Heat flow pattern,base of methane hydrates stability zones and BSRs in Shenhu Area,northern South China Sea

Using the collected 433 heat flow values, we estimated the bases of methane hydrate stability zone （BHSZ）, in northern South China Sea （NSCS）. Through comparing BHSZs with the depths of bottom simulating reflectors （BSRs）, in Shenhu Area （SA）, we found that there are big differences between them. In the north of SA, where the water depth is shallow, many slumps developed and the sedimentation rate is high, it appears great negative difference （as large as -192%）. However, to the southeast of SA, where the water depth is deeper, sedimentation rate is relatively low and uplift basement topography exists, it changes to positive difference （as large as ＋45%）. The differences change so great, which haven＇t been observed in other places of the world. After considering the errors from the process of heat flow measurement, the BSR depth, the relationship of thermal conductivity with the sediments depth, and the fluid flow activities, we conclude that the difference should be not caused by these errors. Such big disagreement may be due to the misunderstanding of BSR. The deviant ＂BSRs＂ could represent the paleo-BSRs or just gas-bearing sediment layers, such as unconformities or the specific strata where have different permeability, which are not hydraterelated BSRs.

Investigation of Miocene Methane Hydrate Generation Potential in the Transylvanian Basin, Romania

In geology we often revise theoretical models;upon finding new evidence,such as the discovery of methane hydrates,the initial model will be challenged immediately.Hereby the authors put forward two postulates:1)There is a third,previously unexplored source of methane in the Transylvanian Basin,based on a new theoretical approach on methane hydrate formation;2)The dissociation of methane hydrates creates a strong chlorinity anomaly.Based on a recent analogy with the Black Sea basin model,we apply our statements to the Transylvanian Basin.Using direct and indirect indicators and the published system tract analysis,we claim that there are substantial grounds to believe that this model of methane hydrate formation applies to the Miocene Transylvanian Basin.Due to the increase of the geothermal gradient as a result of the volcanic activity from the Eastern Carpathians,the clathrates dissociated into methane and freshwater.This process of dilution resulted in a chlorinity anomaly that can be spotted in the formation waters of several gas fields from the Transylvanian Basin.

Methane hydrate formation and dissociation in synthetic seawater

The formation and dissociation of methane gas hydrate at an interface between synthetic seawater （SSW） and methane gas have been experimentally investigated in the present work. The amount of gas consumed during hydrate formation has been calculated using the real gas equation. Induction time for the formation of hydrate is found to depend on the degree of subcooling. All the experiments were conducted in quiescent system with initial cell pressure of 11.14 MPa. Salinity effects on the onset pressure and temperature of hydrate formation are also observed. The dissociation enthalpies of methane hydrate in synthetic seawater were determined by Clausius-Clapeyron equation based on the measured phase equilibrium data. The dissociation data have been analyzed by existing models and compared with the reported data.

The influence of temperature,pressure,salinity and capillary force on the formation of methane hydrate

We present here a thermodynamic model for predicting multi-phase equilibrium of methane hydrate liquid and vapor phases under conditions of different temperature, pressure, salinity and pore sizes. The model is based on the 1959 van der Waals--Platteeuw model, angle-dependent ab initio intermolecular potentials, the DMW-92 equation of state and Pitzer theory. Comparison with all available experimental data shows that this model can accurately predict the effects of temperature, pressure, salinity and capillary radius on the formation and dissociation of methane hydrate. Online calculations of the p-T conditions for the formation of methane hydrate at given salinities and pore sizes of sediments are available on： www.geochem-model.org/models.htm.

Investigation on Gas Storage in Methane Hydrate

The effect of additives (anionic surfactant sodium dodecyl sulfate (SDS), nonionic surfactant alkyl polysaccharide glycoside (APG), and liquid hydrocarbon cyclopentane (CP)) on hydrate induction time and formation rate, and storage capacity was studied in this work. Micelle surfactant solutions were found to reduce hydrate induction time, increase methane hydrate formation rate and improve methane storage capacity in hydrates. In the presence of surfactant, hydrate could form quickly in a quiescent system and the energy costs of hydrate formation were reduced. The critical micelle concentrations of SDS and APG water solutions were found to be 300×10-6 and 500×10-6 for methane hydrate formation system respectively. The effect of anionic surfactant (SDS) on methane storage in hydrates is more pronounced compared to a nonionic surfactant (APG). CP also reduced hydrate induction time and improved hydrate formation rate, but could not improve methane storage in hydrates.

Water transfer characteristics during methane hydrate formation and dissociation processes inside saturated sand

Gas hydrates formation and dissociation processes inside porous media are always accompanied by water transfer behavior, which is similar to the water behavior of ice freezing and thawing processes. These processes have been studied by many researchers, but all the studies are so far on the water transfer characteristics outside porous media and the water transfer characteristics inside porous media have been little known. In this study, in order to study the water transfer characteristics inside porous media during methane hydrate formation and dissociation processes, a novel apparatus with three pF-meter sensors which can detect water content changes inside porous media was applied. It was experimentally observed that methane hydrate formation processes were accompanied by water transfer from bottom to top inside porous media, however, the water behavior during hydrate dissociation processes was abnormal, for which more studies are needed to find out the real reason in our future work.

