含盐蒙脱石中甲烷水合物的生成/分解特性及离子分布研究

目前,天然气水合物成藏和开采是新能源开发应用的热点,但海域沉积物中天然气水合物的形成/分解特性,及盐离子对水合物稳定性的影响等关键问题亟待解决。利用天然气水合物原位取样技术对甲烷水合物在含盐多孔介质中生成和分解过程进行原位扫描电镜(Scanning Electron Microscopy, SEM)测试和能谱分析(Energy Dispersive Spectrometry, EDS),系统研究了含0.5 mol/L NaCl溶液的蒙脱石中甲烷水合物生成、分解过程中形貌的变化及离子分布特征。研究发现水合物生成和分解过程元素分布发生明显改变,水合物生成的排盐效应使得NaCl在水合物颗粒与颗粒交结处以水合盐离子的形成存在,并且Na+和Cl-在蒙脱石表面分层排布。水合物生成后蒙脱石表面呈独立颗粒状,水合物分解后蒙脱石表面凹陷并形成微小的气体通道,并且水合物分解后蒙脱石的骨架堆积结构发生改变。研究得出水合物成核、生长、分解过程均在特定基元颗粒间是独立进行的,并且生长与分解过程与水合物晶胞结构有关。     

多孔载体海绵对CH4-N2-O2瓦斯气体水合分离影响（英文）

The findings were presented from laboratory investigations on the hydrate formation and dissociation processes employed to recover methane from coal mine gas.The separation process of coal mine methane(CMM) was carried out at 273.15K under 4.00 MPa.The key process variables of gas formation rate,gas volume stored in hydrate and separation concentration were closely investigated in twelve THF-SDS-sponge-gas systems to verify the sponge effect in these hydrate-based separation processes.The gas volume stored in hydrate is calculated based on the measured gas pressure.The CH4 mole fraction in hydrate phase is measured by gas chromatography to confirm the separation efficiency.Through close examination of the overall results,it was clearly verified that sponges with volumes of 40,60 and 80 cm 3 significantly increase gas hydrate formation rate and the gas volume stored in hydrate,and have little effect on the CH4 mole fraction in hydrate phase.The present study provides references for the application of the kinetic effect of porous sponge media in hydrate-based technology.This will contribute to CMM utilization and to benefit for local and global environment.     

Investigation of the methane hydrate surface area during depressurization-induced dissociation in hydrate-bearing porous media

The surface area of hydrate during dissociation in porous media is essentially important for the kinetics of hydrate dissociation. In this study, the methane hydrate surface area was investigated by the comparison results of experiments and numerical simulations during hydrate decomposition in porous media. The experiments of methane hydrate depressurization-induced dissociation were performed in a 1D high pressure cell filled with glass beads, an improved and valid 1D core-scale numerical model was devel-oped to simulate gas production. Two conceptual models for hydrate dissociation surface area were pro-posed based on the morphology of hydrate in porous media, which formed the functional form of the hydrate dissociation surface area with porosity, hydrate saturation and the average radius of sand sedi-ment particles. With the establishment of numerical model for depressurization-induced hydrate disso-ciation in porous media, the cumulative gas productions were modeling and compared with the experimental data at the different hydrate saturations. The results indicated that the proposed prediction equations are valid for the hydrate dissociation surface area, and the grain-coating surface area model performs well at lower hydrate saturation for hydrate dissociation simulation, whereas at higher hydrate saturation, the hydrate dissociation simulation from the pore-filling surface area model is more reason-able. Finally, the sensitivity analysis showed that the hydrate dissociation surface area has a significant impact on the cumulative gas production.     

