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.

Methane hydrate formation and dissociation behaviors in montmorillonite

The methane hydrate formation and the methane hydrate dissociation behaviors in montmorillonite are experimentally studied. Through the analyses of the microstructure characteristic, the study obtains the porous characteristic of montmorillonite. It is indicated that methane hydrate in montmorillonite forms the structure I (si) crystal. Meanwhile, molecular dynamics simulation is carried out to study the processes of the methane hydrate formation and the methane hydrate dissociation in montmorillonite. The microstructure and microscopic properties are analyzed. The methane hydrate formation and methane hydrate dissociation mechanisms in the montmorillonite nanopore and on the montmorillonite surface are expounded. Combining the experimental and simulating analyses, the results indicate the methane hydrate formation and methane hydrate dissociation processes have little influence upon the crystal structure of porous media from either micro- or macro-analysis. It is beneficial to the fundamental researches on the exploitation and security control technologies of natural gas hydrate in deep-sea sediments.

Experimental studies of the formation and dissociation of methane hydrate in loess

In order to study the nature of gas hydrate in porous media,the formation and dissociation processes of methane hydrate in loess were investigated.Five cooling rates were applied to form methane hydrate.The nucleation times of methane hydrate formation at each cooling rate were measured for comparison.The experimental results show that cooling rate is a significant factor affecting the nucleation of methane hydrate and gas conversion.Under the same initial conditions,the faster the cooling rate,the shorter the nucleation time,and the lower the methane gas conversion.Five dissociating temperatures were applied to conduct the dissociation experiment of methane hydrate formed in loess.The experimental results indicated that the temperature evidently controlled the dissociation of methane hydrate in loess and the higher the dissociating temperature,the faster the dissociating rates of methane hydrate.

Investigations on methane hydrate formation,dissociation,and viscosity in gas-water-sand system

Understanding the kinetics and viscosity of hydrate slurry in gas-water-sand system is of great significance for the high-efficiency and high-safety development of natural gas hydrates.The effect of micronsized sands with various concentrations and particle sizes on the hydrate formation,dissociation,and viscosity in gas-water-sand system are investigated in this work.The experimental results show that the hydrate induction time in the sandy system is slightly prolonged compared to the pure gas-water system,and the inhibition effect first strengthens and then weakens as the sand concentration increases from0 wt%to 5 wt%.Besides,the difference of hydrate formation amount in various cases is not obvious.The concentration and particle size of sand have little effect on the kinetics of hydrate formation.Both promoting and inhibiting effects on hydrate formation have been found in the sandy multiphase fluid.For the viscosity characteristics,there are three variations of hydrate slurry viscosity during the formation process:Steep drop type,S-type and Fluctuation type.Moreover,appropriate sand size is helpful to reduce the randomness of slurry viscosity change.Meanwhile,even at the same hydrate volume fraction,the slurry viscosity in the formation process is significantly higher than that in dissociation process,which needs further research.This work provides further insights of hydrate formation,dissociation,and viscosity in gas-water-sand system,which is of great significance for safe and economic development of natural gas 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.

Comparison of the water change characteristics between the formation and dissociation of methane hydrate and the freezing and thawing of ice in sand

Hydrate formation and dissociation processes are always accompanied by water migration in porous media, which is similar to the ice. In our study, a novel pF-meter sensor which could detect the changes of water content inside sand was first applied to hydrate formation and dissociation processes. It also can study the water change characteristics in the core scale of a partially saturated silica sand sample and compare the differences of water changes between the processes of formation and dissociation of methane hydrate and freezing and thawing of ice. The experimental results showed that the water changes in the processes of formation and dissociation of methane hydrate were basically similar to that of the freezing and thawing of ice in sand. When methane hydrate or ice was formed, water changes showed the decrease in water content on the whole and the pF values rose following the formation processes. However, there were very obvious differences between the ice thawing and hydrate dissociation.

