•NGH production from methane steam reforming and CO2/H2 replacement was proposed.•H2 is to decrease the partial pressure of methane and break methane hydrate stability.•Higher H2 ratio in feed gas results in higher accumulative gas production ratio but lower CO2 SR.•Gas replacement by CO2/H2 mixture improves gas production ratio and rate.A novel natural gas hydrate production method combined with methane steam reforming and CO2/H2 replacement was proposed to improve the replacement effect and reduce the cost of later gas separation, in which the role of H2 is to decrease the partial pressure of methane in gas phase and help to break the methane hydrate stability. After preparing representative hydrate sediment samples, we conducted a series of experiments to study the characteristics of gas production by the CH4-CO2/H2 replacement method. For the composition of CO2 and H2 in the feed gas, an increase in the mole fraction of H2 would result in a higher accumulative gas production ratio during the gas sweep and replacement stages but decrease the CO2 sequestration ratio, which refers to the amount of CO2 captured by the hydrate versus the gross CO2 injected into the hydrate layer. On the contrary, an increase in the mole fraction of CO2 in the feed gas would have a higher CO2 sequestration ratio, but would sacrifice both the gas production rate and the accumulative methane production ratio. Notably, when the mole fraction of the CO2 ranges from 55% to 72%, the amount of CO2 trapped into hydrate phase is close to the amount of methane dissociated from hydrate. Although the accumulative gas production ratio is not the highest in this range, it can meet the dual function of CO2 replacement.Download high-res image (95KB)Download full-size image

A terrestrial plant fruit extract was adopted to prevent hydrate accumulation. Four groups with different polarities, water-soluble, n-butanol-soluble, ethyl acetate-soluble, and petroleum ether-soluble portions, were obtained by prefractionation of the plant-extract hydrate antiagglomerant (AA). The hydrate antiagglomeration effect was tested in a high pressure transparent sapphire cell, and the active components were found mainly in the ethyl acetate-soluble portion that accounts for approximately 4.0% of the whole plant extract. Through further separation and purification, the five kinds of components obtained proved experimentally to have the effect of preventing hydrate agglomeration. Their molecules and structural formulas were inferred by the high resolution mass spectrum and nuclear magnetic resonance analysis. Compared with the spectrum library, the components were determined to be eriodictyol, apigenin, naringenin, luteolin, and 5-(4-hydroxy-6,7-dimethoxy-3-methylchroman-2-yl) benzene-1,2,3-triol. The tests on the extracted components and the commercial products verified their effect of preventing hydrate agglomeration, where apigenin and luteolin performed better than others.

•Morphology and partial structure of hydrate film on gas bubble surface were studied.•Part of polyhedral structures are holes and others are separated single crystals.•Polyhedral structures appeared for CH4-C2H6 mixture with 81–82% methane.•Single polyhedral crystals and holes after filled with hydrate are sII hydrate.The morphology and structure of hydrate films of pure methane and methane+ethane mixtures were studied by suspending a single gas bubble in liquid water. The methane+ethane gas mixtures, containing 20 mol% or 81–89 mol% methane, were used to form hydrate films with different crystal structures. Polyhedral structures appeared on the background of uniform polycrystalline hydrate films formed by gas mixtures with a high methane mole fraction of 0.81–0.82. Some of the polyhedral structures are holes, and others are separated structure II polyhedral hydrate crystals. However, for pure methane, a gas mixture with a lower CH4 mole fraction of 0.2, or a gas mixture with a higher CH4 mole fraction of 0.89, the morphology is uniform through the whole hydrate film. The crystal structures in the hydrate film were determined using in situ Raman spectra. The results show that both pure methane and the gas mixture with a lower CH4 mole fraction of 0.2 form a hydrate film with structure I. The single polyhedral crystal appeared in the background of the uniform polycrystalline hydrate film formed by gas mixtures with a high methane mole fraction of 0.81–0.82 as a structure II CH4-C2H6 double hydrate crystal. The hole is mainly gas with an initial small amount of structure I hydrate, and that hole will be filled with structure II hydrate after a certain time. The experimental methods and the new findings presented in this work should be significant for the further research of hydrate growth kinetics.