煤层气水合化的基础研究

自行设计制造了一套可用于煤层气水合物生成与分解的可视化实验系统,利用该实验系统研究了阴离子表面活性剂、非离子表面活性剂和多孔介质煤对煤层气水合物生成的影响,进行了煤层气水合物生成相平衡参数和分解热力学方面的研究。结果表明:表面活性剂的加入促进了水合物的生长,但水合物的生成情况与表面活性剂的种类和浓度有关;表面活性剂的加入有效地改变了水合物生成的热力学条件;水合物分解过程所需热量较多,证实了利用煤层气水合化技术预防煤矿煤与瓦斯突出以及进行煤层气固化储运的可行性。     

聚乙烯己内酰胺链端改性及其对甲烷水合物形成的抑制作用研究

注入动力学抑制剂是一种有效缓解天然气水合物管道堵塞的方法。本文以动力学抑制剂聚乙烯基己内酰胺(PVCap)结构为基础,将氧乙基和酯基引入PVCap的分子链端,合成了新抑制剂PVCap-XA1,在高压定容反应釜内评价了PVCap-XA1对甲烷水合物形成的抑制作用,并采用粉末X射线衍射、低温激光拉曼光谱和冷冻扫描电子显微镜研究了抑制剂对甲烷水合物结构和形态的影响。实验结果表明,相同实验条件下PVCap-XA1比PVCap具有更好的抑制作用;微观测试表明PVCap-XA1的加入没有改变甲烷水合物的晶体结构,但会使甲烷水合物晶面扭曲变形,可以降低水合物大小笼占有比(I_L/I_S),使得甲烷分子更难进入水合物大笼,同时PVCap-XA1的加入使甲烷水合物的微观形貌由多孔有序变得更致密而不利于气体通过。     

孔隙尺寸、盐度和气体组分对水合物相平衡的影响(英文)

An experimental device was set up to study the hydrate formation conditions. Effects of pore size, salinity, and gas composition on the formation and dissociation of hydrates were investigated. The result indicates that the induction time for the formation of hydrates in porous media is shorter than that in pure water. The decrease in pore size, by decreasing the size of glass beads, increases the equilibrium pressure when the salinity and temperature are kept constant. In addition, higher salinity causes higher equilibrium pressure when the pore size and temperature are kept constant. It is found that the effects of pore size and salinity on the hydrate equilibrium are quite different. At lower methane concentration, the hydrate equilibrium is achieved at lower pressure and higher temperature.     

Kinetic simulation of methane hydrate formation and dissociation in porous media

The vast amount of hydrocarbon gas deposited in the earth's crust as gas hydrates has significant implications for future energy supply and global climate. A 3-D simulator for methane hydrate formation and dissociation in porous media is developed for designing and interpreting laboratory and field hydrate experiments. Four components (hydrate, methane, water and salt) and five phases (hydrate, gas, aqueous-phase, ice and salt precipitate) are considered in the simulator. The intrinsic kinetics of hydrate formation or dissociation is considered using the Kim-Bishnoi model. Water freezing and ice melting are tracked with primary variable switch method (PVSM) by assuming equilibrium phase transition. Mass transport, including two-phase flow and molecular diffusions, and heat transfer involved in formation or dissociation of hydrates are included in the governing equations, which are discretized with finite volume difference method and are solved in a fully implicit manner. The developed simulator is used here to study the formation and the dissociation of hydrates in laboratory-scale core samples. In hydrate formation from the system of gas and ice (G+I) and in hydrate dissociation systems where ice appears, the equilibrium between aqueous-phase and ice (A-I) is found to have a “blocking” effect on heat transfer when salt is absent from the system. Increase of initial temperature (at constant outlet pressure), introduction of salt component into the system, decrease of outlet pressure, and increase of boundary heat transfer coefficient can lead to faster hydrate dissociation.     

Experimental investigation and modeling on the dissociation conditions of methane hydrate in clayey silt cores

As the majority of global natural gas hydrate reserve, the dissociation conditions of hydrate in clayey silts are of great significance for its efficient production. In this work, the dissociation conditions of methane hydrate in clayey silt cores were experimentally measured by step-heating method at the temperature range of 280.76–289.55 K and pressure range of 8.11–15.03 MPa, respectively. Various cores including quartz powder, montmorillonite, and South China Sea sediments at the water content range of 20%–33% were used for investigation. The results showed that the dissociation temperatures of methane hydrate in clayey silt cores depressed compared to bulk hydrate. The grain size, salinity, and lithology of clayey silt cores significantly affect the dissociation conditions of hydrate. In comparison to grain size, salinity, and lithology had a more significant influence on the equilibrium temperature depression. The dissociation temperature depression of methane hydrate was considered as a consequence of the water activity depression which is caused by the effect of capillary, salt, or clay. A water activity meter was used to measure the water activity in clayey silt cores. The influence of salt component and mineral characteristics on the water activity was investigated. By combining the measured water activity data with the Chen-Guo model, a novel water activity measurement (WAM) method for the hydrate dissociation conditions prediction was proposed. With the maximum deviation less than 12%, the predicted results are in good agreement with the experimental data. It demonstrated that the WAM method could effectively predict the dissociation conditions of methane hydrate in clayey silts with convenience and accuracy.     