Water transfer characteristics in the vertical direction during methane hydrate formation and dissociation processes inside non-saturated media

In order to study water transfer characteristics inside non-saturated media during methane hydrate formation and dissociation processes,water changes on the top,middle and bottom locations of experimental media during the reaction processes were continuously followed with a novel apparatus with three pF-meter sensors.Coarse sand,fine sand and loess were chosen as experimental media.It was experimentally observed that methane hydrate was easier formed inside coarse sand and fine sand than inside loess.Methane hydrate formation configuration and water transfer characteristics during methane hydrate formation processes were very different among the different non-saturated media,which were important for understanding methane hydrate formation and dissociation mechanism inside sediments in nature.

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.

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.

Nuclear magnetic resonance study of the formation and dissociation process of nature gas hydrate in sandstone

In this work,the authors monitored the formation and dissociation process of methane hydrate in four different rock core samples through nuclear magnetic resonance(NMR)relaxation time(T_(2))and 2D imaging measurement.The result shows that the intensity of T_(2) spectra and magnetic resonance imaging(MRI)signals gradually decreases in the hydrate formation process,and at the same time,the T_(2) spectra move toward the left domain as the growth of hydrate in the pores of the sample accelerates the decay rate.The hydrate grows and dissociates preferentially in the purer sandstone samples with larger pore size and higher porosity.Significantly,for the sample with lower porosity and higher argillaceous content,the intensity of the T_(2) spectra also shows a trend of a great decrease in the hydrate formation process,which means that high-saturation gas hydrate can also be formed in the sample with higher argillaceous content.The changes in MRI of the sample in the process show that the formation and dissociation of methane hydrate can reshape the distribution of water in the pores.

Mechanical properties of gas hydrate-bearing sediments during hydrate dissociation

The changes in the mechanical properties of gas hydrate-bearing sediments(GHBS) induced by gas hydrate(GH) dissociation are essential to the evaluation of GH exploration and stratum instabilities. Previous studies present substantial mechanical data and constitutive models for GHBS at a given GH saturation under the non-dissociated condition. In this paper, GHBS was formed by the gas saturated method, GH was dissociated by depressurization until the GH saturation reached different dissociation degrees. The stress–strain curves were measured using triaxial tests at a same pore gas pressure and different confining pressures. The results show that the shear strength decreases progressively by 30%–90% of the initial value with GH dissociation, and the modulus decreases by 50% –75%. Simplified relationships for the modulus, cohesion, and internal friction angle with GH dissociated saturation were presented.

Study of effective parameters for enhancement of methane gas production from natural gas hydrate reservoirs

Natural gas hydrate resources have become the major source of energy in the second half of 21st century.Gas production and fluid behavior in natural gas hydrate reservoirs are different from conventional ones.There are three major methods for methane decomposition such as depressurization,thermal stimulation and inhibitor injection.However,CO2 substitution can also be introduced as an alternative method to inject in sediments containing gas hydrate.All these methods tend to imbalance equilibrium condition via temperature and pressure variation in order to fulfill hydrate decomposition process.This study aims to simulate depressurization method for gas production from a hydrate gas bearing layer.Hence,a sensitivity analysis of reservoir parameters includes porosity,permeability,hydrate saturation,hydrate thickness layer;pressure and temperature of single well hydrate model were investigated to determine how these parameters impact on gas production.Results show that depressurization is an efficient method for gas production from hydrate bearing sediments.Through sensitivity analysis,it has been concluded that if properties of a hydrate layer such as porosity and permeability become greater,methane production will be increased significantly.Moreover,results investigate that the rate of hydrate dissociate is strongly dependent on pressure reduction,and it has a reverse relationship with bottomhole pressure and reservoir temperature.