The separation of methane/ethylene gas mixtures by the adsorption–hydration method was investigated by using a wet zeolitic imidazolate framework (ZIF) microporous material, ZIF-8. The influences of the initial gas–solid ratio (the volume ratio of gas mixture to ZIF-8) and water content of wet ZIF-8 on the separation performance were studied systematically at 274.15 K. The experimental results show that gas–solid ratio of ca. 310 and water content of ca. 38.50 wt % are the suitable conditions for the separation of methane/ethylene mixture with wet ZIF-8 on the premise of hydrate formation. When water content of wet ZIF-8 is 38.50 wt % and the gas–solid ratio is 297, the mole fraction of ethylene in gas phase decreases from 30.91% to 14.44%, the selectivity coefficient reaches 5.57, and the adsorption quantity of ethylene reaches 3.37 mmol/g after one stage of the adsorption–hydration process. Compared with a single adsorption method or hydration method, methane/ethylene gas mixtures can be further separated with the synergistic effect of gas adsorption and hydrate formation. The morphology and crystal structure of ZIF-8 remains after a series of processes of saturation with water, adsorption–hydration, hydrate decomposition, and drying.

The feasibility of evaluating the performance of anti-agglomerants from analyzing the variation of morphologies and chord length of particles was examined using an autoclave installed with a particle video microscope and focused beam reflectance measurement probes. The experimental results from methane + water + diesel oil systems show that the agglomeration of hydrate can be judged directly from two laser probes. With the action of an effective anti-agglomerant, the size of droplets/hydrate particles is uniform and tends to that before hydrate formation when most water converts to hydrate or the system stops stirring for 9 h, although the agglomeration occurs when water droplets initially convert into hydrates. The evaluation method was examined under different anti-agglomerant concentrations, water cuts, and subcoolings for water + diesel oil systems. The action mechanism of anti-agglomerants was also proposed.

The self-preservation effect experiments in water + diesel oil dispersion systems for methane hydrate were carried out with the particle size ranging from tens to more than 100 μm. The influence of water cuts (low water cuts of 10, 20, and 30 vol % or high water cuts of 95, 99, and 100 vol %) and types of inhibitors (tetra-n-butylammonium bromide or Lubrizol) on the dissociation kinetics in oil and water suspensions were examined. The addition of surfactants, especially those able to lower the size of droplets or hydrate particles in low water cut suspension systems, could remarkably hinder the self-preservation effect by surface adsorption and alterations in structures and morphologies of ice film. For higher water cut systems with or without surfactants, the enhanced self-preservation effect was observed in comparison to lower water cuts. Systems with oil exhibited a declined effect in contrast to pure water systems. The ice-shielding mechanism for hydrate dissociation is supported by the effects of surfactants and water cuts on dispersion and agglomeration properties as well as the size of hydrate particles.

Two new kinetic hydrate inhibitors (KHIs) named PVP-B and PVP-BP were synthesized successfully and compared with commercial KHIs such as PVP and Inhibex 55W. PVP-B and PVP-BP are both the ramifications of PVP. They were obtained by introducing a t-butyl group and both t-butyl and phenyl groups into PVP molecules, respectively. The hydrate inhibition performances of KHIs were assessed in a sapphire cell through two kinds of onset times of hydrate formation: TVO, the time when hydrate crystals are initially observed by naked eyes, and TPD, the time when rapid and continuous dropping of system pressure begins. TVO was used to evaluate an KHI’s inhibition performance to hydrate nucleation. The length of time period from TVO to TPD was used to evaluate a KHI’s inhibition performance to hydrate growth. The results demonstrate that both PVP-B and PVP-BP are superior to PVP and Inhibex 55W; PVP-BP has better inhibition performance than PVP-B because of the stronger steric hindrance effects of the phenyl group in PVP-BP molecules than those of the t-butyl group in PVP-B molecules. Additionally, we found TVO does not always increase with the increasing dosage of KHI while TPD does. The most suitable dosage of PVP-BP was determined to 0.5 wt % or so, at which the nucleation of hydrate is inhibited most perfectly. Finally, we demonstrated glycol could be used as synergist to improve the performance of PVP-BP remarkably. The most suitable dosage of glycol was determined to be 0.9 wt % or so. Our work presents not only two new KHIs but also important insights into the inhibition mechanism of KHIs.