Methane hydrates bearing synthetic sediments—Experimental and numerical approaches of the dissociation

The production of methane gas from methane hydrate bearing sediments may reach an industrial scale in the next decades owing to the huge energy reserve it represents.However the dissociation of methane hydrate in a porous medium is still poorly understood and controlled: the melting of methane hydrate involves fluids flows and heat transfer through a porous medium whose properties evolve as the hydrate phase disappears, and is replaced (or not) by an ice phase. Mass and heat transfers can be coupled in a complex way, firstly because of the permeability changes, and secondly due to material conduction changes. In our work, mass and heat transfers have been studied both experimentally and numerically.A 2D numerical model is proposed where heat and mass transfers govern the dissociation of methane hydrate. This model has been used to design an experimental device. Experiments have been obtained and finally the model has been validated.The experimental set-up consists of five cylindrical sand packs having the same diameter but different lengths. Each experiment starts by crystallizing a hydrate phase in a porous medium. Then the hydrate is dissociated by controlling the pressure at one boundary. The kinetic of dissociation is monitored by collecting gases in ballast. Simulations and experiments demonstrate that the dissociation limiting step switches from thermal transfer to mass transfer depending on the initial permeability and conductivity of the porous medium.     

0℃以下含SDS的甲烷水合物生成方式及过程对其分解速率的影响

The dissociation rates of methane hydrates formed with and without the presence of sodium dodecyl sulfate(methane-SDS hydrates),were measured under atmospheric pressure and temperatures below ice point to investigate the influence of the hydrate production conditions and manners upon its dissociation kinetic behavior.The experimental results demonstrated that the dissociation rate of methane hydrate below ice point is strongly dependent on the manners of hydrate formation and processing.The dissociation rate of hydrate formed quiescently was lower than that of hydrate formed with stirring;the dissociation rate of hydrate formed at lower pressure was higher than that of hydrate formed at higher pressure;the compaction of hydrate after its formation lowered its stability,i.e.,increased its dissociation rate.The stability of hydrate could be increased by prolonging the time period for which hydrate was held at formation temperature and pressure before it was cooled down,or by prolonging the time period for which hydrate was held at dissociation temperature and formation pressure before it was depressurized to atmospheric pressure.It was found that the dissociation rate of methane hydrate varied with the temperature(ranging from 245.2 to 272.2 K) anomalously as reported on the dissociation of methane hydrate without the presence of surfactant as kinetic promoter.The dissociation rate at 268 K was found to be the lowest when the manners and conditions at which hydrates were formed and processed were fixed.     

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.     

Methane Hydrate Formation and Dissociation in the presence of Bentonite Clay Suspension

The present work reports the effect of bentonite clay on methane hydrate formation and dissociation in synthetic seawater of salinity 3.55 % of total dissolved salts. Extensive observations of pressure‐temperature equilibrium during formation and decomposition of methane hydrate under different conditions have been made. It is observed that phase equilibrium conditions of hydrate are affected on changing the concentration of bentonite clay in synthetic seawater. Induction time for hydrate nucleation has been measured under different concentrations of clay and subcooling conditions. The presence of bentonite clay in synthetic seawater reduces the induction time of hydrate formation. Enthalpy of hydrate dissociation is calculated by Clausius‐Clapeyron equation using measured phase equilibrium data. The amount of gas consumed during hydrate formation has been calculated using real gas equation. It is found that a larger amount of gas is consumed upon addition of bentonite clay in synthetic seawater.     

