Compressive characteristics of methane hydrate-bearing sands under isotropic consolidation

Depressurization is an effective method to produce methane gas from methane hydrate reservoirs. However, during gas production, sediments consolidate due to increasing effective stress. Revealing the compressive characteristics of methane hydrate-bearing sands during consolidation is essential for an accurate understanding of sediment properties and for the development of a constitutive model. Therefore, a series of isotropic consolidation tests was performed on sand in which methane hydrate was artificially generated, and its compressibility characteristics were evaluated. Furthermore, to assess prolonged production, creep compressive behavior was investigated. The experimental results showed volumetric strain due to increasing confining stress decreased with increasing initial methane hydrate saturation. Particle crushing during consolidation was inhibited by the presence of methane hydrate. It was confirmed that the increase in the effects of methane hydrate on soil compressibility followed a power function with the increase in methane hydrate saturation. Creep deformation was observed during the stress holding period regardless of the presence of methane hydrate. Creep behavior during the stress holding period was related to the extent to which the creep component had dissipated before the stress holding period in the past. A theoretical concept for creep strain was proposed based on the experimental results.     

深海能源土宏观力学性质离散元数值模拟分析

首先,引入笔者等^[16-17]所提出的微观胶结模型用以反映能源土颗粒之间水合物微观胶结接触力学特性;其次,采用C++语言将模型程序化,建立同商业软件PFC^2D的程序接口,将模型引入离散单元法中;然后,通过简化计算方法确定胶结宽度随水合物浓度的变化规律,进而确定水合物微观胶结参数;最后,根据所确定的胶结参数,针对不同水合物浓度试样进行能源土宏观力学特性离散元双轴试验模拟,并从应力应变、体积应变、水合物对能源土弹性模量的影响等方面与Masui等^[4]所进行的能源土室内三轴试验进行对比分析。结果表明：所选择胶结模型及微观胶结参数能有效反映深海能源土宏观力学规律;能源土峰值强度、弹性模量均随水合物浓度增加而增加,体积膨胀随水合物浓度的增加越来越显著。     

Constitutive model for gas hydrate-bearing soils considering different types of hydrate morphology and prediction of strength-band

The present study proposes a new elasto-plastic constitutive model that considers different types of hydrates in pore spaces. Many triaxial compression tests on both methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have been carried out over the last few decades. It has been revealed that methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have different strength and dilatancy properties even though they have the same hydrate contents. The reason for this might be due to the different types of hydrate morphology. In this study, therefore, the effect of the hydrate morphology on the mechanical response of gas-hydrate-bearing sediments is investigated through a model analysis by taking into account the different hardening rules corresponding to each type of hydrate morphology. In order to evaluate the capability of the proposed model, it is applied to the results of past triaxial compression tests on both methane hydrate-containing and carbon dioxide hydrate-containing sand specimens. The model is found to successfully reproduce the different stress–strain relations and dilatancy behaviors, by only giving consideration to the different morphology distributions and not changing the fitting parameters. The model is then used to predict a possible range in which the maximum deviator stress can move for various hydrate morphology ratios; the range is defined as the strength-band. The predicted curve of the maximum deviator stress obtained by the constitutive model matches the empirical equations obtained from past experiments. It supports the fact that the hydrate morphology ratio changes with the total hydrate saturation. These findings will contribute to a better understanding of the relation between the microscopic structures and macro-mechanical behaviors of gas-hydrate-bearing sediments.     

试验反压对深海能源土宏观力学特性影响的离散元分析

在饱和土体三轴试验中,反压常被用于提高试样饱和度,其对常规土体强度特性无影响已广为认同,而已有试验资料表明反压对深海能源土强度、弹性模量等宏观力学参数均存在一定影响,成为困扰国际岩土界的一个难题。首先探讨了试验反压对能源土力学特性的影响机理,通过引入能源土微观接触模型的离散元双轴试验检验上述机理的合理性;然后结合20组离散元双轴试验,进一步探究能源土宏观力学特性随反压的变化规律。结果表明：试验反压对能源土力学特性的影响与水合物作用相关;反压能提高能源土强度,使应变软化和剪胀特性更加明显,并对其弹性参数有一定的影响;试验反压较大时,反压变化对能源土强度参数的影响难以忽略,但对弹性参数的影响可忽略。     