•Hydrate phase equilibrium of H2/CH4/CO2 ternary gas mixtures in pure water system were determined.•There exists little difference for thermodynamic model prediction if taking H2 as a former or not.•The occupancy percentage of hydrogen molecules range from 0.3 to 2.3 percent in small cage.•H2 molecules are indeed trapped in hydrate solid.Hydrogen, methane, and carbon dioxide are co-existence in many cases such as vent gas from the steam reforming of hydrocarbons. For the purpose of H2 recovery and/or CO2 sequestration via separation technology based on hydrate, the hydrate phase equilibrium of four groups of H2/CH4/CO2 ternary gas mixtures in pure water system were determined using the isothermal pressure-search method in a temperature range of (274.7–284.5) K. During modeling hydrate formation conditions, there is little difference with experimental data no matter whether H2 works as a hydrate former or not. Hydrogen can be regarded as a kind of diluent gas for convenience when presetting the operation conditions. To meet the mass balance of each gas species, the cage occupancy percentages of hydrogen in hydrate have been evaluated. The result illustrates that the occupancy percentage of hydrogen molecules ranges from 0.3 to 2.3 percent in small cage and (2.16–13.6) × 10−7 percent in large cage, which deduces that H2 molecules are indeed trapped in hydrate solid.

CO2-in-water emulsions formed from polyoxyethylene sorbitan monooleate (Tween 80) and sodium dodecyl sulfate (SDS) were evaluated in terms of stabilization time and emulsion droplet distribution. A CO2 emulsion with 0.5 wt % SDS + 5 wt % Tween 80 was found to be more stable than the other emulsions. With an increase in the stirring rate or pressure or a decrease in the temperature, the size of CO2 emulsion droplets tended to be smaller, and the stabilization time of the CO2 emulsion increased. With the optimized CO2 emulsion, the CH4–CO2 replacement reaction in a hydrate-bearing quartz sand sample was performed in a three-dimensional reactor. In the emulsions, the function of Tween 80 is to make the emulsion much more hydrophilic and reduce the flow resistance of the emulsion, whereas that of SDS is to make the newly formed hydrate particles/layers more granular and looser. The results indicate that, for hydrate reservoirs located in the stability fields of both CO2 hydrate and methane hydrate, the replacement efficiency of the CO2 emulsion can reach 47.8%, higher than those of liquid CO2 and gaseous CO2. The addition of 3.35 wt % salt to the emulsion can prevent the pore spaces of sediments from being blocked.

•Viscosity of water-in-oil emulsions containing hydrate anti-agglomerants was measured.•Relative viscosity of emulsion increases exponentially with decreasing of temperature.•Relative viscosity of emulsion increases almost linearly with increasing of pressure.•A viscosity correlation involved temperature, pressure, droplet size, and water cut was built.The viscosities of water-in-oil emulsions containing two kinds of hydrate anti-agglomerants were experimentally measured at three water cuts (10, 20, and 30 vol%) under (275.2–293.2) K and pressures up to 20 MPa. The experimental results indicate that the relative viscosity of the emulsion shows an exponential growth to some extent with the decrease of temperature for a given pressure and water cut, but increases almost linearly with the increase of pressure for all isotherms investigated. It also increases with increasing water volume fraction. The mean droplet sizes of water-in-oil emulsions show an exponential growth with the increase of temperature for a given water cut. An improved viscosity correlation as a function of temperature, pressure, mean droplet size, and water cut was proposed. The average relative deviations of the calculated results from the viscosity correlation were 4.4% and 4.0% for water-in-oil emulsions containing two kinds of anti-agglomerants, respectively.

•Hydrate formation kinetics for methane in water-in-oil emulsions are studied.•The induction time varies inversely with initial pressure, and directly with temperature.•Hydrate growth rate increases with increase of initial pressure and water cut, but decrease of temperature.•A mathematical model was developed for describing methane hydrates kinetic in dispersed system.Experimental data on hydrate formation kinetics for methane in water-in-oil emulsions are presented using a stirred batch reactor in the pressure range of 6.48–8.76 MPa and temperature range of 274.2–278.2 K. The influences of system temperature, initial pressure, and water cut of water-in-oil emulsion on the induction time of hydrate formation and hydrate growth rate were investigated, respectively. Experimental results show that the induction time varies inversely with initial pressure, and directly with temperature. The hydrate growth rate increases with increase of initial pressure and decrease of temperature. In the water-in-oil dispersed systems, the increase of water cut increases the hydrate growth rate. A mathematical model was developed for describing the formation kinetic behavior of methane hydrates in the dispersed system. The parameters of the model were determined by correlating the formation rate data, and the activation energies were further determined. The model was found to be able to calculate the formation kinetic behavior of methane hydrate in the dispersed system.