X射线衍射分析聚乙烯吡咯烷酮对水合物分解过程的影响

深水油气资源的勘探开发以及开采过程中的环保要求,使得天然气水合物动力学抑制剂使用不可避免,含动力学抑制剂的分解研究对水合物生成后的解堵具有重要的指导意义。本文在高压反应釜内采用甲烷和丙烷混合气,合成天然气水合物,并用X射线粉晶衍射仪分析了含动力学抑制剂聚乙烯吡咯烷酮的水合物分解过程。结果显示甲烷和丙烷气体会形成SⅡ型水合物,但伴随有SⅠ型甲烷水合物的生成;添加动力学抑制剂后,水合物的分解速率变慢,在-60℃,添加0.5%的聚乙烯吡咯烷酮后,分解起始的20 min内,无抑制剂体系水合物分解可达69%,而在含抑制剂体系分解约18%;SⅡ型甲烷丙烷混合气水合物分解过程中晶胞各晶面分解速率相同,没有偏好性,水合物笼作为一个整体分解,添加抑制剂不改变这种分解方式,仍以整体分解。     

Natural gas hydrate as a potential energy resource: From occurrence to production

Natural gas hydrate reservoirs have been strongly suggested as a potential energy resource. However, this potential is expected to be limited by geological factors, reservoir properties, and phase-equilibria considerations. Accordingly, sufficient understanding and accurate analyses for the complex surroundings in a natural gas hydrate system have to occur before methane recovery. In this paper, we discuss the formation and structure patterns of global natural gas hydrate, including the origins of hydrocarbon, crystal structures, and unique structure transition. We also summarize two important anomalies related to methane occupancy and chlorinity which were revealed very recently. Furthermore, we review the geological and chemical surroundings of the shallow hydrate deposits, the so-called brine patch discovered in the Cascadia Margin and Ulleung Basin, which are significantly related to tectonic conduits for methane gas and positive chlorinity.     

甲烷水合物分解过程的微尺度测量

实验采用激光拉曼和X射线粉末衍射(PXRD) 在253 K，常压条件下对甲烷水合物的分解过程分别进行了原位测量。研究发现，位于表层的甲烷水合物在前30～50 min内发生分解并生成Ⅰh冰相，随后表层冰相对内层水合物相的包覆引起了“自保护”效应的产生并导致甲烷水合物分解速率显著降低。分解过程中，甲烷在水合物大小笼中的含量之比始终保持在3.2左右，同时水合物晶面特征峰峰面积也按照相同的曲线下降，表明甲烷水合物以晶胞为单位进行整体分解。Ⅰh冰的各个晶面特征峰峰面积差异化的增长曲线表明形成的Ⅰh冰相倾向于片状生长，有助于在水合物表面生成一层冰膜，进而产生“自保护”效应。     

Study on methane hydrate formation using ultrasonic waves

Methane hydrate is a kind of gas hydrate and has the crystal structure I. 1 m³ of methane hydrate can be decomposed to a maximum of 172 m³ of methane gas in standard conditions. If this characteristic of methane hydrate is reversely utilized, natural gas, which mainly consists of methane gas, is fixed into water in the form of hydrate solid. However, when methane hydrate is formed artificially by simply reversing its process of natural generation, the amount of methane gas consumed owing to hydrate formation is fairly low, which would be problematic for its massive synthesis and application. In this study, experiments are carried out with the goal of increasing the amount of gas consumed by using ultrasonic waves. The power for maximum gas consumption was observed at 150 W, and the amount of gas consumed was four times higher than that at 0 W at the subcooling temperature of 0.5 K. The ultrasonic waves are more effective at the subcooling temperature of 5.7 K than at the subcooling temperature of 0.5 K, and are another effective method for enhancing methane hydrate formation and reducing the hydrate formation time.     

离子液体与PVP K90复合抑制剂对甲烷水合物的生成影响

注入抑制剂是油气行业解决管道输送过程因水合物生成而引发的堵塞问题最常用的方法。但现有大多数动力学抑制剂（KHIs）存在抑制性能不足、高过冷度条件下会失效等问题，可应用场合大大受限。离子液体作为绿色溶剂对甲烷水合物具有良好的热力学抑制作用。为改进KHIs的性能，提出将离子液体与KHIs复合。本文实验考察8.0 K过冷度、两种浓度下[1.0%(质量)、2.0%(质量)]离子液体N-丁基-N-甲基吡咯烷四氟硼酸盐([BMP][BF₄])、聚乙烯基吡咯烷酮（PVP K90）以及二者复配构成的复合型抑制剂对甲烷水合物抑制规律，得到了最佳组分配比。利用粉末X射线衍射（PXRD）和低温激光拉曼光谱测量了不同抑制剂体系中形成的甲烷水合物晶体微观结构和晶穴占有率，发现添加抑制剂不会改变sI型甲烷水合物晶体结构，但会影响水合物晶体的大、小笼占有率和水合数。结合宏观动力学实验和微观结构测试结果，揭示离子液体与PVP K90复合抑制剂的抑制机理。     