Effect of temperature cycle on mechanical properties of methane hydrate-bearing sediment

In this study, methane hydrate-bearing sand (MHBS) was created in the laboratory following two methods in order to obtain two types of gas hydrate morphology in sandy sediment. The hydrate morphology in the sediment was assessed by measuring the compressional wave velocity combined with models to predict the wave velocities of the sediment containing gas hydrates. The mechanical properties of the MHBS were investigated by triaxial compression tests. The results obtained by the compressional wave velocity show that after saturating the MHBS sediment (created by the excess gas method) with water, the methane hydrates are partly or completely converted from grain contacts to pore spaces depending on the hydrate saturation (ranging from 0 to 50%). A subsequent temperature cycle completes this conversion process for high hydrate saturation. The results obtained with the triaxial compression tests show higher shear strength, a higher secant Young’s modulus, and a higher dilation angle at higher hydrate saturation. In addition, the effects of hydrate saturation on the mechanical properties of the MHBS obtained by the two procedures (with and without the thermal cycle) are similar at low hydrate saturation. The effect of gas hydrate morphologies can only be detected in the case where the conversion (and/or redistribution) of gas hydrates from grain contacts to pore spaces is not complete (at high hydrate saturation).     

不同温压环境下深海能源土力学特性离散元分析

深海能源土宏观力学特性与所处温度与水压环境密切相关,明晰温压对能源土力学特性的影响对水合物的安全开采具有重要意义。首先,介绍深海能源土的温度-水压-力学微观胶结模型,用以描述能源土粒间的水合物胶结接触力学特性;其次,将该模型导入至离散元商业软件PFC2D中,开展不同温度与水压环境下的离散元双轴试验;最后,结合离散元双轴试验结果及同已有室内试验结果的对比分析,探讨温度与水压对深海能源土宏观力学特性的影响规律和微观作用机理。结果表明：引入胶结模型的离散元双轴试验可较好地描述深海能源土强度、变形等力学特性随温度与水压的变化关系;温度与水压影响深海能源土宏观力学特性的微观机理是颗粒间水合物胶结强度与刚度同温度与水压间的相关性;建议采用温压距离参数L（在无量纲化的温度-水压坐标平面内,水合物赋存温度与水压点至其相平衡线的最小距离）评价实际复杂温压场下的深海能源土宏观力学特性。     

甲烷水合物三维离散元模拟参数反演初探

含填充型水合物的砂性能源土可视为特殊的散粒体材料（砂粒和水合物颗粒混合物）,具有明显的非连续特征。在离散元中若采用团粒（胶结成团的颗粒组）模拟填充水合物颗粒则需合理确定团粒结构内颗粒间胶结模型参数。为此,基于前人的室内纯水合物三轴试验资料进行离散元建模与参数反演。结果表明,宜采用松散且颗粒间摩擦系数较小的试样模拟水合物块体,当颗粒间摩擦系数小于等于0.0时,可确保无胶结试样的内摩擦角小于室内试验获得的纯水合物内摩擦角。胶结刚度只需在较小范围变化即可反映相同温度不同围压条件下的弹性特性,且微观刚度参数与胶结强度参数的相互作用较小,可以假定二者相互独立。通过选取不同的微观胶结强度值进行不同围压下的三轴压缩试验,建立微观胶结强度参数与宏观参数（内摩擦角和黏聚力）之间的关系,从而确定与室内试验强度特性相符合的微观胶结强度值,实现甲烷水合物三轴试验离散元模拟;由体变规律可知,甲烷水合物在发生剪胀前均存在一个初始的体积收缩阶段,且剪胀特性随着围压的减小而呈现增强趋势。通过微观变量颗粒接触方向组构的分布图可知,随着轴向应变增大,颗粒间接触主方向朝竖直方向偏转,表现出明显的各向异性特性。随着轴向应变的增大,颗粒间胶结残余率变小,表明试样逐步破坏。     

稳定型橡胶改性沥青混合料动态模量预估

测试了8种混合料类别、12种级配组成的稳定型橡胶改性沥青混合料的动态模量,分析了稳定型橡胶改性沥青混合料的动态力学性能特点.针对Witczak模型对稳定型橡胶改性沥青混合料动态模量预测效果不理想的问题,采用Levenberg-Marquardt非线性回归方法修正了Witczak模型.结果表明:稳定型橡胶改性沥青混合料具有良好的动态力学性能;修正后的Witczak模型可以较好地预测稳定型橡胶改性沥青混合料的动态模量.     