An absorption–hydration hybrid method was employed for separating C2 components (C2H4 + C2H6) from low-boiling gas mixtures such as refinery dry gas using water-in-diesel emulsions under hydrate formation conditions. Span 20 was used to disperse the water or hydrate in diesel to form the emulsion or hydrate slurry. To simulate a three-stage separation process, three (CH4 + C2H4 + C2H6 + N2) feed gas mixture samples with different gas molar compositions were prepared. Separation experiments were performed under different conditions to investigate the influences of feed composition, temperature, pressure, initial water cut in the emulsion, and initial gas/liquid volume ratio on separation efficiency. The experimental results show that the absorption–hydration hybrid method is obviously superior to the single-absorption method. After three stages of separation at appropriate operating conditions, we found that C2 compounds can be enriched from ∼15 to more than 50 mol % in the (hydrate + diesel) slurry phase and that the content of C2 compounds in the residual gas phase can be reduced to lower than 2 mol %. Low temperature, low initial gas/liquid volume ratio, high pressure, and high water cut were found to be favorable for the recovery of C2 compounds. However, when the temperature was lower than 270.2 K and the water cut was higher than 30 vol %, the formation of flowable hydrate slurry became difficult.

The anti-agglomeration performance of single or compounded commercial chemical additives with/without the addition of alcohol as a co-surfactant was evaluated using a sapphire cell and an autoclave reactor with a focused beam reflectance measurement (FBRM) probe. Five kinds of gas hydrate morphologies, clumpy-like, slush-like, flocculent-like, slurry-like, and powder-like, were found during evaluating the effect of the commercial additives in (water + oil) systems. The experimental results showed that AEO-3 combined with some commercial chemical additives, especially Span 20, exhibits good anti-agglomeration performance. The hydrate slurry thus formed has a high stability and will not result in agglomeration for a long period of time. A compounded inhibition mechanism, in which one of the components disperses water droplet in the oil phase and the other component prevents formed hydrate from agglomeration, was proposed. A new structure of hydrate anti-agglomerant was designed according to the evaluating results of the single or compounded commercial additives with/without the addition of alcohol material.

Formation and reformation of methane hydrate in (water + diesel oil + sorbitan monolaurate) dispersed systems have been investigated to test the memory effect using particle video microscope and focused beam reflectance measurement probes. The factors which would affect methane hydrate formation, the dosage of sorbitan monolaurate, water cut, and initial experimental temperature, were examined. The results show that there exists obvious memory effect when methane hydrate reformed in water/diesel oil dispersed systems at initial temperature near methane hydrate formation zone, even maintaining at the initial temperature for 168 h after methane hydrate dissociation. The subcooling will increase with prolonging of maintaining time, which suggesting that memory effect would disappear if time is long enough. When initial temperature increases to 5 K higher than the equilibrium value, the subcooling of reformation of methane hydrate is similar to that of hydrate first formation, which implying that memory effect disappears.

Constant composition expansion measurements were performed using a high pressure fluid PVT system for two gas condensate samples collected from two reservoir fields in China to determine the dew point pressures and compressibility factors at different temperatures, where the experimental pressures were up to 95 MPa. For the samples studied, the experimental results showed that the dew point pressures decrease with the increase of temperature and the gas compressibility factors increase with the increase of pressure in single phase zone at four temperatures. Thermodynamic models that combining equations of state with different kinds of plus-fraction splitting and critical properties of pseudo-components characterization methods were compared and optimized to describe the phase behavior and volumetric properties of gas condensate samples under high-pressure and high-temperature conditions. The preferred model has a better description for dew point pressures and gas compressibility factors with the total average absolute deviation for two samples of 0.64% and 1.56%, respectively.Highlights► Constant composition expansion tests are performed under pressure up to 95 MPa. ► Dew point pressure and compressibility factor for two gas condensates are obtained. ► Thermodynamic models that combined EOS with characterization methods are compared. ► Models to describe gas condensate samples under HPHT conditions are optimized.

•Fractal theory is introduced to describe hydrate phase equilibria in porous media.•The shape of pore is supposed to hold a fractal feature as von Koch curve.•Laplace equation is adopted by considering the surface effect of the pore edge shape.•The hydrate phase equilibria calculations for silica gel are superior to those of VDW-Platteeuw type models.A thermodynamic model based on the reaction-adsorption two-step formation mechanism was improved by introducing fractal theory to predict hydrate phase equilibria in porous media. The surface effect on phase equilibrium of porous media system was modified by considering the shape of the pore edge, which was supposed to hold a fractal feature as von Koch curve. A fractional dimension Laplace equation was established in describing the phase equilibrium conditions of methane, ethane, propane, and carbon dioxide hydrates in silica gel pores. The calculated results showed that when the shape of the pore edge is assumed as spherical, the calculations by the thermodynamics model developed are close or a little superior to those of traditional van der Waals-Platteeuw type models. When the surface effect of the pore edge shape is introduced, the absolute average deviations for silica gel systems can be further decreased.