A comprehensive analysis of formation conditions,intrinsic properties,and mechanical responses of gas hydrate-bearing sediments

Natural gas hydrates(NGH)stored in submarine deposits are a promising energy resource,Yet,the deterioration in sediment strength can trigger geological disasters due to drilling-induced hydrate dissociation.Hence,an in-depth investigation on geo physical-mechanical performance of gas hydrate-bearing sediments(GHBS)is crucial for recovery hydrates safely and efficiently.This paper provides a comprehensive assessment of the research progress on formation conditions,intrinsic properties,and mechanical responses of GHBS.The key findings have been presented:gas composition,inhibitors and promoters alter hydrate formation by modifying the thermodynamic equilibrium of temperature and pressure.Also,we identified the key determinants of porosity of GHBS and revealed the correlation between permeability,hydrate saturation,and hydrate morphology.Moreover,we highlighted the differences in mechanical behavior between hydrate-free sediments and GHBS along with their underlying mechanisms.Furthermore,we examined the methods for GHBS preparation as well as the employed test apparatuses,providing critical insights into the limitations and recommendations.By synthe-sizing data from existing literature,we conducted a comprehensive analysis of the dependence of mechanical parameters of GHBS on factors such as hydrate saturation,effective confining stress,and temperature,and dis-cussed the mechanical responses subjected to various hydrate dissociation methods.Finally,we offer a perspective for future research to focus on the micro-scale aspects,heterogeneous distribution,and long-term stability of GHBS.The discerned patterns and mechanical mechanisms are expected to guide the improvement of predictive model for geo physical-mechanical behavior of GHBS and establish a reference for developing effective strategies for recovery hydrates.

水泥水化影响水合物层稳定性定量评价装置及影响程度的评测

针对海洋深水水合物层在固井过程水泥水化放热对水合物分解量的影响缺乏量化评价装置及方法的问题,充分考虑固井过程中水泥浆体系与水合物层的接触方式,建立了一套模拟水合物层固井的水合物稳定性评价实验装置,该装置实现了低温高压下水合物的生成、带压条件下水泥浆与水合物层接触的流动泵入,直观地测试了与水合物层直接接触下的水泥浆水化放热对地层温度、压力的影响。根据设计的实验装置,通过对水合物饱和度、水合物分解气量的推导计算,建立了一套水合物模拟地层的制作方法,并建立了水泥浆水化影响水合物层稳定性评价方法。根据南海地区浅层地质条件建立了模拟水合物地层,泵入G级油井水泥净浆、低密水泥浆体系和低热水泥浆体系等3组水泥浆体系,得出单位体积油井水泥候凝过程中分解水合物的气体的量分别为0.7356、0.1091和0.0649 mol/L,且评测表明,低热水泥浆体系能够大幅度缩短固井候凝的等待时间。该研究为油气固井过程中对浅层水合物的影响提供了直观测试方法,也证明了海洋深水水合物地层中使用低热水泥浆体系的必要性。

多孔介质中甲烷水合物形成与分解实验研究

基于实际海洋水合物资源的赋存状态及温度、压力条件,在人工多孔介质中物理模拟海底水合物稳定带的水合物藏进行了水合物的形成与分解的实验研究。分析甲烷水合物在多孔介质中的形成与降压开采过程,揭示了其温度、压力和产气速率的变化规律。采用逐步降压的方法测定了多孔介质中水合物在特定温度下最小分解推动力,比较了不同降压模式下的累计产气量。结果表明,水合物形成过程中通过不断注水保持系统压力,甲烷可完全生成水合物,最终水合物藏中仅有水和水合物两相;实验条件下水合物的分解主要受压差影响,压差越大,分解速率越大,累计产气量越高;在一定温度下水合物的分解需有一个最小推动力。比较不同降压模式发现,累计产气量只与压差有关,而与降压模式无关。

5A分子筛粉末对四氢呋喃水合物的生成及分解过程的影响

在低于0℃和常压下,将粉碎并筛分后的成型5A分子筛粉末加入四氢呋喃-水(二者质量比为19∶81)体系中,用显微镜观察5A分子筛粉末的存在对四氢呋喃水合物生成和分解过程的影响.结果表明,5A分子筛粉末能够促进四氢呋喃水合物的生成.5A分子筛粉末存在下,四氢呋喃水合物生成方式主要表现为两种,脉状生成和块状生成;同时5A分子筛粉末能够提高四氢呋喃水合物结晶所需要的温度,降低四氢呋喃水合物的分解温度;而且5A分子筛粉末粒径的大小及分布对四氢呋喃水合物生成及分解的特性也有很大影响.