深海能源土剪切带形成机理离散元分析

天然气水合物的分解开采过程将会劣化深海能源土的力学性能,从而引发一系列岩土工程问题。因此,要实现天然气水合物的安全开采,需要对能源土的强度和变形特性开展研究。结合深海能源土微观胶结模型,通过平面应变双轴试验的离散元模拟,研究了深海能源土剪切带形成机理以及剪切带内外的宏微观变量特征。结果表明：水合物胶结提升了深海能源土的强度,且使其呈现出明显的应变软化特性;剪切带在峰值应力后开始产生,伴随着胶结的大量破坏以及各宏微观变量的局部化;剪切带内外各宏微观变量差异明显,随着轴向应变的增加,土体微观结构也随之发生变化。     

Effects of gas hydrates dissociation on clays and submarine slope stability

Gas hydrate dissociation is often considered as a precursor or triggering factor for submarine slope failures occurring in relatively deep waters where the bulk of the gas hydrate is found in fine-grained sediments. However, there are actually relatively few studies that focus on the effect of gas hydrate dissociation on the behavior of clays, and very few on what physically happens to clay during and after the dissociation process and how gas hydrate dissociation affects the geotechnical properties of clays. In this paper, we illustrate the effects of hydrate dissociation in clays from laboratory strength tests (direct simple shear) combined with visualization including very-high-resolution 3D imaging (computed tomography), using R11 as the hydrate forming fluid in both laponite and Onsøy clay. The test results reveal that the hydrate dissociation creates bubbles in the surrounding clay matrix and around pipe/well models. In addition, we use CO₂-saturated water as the pore fluid in soft clay, and test results show that cracks may develop, allowing gas migration to take place after reducing back pressure in an oedometer cell. Direct simple shear tests show that the undrained shear strength decreases by up to ～15% due to this process. The test results were then implemented in a 2D finite element model to assess the influence of hydrate dissociation on submarine slope stability. We chose a slope segment west of Svalvard—an area where methane gas bubbles escape from the seabed. The gas bubbling in this area is likely due to climate-controlled hydrate-dissociation (warming of bottom water masses). In the finite-element model, we include the change of methane hydrate stability zone (MHSZ) with time as well as the hydrate-dissociation-induced failure zone, which may be a potential leakage pathway. The numerical study indicates that the hydrate dissociation caused by bottom water warming is unlikely to be the main cause generating a leakage pathway or failure plane. However, the hydrate dissociation causing the reduction in shear strength facilitates a potentially unstable condition. The results imply that the hydrate dissociation may contribute to slope failure as a secondary driver, but are unlikely the main driving force. The aim of this study was to improve our understanding of the physical processes of gas expansion, migration and effect of hydrate dissociation through visualization and a finite element model. In addition, this study discussed methods to detect gas hydrate through a case study, and it was found possible to predict average gas hydrate saturation at sites where the sulfate-methane transition depth is known.     

深海能源土地基承载特性离散元分析

天然气水合物以胶结形式广泛赋存于深海能源土中,水合物的饱和度对能源土地基的承载特性影响巨大,水合物的开采也必将使能源土的承载特性发生重大改变。采用考虑水合物胶结厚度的微观胶结模型,分别对3种不同水合物饱和度的能源土地基进行载荷试验离散元模拟。分析水合物开采前后能源土地基的承载特性,研究水合物开采对能源土地基承载特性的影响,探讨基底压力的分布规律。结果表明:水合物开采前,能源土地基的承载力随饱和度的增加而增大。开采后,地基的承载力急剧降低,且原有水合物的饱和度越大,开采后承载力的降低量也越大;水合物饱和度越高,达到极限承载力后,p–s曲线越接近于竖直向下;胶结破坏存在临界荷载,且不同水合物饱和度地基的胶结破坏规律不相同;水合物饱和度对基底压力的分布形状影响不大,但不同沉降量下基底压力的分布形状明显不同。     