•Volumetric properties of two reservoir fluid samples were measured with pressure up to 116 MPa.•Dew point pressures at four temperatures for condensate gas sample are obtained.•Correlations and thermodynamic model for describing gas compressibility factor under high pressure were compared.•The thermodynamic model recommended is most suitable for fluids produced from reservoirs with a wide pressure range.The volumetric properties of two reservoir fluid samples collected from one condensate gas well and one natural gas well were measured under four groups of temperatures, respectively, with pressure up to 116 MPa. For the two samples examined, the experimental results show that the gas compressibility factor increases with the increase of pressure. But the influence of the temperature is related to the range of the experimental pressure. It approximately decreases with the increase of temperature when the pressure is larger than (45 to 50) MPa, while there is the opposite trend when the pressure is lower than (45 to 50) MPa. The dew point pressure was also determined for the condensate gas sample, which decreases with the increase of temperature. The capabilities of four empirical correlations and a thermodynamic model based on equation of state for describing gas compressibility factor of reservoir fluids under high pressure were investigated. The comparison results show that the thermodynamic model recommended is the most suitable for fluids whatever produced from high-pressure reservoirs or conventional mild-pressure reservoirs.

The exploitation of methane hydrate formed under the same conditions is simulated experimentally to investigate the gas production behavior at different injected solutions (hot water, saline solution, and ethylene glycol) with continuous injection mode using a three-dimensional quiescent reactor. The influence of hot water temperature, injection rate, and injected solution type on the gas production and the energy efficiency are examined. The results show that the gas production increases with the increased hot water temperature, but the influence of temperature is weakened when it is beyond 313.0 K. An optimal injection rate also exists for the gas production. Hydrate dissociation with NaCl solution has the highest gas production compared with Na2SO4 and ethylene glycol when under the same conditions. The comparison of energy efficiency between the continuous injection mode and the interval injection mode shows that the former mode with double wells is more in favor of hydrate dissociation and gas production.Highlights► Hydrate exploitation is simulated with continuous injection mode using a 3-D reactor. ► An optimal water temperature and injection rate exists for gas production of hydrate. ► Compared with Na2SO4 and EG, injecting NaCl solution has the highest gas production. ► The continuous injection mode is more in favor of gas production of hydrate.

An experimental setup was developed to in situ measure the evolvement of the electrical resistivity during hydrate formation process to aid the interpretation of the influence of hydrate saturation on the electrical properties of the sediment. Five hydrate samples under different initial brine saturations of 12, 20, 30, 40, and 50% were formed from free methane gas and brine with 3.35 wt % NaCl in the 60–80 mesh sandy sediment, during which the variations of electrical resistivity were in situ measured. It was observed that the resistivity of the hydrate-bearing sediment increases with the increase of hydrate saturation during the hydrate formation process and finally achieves a constant value for each group of hydrate sample. For hydrate samples at different initial brine saturations, the values of electrical resistivity are larger, even though hydrate saturation is lower if free methane gas exists in sediment pores. Based on the measured electrical resistivity of hydrate-bearing sediment before and after brine injection at different hydrate saturations, parameters in Archie equation were determined to describe the relationship between resistivity and hydrate saturation, which would be helpful for mapping the hydrate concentration in the sediment through resistivity logging data from hydrate deposits.

Four reservoir samples under ultra-high-pressure and high-temperature conditions were collected from condensate gas fields in China. Constant-composition expansion tests were performed to determine the phase behavior and volumetric properties of reservoir fluid using an ultra-high-pressure fluid PVT test system. The compressibility factor and dew-point pressure were obtained at four temperatures for four samples. The range of pressure was from 22.03 to 118.89 MPa. For the samples studied, the experimental results showed that the dew-point pressure decreased with increasing temperature and the compressibility factors increased with increasing pressure but decreased with increasing temperature at a given high reduced pressure. A thermodynamic model based on an equation of state was developed to describe the volumetric properties and phase behavior of the condensate gas under ultra-high-pressure conditions. The calculated results are in good accordance with the experimental data, which is important for the development of condensate gas reservoirs in ultra-high-pressure environments.