气体水合物分解与生成技术应用研究进展

水合物技术应用可归纳为分解应用和生成应用,本文就这两大应用方向对水合物进行了分类综述。从水合物分解角度,阐述了天然气水合物资源勘探开发、管道水合物解堵、水合物抑制防控等技术应用的研究进展;从水合物分解的逆过程(生成)角度,阐述了水合物储气、二氧化碳捕获与封存、海水淡化、溶液提浓、污水处理、混合气体分离、蓄冷等应用技术。同时论文结合气体水合物发展历程,概括了气体水合物技术在诸多领域的应用,指出了水合物技术发展取得的诸多成果,也提出了新形势下水合物发展所面临的问题,希望能为今后水合物技术的发展带来一定指导。

钻井液侵入含天然气水合物地层的机理与特征分析

海洋含天然气水合物地层是具有渗透性的多孔介质体,钻进过程中钻井液不可避免地会与它发生能量和物质交换,从而影响测井响应、井壁稳定和储层评价。在过压钻井条件下,水基钻井液驱替侵入含天然水合物地层和温差下热传导导致的天然气水合物分解是耦合在一起的,其侵入可描述为一个包含相变的非等温非稳态渗流扩散过程。分析了天然气水合物在多孔介质中的分解特性,指出了多孔介质中天然气水合物分解的影响因素,通过钻井液侵入含天然气水合物地层与侵入常规油气地层的比较,提出了借鉴天然气水合物开采渗流模型建立钻井液侵入数值模型的思路。

电阻率测试技术在沉积物-盐水-甲烷水合物体系中的应用

利用自制的一维天然气水合物(NGH)开采模拟实验系统,通过对NGH生成过程中电阻率、温度、压力等的变化规律进行分析,验证了在海洋沉积物体系中用电阻率法测试NGH的可行性。实验结果表明,电阻率可以清楚地反映NGH的成核及生长过程。NGH开始成核时消耗了溶液中的水分和甲烷气体,增加了离子之间的连通性,使电阻率降低;晶核开始生长为NGH后,由于生成的NGH类似于绝缘体,并且使得孔隙连通性变差,电阻率逐步升高,NGH大量生成后电阻率保持稳定。然后进行NGH降压分解实验,随着NGH的分解,NGH饱和度下降,电阻率也逐渐下降。

基于电化学阻抗谱测试方法研究四氢呋喃水合物的生成和分解过程

含水合物沉积物的电学性质极为复杂且影响因素众多,目前所采用的电阻率参数尚不能充分刻画含水合物沉积物的电学特性。本文提出应用电化学阻抗谱法对四氢呋喃水合物的生成和分解过程进行测试,采用等效电路模型拟合阻抗谱,通过分析等效电路元件参数的变化规律研究水合物生成和分解过程的阻抗谱特征。研究发现:1四氢呋喃水溶液测试体系的电化学阻抗谱可用传荷与扩散共同控制的电极过程的等效电路进行拟合;2在降温、水合物生成前和生成后的过程中,电荷传递电阻和韦伯阻抗能够显著地反映以上三个过程的转换;3升温过程中各电路元件参数总体变化趋势与降温过程相反。研究表明电化学阻抗谱测试法是研究水合物的电学性质及其生成和分解过程动力学机理的一种有效方法,等效电路元件参数能够从不同的角度揭示水合物生成和分解过程的内在信息。