含水合物砂土力学特性及本构模型

天然气水合物(以下简称水合物)的不当开发可能会带来一系列的地质灾害和环境问题,因此,开展含天然气水合物地层开采过程中的安全稳定性评估显得尤为重要,而建立能有效描述含水合物沉积物的力学行为的本构模型是安全稳定性分析的核心前提。在分析含CO₂水合物砂土的三轴力学特性的基础上,把含水合物沉积物视为水合物和土颗粒骨架组成的复合胶结性材料,参考胶结土体的建模思路,引入附加内变量描述水合物对土体的胶结影响,建立了含水合物砂土的屈服函数和非关联流动法则,建立了含水合物砂土的本构模型。通过模型验证及分析,模型能较好地模拟不同围压下和不同水合物含量下含水合物砂土的应力应变曲线,反映含水合物砂土的力学特性。     

一个深海能源土的温度-水压-力学二维微观胶结模型

天然气水合物以胶结形式赋存时,对深海能源土的强度和变形特性影响显著,且其影响程度与所处温度、水压与力学环境密切相关。旨在建立可考虑温（温度）-压（水压）-力（力-位移与胶结破坏准则）耦合影响的深海能源土微观胶结模型。首先,依据能源土中水合物胶结可发生于两种接触形式（直接接触与有间距）的土颗粒间,提出适用于两种胶结模式的力-位移准则和胶结破坏准则。其次,提出温压距离参数L（表示在无量纲化处理后的温度-水压坐标平面,土体所处温度与水压点到水合物相平衡线的最小距离）,并依据文献资料分析,建立水合物胶结强度、刚度与参数L间的关系。最后,建立由水合物饱和度确定的粒间水合物胶结尺寸计算方法,并据此进一步建立了胶结强度与刚度同水合物饱和度间的关系。该模型可以方便地植入离散元程序,从而用于深海能源土的宏微观力学分析。     

深海能源土微观力学胶结模型及参数研究

天然气水合物主要以胶结物形式存在深海能源土颗粒之间,对能源土强度影响显著,因此研究水合物胶结接触力学特性对能源土力学性质研究有重要作用,而其中的关键是水合物胶结模型及胶结参数的确定。首先,引入并讨论了一种微观胶结接触模型及其对于能源土胶结接触力学特性的适用性;其次,通过文献资料系统分析,获取不同温度、压力及水合物密度条件下天然气水合物的强度与弹性模量表达式;最后,进一步研究了水合物微观胶结模型中的胶结参数,该类水合物微观胶结参数取决于能源土中水合物埋藏深度（赋存环境压力）、温度、水合物密度,这些宏观参量容易确定。     

基于水合物技术的煤层气储运研究

利用静态的和带有搅拌装置的水合物实验系统研究了不同浓度煤层气的水合化过程,分析了水合物储存的稳定性,介绍了水合物的分解工艺和储运的经济可行性,指出了水合物技术在煤层气储运方面应加强研究的内容.     

污泥与城市生活垃圾混填的力学特性及稳定性

工业污水和生活污水排放量日益增多,污水处理厂污泥产量日趋加大,污泥填埋关键技术问题已成为当今环境工程以及环境岩土工程的热点问题之一。污泥和垃圾混合填埋在其他国家已得到应用,但由于目前中国关于污泥及其城市生活垃圾混合样的基础性室内试验成果不多,对污泥与城市垃圾混合样的工程力学特性及其混合填埋的适宜比例等不甚了解,从而导致中国污泥垃圾混合填埋工程事故频繁发生。对不同配比的污泥与城市生活垃圾混合样进行了固结压缩实验、三轴固结不排水实验、无侧限抗压强度实验,得出污泥与城市生活垃圾混合样的工程力学特性;同时,用ANSYS数值模拟方法,对不同配合比的污泥城市生活垃圾混合填埋边坡的稳定性进行分析,从而对污泥与城市生活垃圾混合填埋的适宜配合比及其稳定安全问题提出理论支撑。     