The gas production from methane-hydrate-bearing sediment by injecting ethylene glycol (EG) solution was investigated using a three-dimensional experimental apparatus. Eight experimental runs were performed to examine the influence of operation conditions on hydrate dissociation by EG injection. The variations of pressure and temperature distribution in the reactor stimulated by the injected EG were obtained for the gas production process of the hydrate. The variation trend of temperature in the injection stage shows a shape of a “well” because of heat transfer and hydrate dissociation. The appearance sequence of temperature “well” and “well” depth is different for every port at different depths and radii. The effects of the concentration and quantity of EG and soaking time on the gas production ratio are examined. It shows that there exists an optimal value of the mass ratio of injected EG solution to initial water, where a maximum gas production ratio appears. When other conditions are similar, the gas amount produced by hydrate dissociation increases with the increase of the inhibitor concentration. The gas production efficiency increases with the decrease of the EG quantity and the increase of the EG concentration.

An experimental apparatus was developed to measure P-wave velocity (VP) of gas-hydrate-bearing sediment. Tetrahydrofuran (THF) was added to quicken the hydrate formation in the porous media and to synthesize hydrate-bearing sediments with uniform distribution. Methane acted as a free gas to participate in the hydrate formation. Five experimental runs were performed to examine the influence of sediment grain size and THF concentration on VP. The P-wave velocity and the amplitude for the first arrival wave signal were collected in real time during hydrate formation process. The experimental data showed that VP increases monotonically with the increase of hydrate saturation in the sediment pore space and finally tends to be a constant value. This final VP value increases with the increase of initial THF content, but the effect of sand grain size on VP is inconclusive. The variations of amplitude for the first arrival wave signal with elapsed time during hydrate formation illustrates that the amplitude increases with the increase of hydrate saturation until it attains a maximum value and then decreases gradually due to the effect of free methane gas penetrating into the hydrate-bearing sediment. The acoustic velocity of THF-hydrate filled sediment was also predicted based on the extended contact cement theory. The predicted results were close to the experimental data obtained in this work.

The equilibrium and dynamic interfacial tension of water/n-octane plus sorbitan monolaurate (the commercial name of Span 20) were measured using the pendant drop technique at four temperatures, (274.2, 278.2, 282.2, and 293.2) K. The concentration range of Span 20 is from (0.014 to 1.41) g·kg−1. The experimental results showed that Span 20 has excellent interface activity and a pronounced dynamic effect on the water/n-octane interface. The critical micelle concentration of Span 20 at different temperatures was determined, and it shifts toward lower value with the increase of temperature. The dynamic interfacial tension data show that, at the beginning of the adsorption process, it was only diffusion-controlled. At the near-equilibrium stage, there exists an adsorption barrier due to the strong molecular interaction of Span 20.

The solubility of five groups of natural gas in reservoir formation water was measured under (333.2 to 393.2) K and (15.0 to 43.6) MPa. The formation water and corresponding natural gas were sampled in situ from a China oilfield. The range of total dissolved solids (TDS) of the reservoir formation water samples is from (8744 to 80634) mg·kg−1. The mole fraction of methane in natural gas ranges from 0.7575 to 0.9449. The experimental results show that, with the increase of temperature and pressure, the solubility of natural gas in formation water goes through a minimum value under the experimental conditions. The solubility of natural gas in formation water was also influenced by a composition of gas phase and TDS of liquid phase. The salt effect becomes more significant in the high-pressure region.

The pressure dependence of interfacial tension between methane and octane at five temperatures has been determined using the pendant-drop method. The experimental results show that the interfacial tension value decreases with the increased temperature and pressure. The higher temperature and pressure has a positive contribution on weakening the intermolecular interaction between methane and octane. The surface excess concentration for methane on octane at different temperatures and pressures and the surface free energies of adsorption for (methane + octane) were calculated and compared with the (methane + water) system. The calculated results show that methane is more preferred for the adsorption on octane than on water.

