model for extrapolation by accurately describing phase equilibria scenarios involving liquid and vapor mixtures not included in the regression procedure, in addition to scenarios involving conventional ice and hydrates of THF with some industrial gas or natural gas components: H2, CO2 or CH4.

Tetrahydrofuran Hydrate Crystal Growth Inhibition with Synergistic Mixtures: Insight into Gas Hydrate Inhibition Mechanisms

Various mixtures of two chemicals have been tested for their ability to prevent tetrahydrofuran (THF) hydrate crystal growth and compared to their ability as gas hydrate kinetic hydrate inhibitors (KHIs) using a structure-II-forming natural gas mixture, the same structure obtained with THF hydrates. The THF hydrate results for mixtures of poly(N-vinylcaprolactam) (PVCap) with tetra-n-butylammonium bromide (TBAB), tetra-n-butylphosphonium bromide (TBPB), or hexa-n-butylguanidinium chloride (Bu6GuanCl) showed clear synergy effects in two different types of tests: an isothermal test at −0.5 °C and a variable temperature test. In these tests, the key parameters are the lowest concentration of a mixture and the highest subcooling for which complete THF hydrate inhibition is observed, respectively. The synergistic similarities between the THF and previously obtained structure-II-forming gas hydrate results suggest that the dominant inhibition mechanism operating in these mixtures in the gas hydrate system is cr...

2-2-2 一方向凝固法による動的インヒビタの成長抑制および分解速度に与える影響の評価

2-2-2 一方向凝固法による動的インヒビタの成長抑制および分解速度に与える影響の評価