Development of high-pressure low-temperature plane strain testing apparatus for methane hydrate-bearing sand

A high-pressure low-temperature plane strain testing apparatus was developed for visualizing the deformation of methane hydrate-bearing sand due to methane hydrate production. Using this testing apparatus, plane strain compression tests were performed on pure Toyoura sand and methane hydrate-bearing sand with localized deformation measurements. From the results, it was observed that the methane hydrate-free specimens, despite their relatively high density, showed changes in compressive volume. Marked increases in the initial stiffness and strength of the methane hydrate-bearing sand were observed (methane hydrate saturation of S_MH=60%). Moreover, the volumetric strain changed from compressive to dilative. For the specimens with methane hydrate, a dilative behavior above S_MH=0% was observed. An image analysis showed that the shear bands of the methane hydrate-bearing sand were thinner and steeper than those of the host sand. In addition, the dilative volumetric strain in the shear band increased markedly when methane hydrate existed in the pore spaces.     

建筑固废物再生粒料基层混合料的性能研究

在研究天然集料(NA)、再生混凝土集料(RCA)、再生砖渣与混凝土集料(RBCA)物理力学性质的基础上,开展了不同RCA掺量(质量分数,下同)及掺30%砖渣的混合料CBR(California bearing ratio)试验,探讨了压实度、含水率对混合料动回弹模量的影响.结果表明:RCA掺量越大,混合料CBR值与动回弹模量均越大;掺入30%砖渣后,混合料CBR值与动回弹模量均降低明显,表明工程应用中应适当控制砖渣掺量;压实度对RCA混合料的动回弹模量影响最大,对RBCA混合料的影响次之,对NA混合料的影响最小;含水率对NA混合料的动回弹模量影响较小,而对RCA、RBCA混合料的影响较大.该研究可为建筑固废物混合料用作道路结构层提供参考.     

Effects of cement type,water/cement ratio and cement content on sea water resistance of concrete

In this study, effects of cement type, cement content and water/cement (W/C) ratio level on the sea water resistance of concrete were investigated. Test samples were exposed to sea water by wetting–drying manner. Residual splitting tensile and compressive strength, and chloride penetration depths of specimens after exposure were determined. Besides, energy dispersive spectrometer (EDS) analyses were performed on scanning electron microscope (SEM) images of selected mixtures. Test results indicate that blast furnace slag cement (SC) mixtures have considerably greater resistance to sea water than portland cement (PC) mixtures both from the point of mechanical properties and chloride penetration.     

Undrained creep behavior of CO2 hydrate-bearing sand and its constitutive modeling considering the cementing effect of hydrates

A technique for carbon dioxide (CO₂) capture and storage using CO₂ hydrates where CO₂ is stored as solid hydrates in the seabed ground, is attracting attention. Shallow sediments may be the most suitable seabed ground for CO₂ hydrate storage because these unconsolidated soil sediments satisfy the limitation for the low-temperature condition. Hence, the deformation properties and long-term stability of gas hydrate-bearing sediments during and after gas storage must be investigated. In this study, a series of undrained triaxial creep tests were conducted on artificially made CO₂ hydrate-bearing sand specimens to study the fundamental time dependent property of hydrate-bearing sediment. We extended an elasto-viscoplastic constitutive model by introducing a cohesion component and its degradation on surfaces and applied the proposed model to creep tests on gas hydrate-bearing sand.Three findings were obtained from the experiments and modelling. First, CO₂ hydrate-bearing sand specimens showed accelerated creep behavior, which was characterized by the creep stress ratio level, regardless of the hydrate saturation. Second, creep accelerated under undrained conditions before the stress reached the critical state line obtained from the monotonic loading tests, and the stress ratio at the occurrence of acceleration creep was higher for specimens with a higher hydrate saturation. Third, the elasto-viscoplastic constitutive model which considered the cementing effect of hydrates was able to well reproduce the undrained creep behavior of hydrate-bearing sand with different hydrate saturations under relatively high creep stress levels.