A thermodynamics model is improved to predict the hydrate−water−gas equilibria in micropores based on the reaction−adsorption two-step formation mechanism by considering the effect of capillarity. The interfacial tension values between hydrate and water for different gas species are determined by the Gibbs−Thomson relationship or generalized as a linear function of temperatures. The hydrate phase equilibrium conditions for carbon dioxide, methane, ethane, propane, and (6.7% methane + 2.1% ethane + 91.2% propane) gas mixtures in porous media with different pore diameters predicted by the thermodynamics model developed in this work are equivalent to or superior to those of the traditional van der Waals−Platteeuw type models if the interfacial tension values are fixed. The absolute average deviations for all porous media systems can be decreased to 5.08% after a linear relation of interfacial tension and temperatures are introduced. It was found that the interfacial tension values between hydrate and water are in the magnitude order of propane, ethane, carbon dioxide, and methane, which is related to hydrate structure type and the occupancy of linked cavities for different gas species.

A three-dimensional middle-size reactor was used to simulate gas production from methane hydrate-bearing sand by hot-water cyclic injection. The gas production process and energy efficiency in the whole process, which was divided into injecting hot water, closing well, and producing gas (three steps), were investigated using 16 thermocouples distributed in hydrate-bearing sand samples. The experimental results indicates that the overall temperature trend increases with hot-water injection and decreases with gas production. The temperature distribution and fluctuation in the reactor depend upon the location of the injecting/producing well as well as the porosity and permeability of hydrate samples. Heat transfer is controlled by hot-water seepage flow during the injection of hot water. The affecting factors on the energy efficiency, such as hydrate saturation, hydrate sample temperature, hot-water temperature, mass of hot water injected, and well pressure, were examined. It was found that, when other conditions are similar, the energy efficiency ratio increases with the increase of the hydrate-bearing sand saturation and hydrate sample temperature but decreases with the increase of the hot-water temperature and well pressure.

The hydrate inhibition effect of three kinetic inhibitors (inhibex 301, 501, and 713) was assessed from (CH4 + C2H6 + C3H8) gas mixture + brine systems using a high pressure sapphire cell. The onset time of hydrate formation was determined by visual observation method and pressure drop profile method, respectively. The experimental results demonstrated that the onset time was able to be determined by the visual observation method all the time while the pressure drop profile method failed to detect the onset time clearly and correctly at lower temperatures. In some cases, the initial appearance of hydrate crystals cannot induce a clear break in the pressure–time relationship curve. The onset time measured by the visual observation method is usually shorter than or at least the same as that determined by the pressure drop profile method. The inhibiting effect on the growth of hydrate crystals can be shown by the difference of the onset time obtained by the two methods. The maximum tolerated subcooling of each inhibitor was also investigated based on the onset time. It was found that inhibex 301 behaves as the best inhibitor that can tolerate the maximum subcooling of 8.3 K at 0.5 wt% and 10.6 K at 1.0 wt%, respectively. The maximum subcooling for inhibex 501 is 6.8 K at 0.5 wt% and 6.6 K at 1.0 wt%, respectively. Inhibex 713 has relatively poor inhibiting effect among the three inhibitors with the maximum subcooling of less than 3.5 K at 0.5 wt% and 5.1 K at 1.0 wt%, respectively.

Three-phase equilibrium conditions of (CH4 + C2H6 + C3H8) mixtures (V) + hydrate (H) + aqueous solutions of sodium dodecyl sulfate (LW) are determined using a sapphire cell apparatus according to an isochoric isothermal dissociation method at four temperatures, 275.2 K, 277.0 K, 278.8 K, and 280.6 K. The propane mole fraction in each equilibrium state is from 0.45 % to 16.98 %. Sodium dodecyl sulfate was used as a kinetic promoter as it has been shown that it has no influence on the phase equilibrium. The experimental data are in accord with those calculated by the hydrate thermodynamic models.

In order to simulate the behavior of gas hydrate formation and decomposition, a 3-Dimension experimental device was built, consisting of a high-pressure reactor with an inner diameter of 300 mm, effective height of 100 mm, and operation pressure of 16 MPa. Eight thermal resistances were mounted in the porous media at different depthes and radiuses to detect the temperature distribution during the hydrate formation/decomposition. To collect the pressure, temperature, and flux of gas production data, the Monitor and Control Generated System (MCGS) was used. Using this device, the formation and decomposition behavior of methane hydrate in the 20∼40 mesh natural sand with salinity of 3.35 wt% was examined. It was found that the front of formation or decomposition of hydrate can be judged by the temperature distribution. The amount of hydrate formation can also be evaluated by the temperature change. During the hydrate decomposition process, the temperature curves indicated that the hydrate in the top and bottom of reactor dissociated earlier than in the inner. The hydrate decomposition front gradually moved from porous media surface to inner and kept a shape of column form, with different moving speed at different surface position. The proper decomposition pressure was also determined.