Effect of complex inhibitors containing ionic liquids and PVP K90 on formation of methane hydrate

Injecting inhibitors is the most commonly used method in the oil and gas industry to solve the problem of blockage caused by hydrate formation during pipeline transportation. However, most of the kinetic hydrate inhibitors (KHIs) are strictly limited by weak inhibition performance and low subcooling. Ionic liquids, a kind of green solvent, have been recognized to act as excellent thermodynamic inhibitors on methane hydrate formation. So, it is proposed to add the ionic liquids into KHIs to improve their overall performance. In this paper, the kinetic effects of an ionic liquid N-butyl-N-methylpyrrolidine tetrafluoroborate ([BMP][BF₄]), a commercial kinetic inhibitor polyvinyl pyrrolidone (PVP K90) and their mixtures with different mass ratios on the methane hydrate formation were experimentally studied at 8.0 K subcooling and two concentrations [1.0%(mass) and 2.0%(mass)]. The best mass ratio of the compound inhibitor was determined. Moreover, the crystal structures and cage occupancy characteristics of methane hydrates formed without and with inhibitors at different mass concentrations and composition ratios were measured by using powder X-ray diffraction (PXRD) and low-temperature Laser Raman spectrometers. It was found that the addition of inhibitors did not change the crystal structure of methane hydrate, but affected the cage occupancies and hydration numbers. Based on the results from macroscopic kinetics and microscopic structure tests, the inhibition mechanism of compound inhibitors was proposed.     

Comparison between equilibrium and kinetic models for methane hydrate dissociation

Current literature agrees that the equilibrium and kinetic models for methane hydrate dissociation are almost indistinguishable. In this comparison, we used the equilibrium and kinetic models with two kinds of thermal boundary conditions to study the dissociation of methane hydrates in porous media. We found significant deviations between the two models. A systematic parametric study of the kinetic reaction constants clearly shows that the kinetic model results approach the equilibrium model when the intrinsic mole dissociation constant excessively exceeds the range found in the literature. Further, we showed deviations in the dissociation pattern between the equilibrium and kinetic models for both boundary conditions. The equilibrium model exhibits a moving front pattern for hydrate dissociation while the kinetic model shows a moving zone pattern under adiabatic boundary conditions. As for the constant temperature boundary condition, the hydrate dissociates by shrinking in all dimensions for the equilibrium model while, for the kinetic model, it dissociates with no specific pattern throughout the entire reservoir. The parametric studies show that higher activation energy results in a lower rate of hydrate dissociation.     

Kinetic Modeling of Methane Hydrate Formation in the Presence of Low‐Dosage Water‐Soluble Ionic Liquids

The kinetic and thermodynamic effects of three typical low‐dosage imidazolium‐based ionic liquids (ILs) on methane hydrate formation and dissociation were investigated, considering the anion nature and subcooling and/or overpressure driving forces. Isochoric hydrate formation and dissociation data were obtained by the modified slow step‐heating method. ILs proved to have a dual effect on both formation and dissociation of methane hydrate including thermodynamic and kinetic inhibition. Kinetic modeling of methane hydrate inhibition by low‐dosage ILs was performed. Kinetic analysis showed that IL inhibitors mainly cause a delay in the nucleation or hydrate growth step. The related inhibition mechanism was resolved regarding the ionic nature and electrostatic interactions of ILs with water molecules. Two binomial exponential kinetic relations were derived and used for simple methane hydrate formation in the presence of ILs as kinetic hydrate inhibitors. The proposed relations can serve for a quick estimation of the nature, extent, strength, and effectiveness of ILs on various gas hydrates.