Estimation of ultra-stability of methane hydrate at 1 atm by thermal conductivity measurement

Thermal conductivity of methane hydrate was measured in hydrate dissociation self-preservation zone by means of the transient plane source（TPS） technique developed by Gustafsson.The sample was formed from 99.9%（volume ratio） methane gas with 280 ppm sodium dodecyl sulfate（SDS） solution under 6.6 MPa and 273.15 K.The methane hydrate sample was taken out of the cell and moved into a low temperature chamber when the conversion ratio of water was more than 90%.In order to measure the thermal conductivity,the sample was compacted into two columnar parts by compact tool at 268.15 K.The measurements are carried out in the temperature ranging from 263.15 K to 271.15 K at atmospheric pressure.Additionally,the relationship between thermal conductivity and time is also investigated at 263.15 K and 268.15 K,respectively.In 24 h,thermal conductivity increases only 5.45% at 268.15 K,but thermal conductivity increases 196.29% at 263.15 K.Methane hydrates exhibit only minimal decomposition at 1 atm and the temperature ranging from 263.15 K to 271.15 K.At 1 atm and 268.15 K,the total gas that evolved after 24 h was amounted to less than 0.71% of the originally stored gas,and this ultra-stability was maintained if the test was lasted for more than two hundreds hours before terminating.

P-T stability conditions of methane hydrate in sediment from South China Sea

For reasonable assessment and safe exploitation of marine gas hydrate resource, it is important to determine the stability conditions of gas hydrates in marine sediment. In this paper, the seafloor water sample and sediment sample （saturated with pore water） from Shenhu Area of South China Sea were used to synthesize methane hydrates, and the stability conditions of methane hydrates were investigated by multi-step heating dissociation method. Preliminary experimental results show that the dissociation temperature of methane hydrate both in seafloor water and marine sediment, under any given pressure, is depressed by approximately -1.4 K relative to the pure water system. This phenomenon indicates that hydrate stability in marine sediment is mainly affected by pore water ions.

Structural stability of methane hydrate at high pressures

The structural stability of methane hydrate under pressure at room temperature was examined by both in-situ single-crystal and powder X-ray diffraction techniques on samples with structure types I, II, and H in diamond-anvil ceils. The diffraction data for types II （slI） and H （sH） were refined to the known structures with space groups Fd3m and P63/mmc, respectively. Upon compression, sl methane hydrate transforms to the sll phase at 120 MPa, and then to the sH phase at 600 MPa. The slI methane hydrate was found to coexist locally with sI phase up to 500 MPa and with sH phase up to 600 MPa. The pure sH structure was found to be stable between 600 and 900 MPa. Methane hydrate decomposes at pressures above 3 GPa to form methane with the orientationally disordered Fm3m structure and ice VII （Pn3m）. The results highlight the role of guest （CH4）-host （H2O） interactions in the stabilization of the hydrate structures under pressure.

Experimental study on characteristics of pore water conversion during methane hydrates formation in unsaturated sand

Understanding the pore water conversion characteristics during hydrate formation in porous media is important to study the accumulation mechanism of marine gas hydrate.In this study,low-field NMR was used to study the pore water conversion characteristics during methane hydrate formation in unsaturated sand samples.Results show that the signal intensity of T_(2) distribution isn’t affected by sediment type and pore pressure,but is affected by temperature.The increase in the pressure of hydrogen-containing gas can cause the increase in the signal intensity of T_(2) distribution.The heterogeneity of pore structure is aggravated due to the hydrate formation in porous media.The water conversion rate fluctuates during the hydrate formation.The sand size affects the water conversion ratio and rate by affecting the specific surface of sand in unsaturated porous media.For the fine sand sample,the large specific surface causes a large gas-water contact area resulting in a higher water conversion rate,but causes a large water-sand contact area resulting in a low water conversion ratio(C_(w)=96.2%).The clay can reduce the water conversion rate and ratio,especially montmorillonite(C_(w)=95.8%).The crystal layer of montmorillonite affects the pore water conversion characteristics by hindering the conversion of interlayer water.

Thermal Transport in Methane Hydrate by Molecular Dynamics and Phonon Inelastic Scattering

The heat conduction and thermal conductivity for methane hydrate are simulated from equilibrium molecular dynamics. The thermal conductivity and temperature dependence trend agree well with the experimental results. The nonmonotonic temperature dependence is attributed to the phonon inelastic scattering at higher temperature and to the confinement of the optic phonon modes and low frequency phonons at low temperature. The thermal conductivity scales proportionally with the van der Waals interaction strength, The conversion of a crystal-like nature into an amorphous one oecurs at higher strength. Both the temperature dependence and interaction strength dependence are explained by phonon inelastic scattering.