•Hydrate slurry presents obvious shear-thinning behaviour with increase of hydrate ratio.•Hydrate slurry is transported as a solid dispersion system with addition of AAs.•A Herschel-Bulkley-type equation was built by considering the hydrate volume fraction.•Shutting-down/restarting tests show that the hydrate slurry is easily and safely restarted.•Hydrate slurry exhibits obvious thixotropic behaviour with increasing shutting-down time.The flow characteristics and rheological properties of natural gas hydrate slurry, with initial water cuts ranging from 5 to 30 vol%, were investigated in a flow loop. The experimental results indicate that the hydrate slurry can be considered a pseudoplastic fluid and presents more obvious shear-thinning behaviour with the increase in the hydrate volume fraction. The study on the fluid morphology demonstrated that the original structure of the water-in-oil emulsion is destroyed by the formation of gas hydrate, and the hydrate slurry is ultimately transported as a solid dispersion system. An empirical Herschel–Bulkley-type equation that considers the hydrate volume fraction was developed to improve the description of the rheological properties of the hydrate slurry. The apparent viscosities of the hydrate slurry calculated by the new equation were in accordance with the experimental data. Shutting-down/restarting tests using three shutting-down times (2 h, 4 h, and 8 h) were performed. The experimental results indicate that the hydrate slurry can be easily and safely restarted from the static state after a long shutting-down period and exhibits obvious thixotropic behaviour with increasing shutting-down time.

•A one-dimensional visual hydrate simulator was built to study NGH formation.•Hydrate grows from film formation, thickening, water adsorption intermittently.•A colloidal solution formed from hydrate and salt water.•Massive hydrate deposits in coarse sand and silt but not in clay marine sediment.•Perturbation from flow or temperature can accelerate hydrate formation process.To study natural gas hydrate formation in sediments, a one-dimensional visual hydrate simulator with three visual sapphire tubes and twelve electrode tips was designed and constructed. Using this device, the hydrate formation process was experimentally studied in coarse sands, silts, and natural clay marine sediments from free methane gas. During experiments, each of which lasting up to three weeks, a hydrate film formed initially at the gas-water interface and spread out to form a film network in sediments. The hydrate film grew in thickness by adsorbing water, where film thickening and water adsorption occur intermittently. The salting-out effect and the osmosis effect play a critical role in the hydrate film thickening-water adsorption process, and the perturbation arising from the fluctuation such as fluid flow and temperature may accelerate the hydrate formation. A colloidal solution formed from hydrate and salt water was also observed, which may be a new form of existence for marine natural gas hydrate. Electrical resistivity of the sediments initially decreased as hydrate formation increased the remaining water salinity, and then increased when additional hydrate formation restricted the water distribution in the sediment. Among three studied sediments, massive hydrate deposits were observed in coarse sand and silt but not in clay marine sediment, while fractures appeared when massive hydrates formed in sediments.

•Kinetic experiments were performed to optimize the CP hydrate desalination conditions.•Excess CP addition into brine and CP dispersion increase the yield of dissociated water.•Controlled encapsulation/entrapping of residual brine into hydrate phase was attained.•An inverse correlation between removal efficiency and yield of dissociated water exists.To make application of hydrate based desalination technology practical, the kinetic behaviors and separation capability were examined for cyclopentane hydrates formed from brine in cyclopentane dispersion systems at 274.1 K/277.1 K with initial salinity of 3 to 5 wt% for water fractions from 20.0 to 90.0 vol% and rpms from 300 to 500. The excess cyclopentane addition into brine and cyclopentane dispersion significantly increased the yield of dissociated water when the removal efficiency stayed around 80%. This implies enhanced kinetics and controlled encapsulation of residual brine into the hydrate phase for brine in cyclopentane dispersion systems. More hydrates formed under lower temperature or initial salinity. Higher rpm could further promote hydrate formation but would adversely affect the removal efficiency. Desalination efficiency was enhanced with washing during vacuum filtration and effects of ratio of washing water/dissociated water on the removal efficiency were considerable. An inverse correlation exists between the removal efficiency and the yield of dissociated water under the same conditions. Lower temperature (274.1 K) and 60% water cut should be optimal for higher yield of dissociated water from brine in cyclopentane dispersion. The ratio of washing water/dissociated water and rpm conditions should be adjusted depending on the situation.