Experimental and modeling study of the kinetics of methane hydrate formation and dissociation

In this work,several experiments were conducted at isobaric and isothermal condition in a CSTR reactor to study the kinetics of methane hydrate formation and dissociation.Experiments were performed at five temperatures and three pressure levels(corresponding to equilibrium pressure).Methane hydrate formation and dissociation rates were modeled using mass transfer limited kinetic models and mass transfer coefficients for both formation and dissociation were calculated.Comparison of results,shows that mass transfer coefficients for methane hydrate dissociation are one order greater than formation conditions.Mass transfer coefficients were correlated by polynomials as relations of pressure and temperature.The results and the method can be applied for prediction of methane production from naturally occurring methane hydrate deposits.

In situ High-pressure NMR Observations on the Formation and Dissociation of Methane Hydrate

A home-made static NMR cell with pressure up to 10 MPa was employed to observe the formation and dissociation processes of methane hydrate by in situ ^1H and ^13C NMR spectroscopies. Methane hydrate can be formed or decomposed in the temperature range of -5 to -13℃ at pressures between 4.0 and 7.0 MPa. The higher methane pressure, the formation or dissociation temperature of methane hydrate was higher. In situ ^1H NMR experiments indicated that the critical size of the hydrate clusters is crucial for the formation of methane hydrate.

Molecular dynamics simulation of decomposition and thermal conductivity of methane hydrate in porous media

The hydrate has characteristics of low thermal conductivity and temperature sensitivity. To further analysis the mechanism of thermal conductivity and provide method for the exploitation, transportation and utilization of hydrate, the effect of decomposition and thermal conductivity of methane hydrate in porous media has been studied by using the molecular dynamics simulation. In this study, the simulation is carried out under the condition of temperature 253.15 K-273.15 K and pressure 1 MPa. The results show that the thermal conductivity of methane hydrate increases with the increase of temperature and has a faster growth near freezing. With the addition of porous media, the thermal conductivity of the methane hydrate improves significantly. The methane hydrate-porous media system also has the characteristics of vitreous body.With the decrease of the pore size of the porous media, thermal conductivity of the system increases gradually at the same temperature. It can be ascertained that the porous media of different pore sizes have strengthened the role of the thermal conductivity of hydrates.

Formation mechanism,experimental method,and property characterization of graindisplacing methane hydrates in marine sediment:A review

Grain-displacing hydrate deposits exist at many marine sites,which constitute an important part of methane hydrate resources worldwide.Attributed to the difficulties in acquiring field data and synthesizing experimental samples,the formation and property characterization of grain-displacing hydrate remains less understood and characterized than the pore-filling hydrate in current literature.This study reviews the formation mechanisms of grain-displacing hydrate from the perspective of geological accumulation and microscale sedimentary property.The experimental methods of synthesizing grain-displacing hydrate in the laboratory and the current knowledge on the property of grain-displacing hydrate sediment are also introduced.Shortcomings in current theories and suggestions for future study are proposed.The work is hoped to provide valuable insights for the research into the hydrate accumulation,geophysics,and hydrate exploitation targeted at the grain-displacing hydrate in the marine sediments.

Methane hydrate formation comparisons in media with and without a water source supply

Water is a necessary element during gas hydrate formations. Therefore, by analyzing water depletion changes in media, the reaction characteristics of methane hydrate in media can be studied. In this study, two water sources supplying some liquid water which may be consumed by the methane hydrate formation reactions were designed and assembled. Using them, the full formation processes of methane hydrate was studied. Experimental results show the following： If heat released from nucleation reaction of methane hydrate is diffused rapidly, the nucleation ratios will be enhanced discernibly. While the hydrate is formed, a force is generated that sucks fresh water from the source into the vicinity of the hydrate, slowing down the cementation process and causing some hydrate grain dissociation. As a result of cementation differences, the hydrate reaction processes with different water sources present linear or quadratic equation characteristics. After a few repeated dissociation and formation processes of some hydrate grains caused by the fresh water, the gas amounts contained in hydrate will be significantly enhanced.

Pore capillary pressure and saturation of methane hydrate bearing sediments

To better understand the relationship between the pore capillary pressure and hydrate saturation in sediments, a new method was proposed. First, the phase equilibria of methane hydrate in fine-grained silica sands were measured. As to the equilibrium data, the pore capillary pressure and saturation of methane hydrate were calculated. The results showed that the phase equilibria of methane hydrates in fine-grained silica sands changed due to the depressed activity of pore water caused by the surface group and negatively charged characteristic of silica particles as well as the capillary pressure in small pores together. The capillary pressure increased with the increase of methane hydrate saturation due to the decrease of the available pore space. However, the capillary-saturation relationship could not yet be described quantitatively because of the stochastic habit of hydrate growth.