基于微观胶结厚度模型的深海能源土宏观力学特性离散元分析

首先,引入蒋明镜等提出的考虑水合物胶结厚度的深海能源土粒间微观胶结模型,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,并将其引入离散单元法中;然后,对选定的水合物饱和度经过实际二维离散元模拟调算,得出相应的水合物胶结尺寸,以修正水合物临界胶结厚度、最小胶结厚度及胶结宽度,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,针对不同水合物饱和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、体变、剪胀角等方面与Masui等所进行的能源土室内三轴试验进行对比分析。结果表明：采用考虑粒间胶结厚度的水合物微观胶结模型,能够定性反映深海能源土的宏观力学特性,能源土试样的峰值强度、黏聚力和剪胀角均随水合物饱和度的增加而增加,但水合物饱和度对内摩擦角的影响规律不明朗;能源土试样的峰值强度、残余强度及体积剪缩量随着有效围压的增大而增大;剪胀角随有效围压的增大而减小。     

三维离散元单轴试验模拟甲烷水合物宏观三轴强度特性

填充型水合物的砂性能源土试样可视为特殊的散粒体材料,即砂粒和水合物颗粒混合物,具有明显的非连续特征。为研究填充型水合物的能源土力学特性,初步探索了甲烷水合物在不同温度、反压条件下加荷模式的离散元模拟方法。离散元模拟中,将水合物块体视为由大量颗粒通过强胶结作用凝聚而成的整体,室内试验中的内部孔隙水压作用转化为水合物颗粒间的胶结力,故需要合理确定颗粒间胶结模型参数来实现反压的影响作用。通过参数反演建立了宏观强度、刚度参数与平行胶结模型的微观胶结参数间的宏、微观关系,基于已有室内甲烷水合物三轴试验资料,确定了给定温度和反压条件下的微观胶结参数取值,随后进行离散元单轴压缩试验。离散元单轴压缩试验模拟获得的水合物强度特性,与室内三轴试验结果符合较好;通过建立的宏、微观参数间的关系,实现了不同温度、反压下的水合物加荷模式的模拟。为进一步提出深海能源土离散元数值试验成样方法--孔隙填充水合物生成技术,形成含填充型水合物的能源土试样,研究其力学和变形特性奠定基础。     

水合物沉积物力学性质的三维离散元分析

水合物沉积物力学特性研究是天然气水合物开采领域中的热点问题。为深入了解水合物对沉积物力学特性的影响,在提出一个新的水合物沉积物离散元数值试样制备方法的基础上,模拟了不同水合物饱和度沉积物试样的三轴排水试验,并从其应力-应变关系、体变特性、弹性模量及峰值强度等方面对模拟结果及已有室内三轴试验结果进行了对比,然后利用该方法对具有不同微观胶结参数的水合物沉积物样进行了三轴离散元数值试验。研究结果表明：所提出的离散元模拟方法能较好地反映水合物沉积物的主要力学特性;天然气水合物与土颗粒间胶结性能的改变会对水合物沉积物的力学响应产生一定的影响;水合物沉积物强度和模量的增加是孔隙填充水合物和粒间胶结水合物共同作用的结果。     

结构性黄土一维湿陷特性的离散元数值模拟

针对结构性湿陷性黄土大孔隙和胶结特性,应用离散元生成了不同含水率结构性黄土试样,研究试样的一维湿陷特性。首先,根据已有的结构性黄土试验资料和胶结颗粒材料离散元数值试验成果,建立胶结强度和初始饱和度之间的关系。其次,采用蒋明镜等提出的分层欠压法[1]和胶结模型[2]制得不同含水率结构性黄土离散元试样,然后进行不同含水率双线法和同一含水率4个压力下单线法湿陷试验的离散元数值模拟。数值模拟结果表明,提出的离散元分析方法能模拟天然结构性湿陷性黄土的主要力学性质,随着含水率的减少,结构屈服应力和最大湿陷压力增加,湿陷系数随着压力先增加后减小,湿陷起始压力为饱和试样的结构屈服应力,单线法湿陷后压缩曲线与饱和试样的压缩曲线接近。此外,模拟结果还表明,不同含水率结构性黄土离散元试样的最大湿陷系数与天然结构性湿陷性黄土相差较远,但在最大湿陷系数与孔隙比的比值上相接近;结构屈服对应着胶结的逐步破坏,湿陷伴随着大量的胶结破坏。提出了基于胶结点数目的损伤变量,研究了其在加载和湿陷过程中的变化规律。研究成果为认识黄土复杂力学特性和建立其本构理论提供了基础。     

各向异性结构性砂土离散元分析

天然沉积砂土力学特性受各向异性及结构性影响明显,实际工程中不能忽视。为探究二者的影响,首先在二维离散元程序NS2D中采用椭圆颗粒模拟了重力场中颗粒长轴主方向为水平的各向异性净砂样,随后基于结构性砂土胶结厚度分布规律及室内试验提出了一个新的微观胶结接触模型并将其引入各向异性净砂样以模拟天然各向异性结构性砂土,最后对该离散元试样进行了双轴试验模拟,将模拟结果与室内试验结果对比以验证该模型的适用性,并对其微观力学特性变化进行研 究。分析结果表明,随着剪切进行,各向异性结构性砂土呈明显应变软化及剪胀现象;胶结接触逐渐减少,且主方向始终为竖向方向;胶结破坏速率及胶结破坏率变化情况与宏观力学响应较一致,且胶结物多为拉剪破坏;土颗粒排列主方向始终为水平向,且水平向排列颗粒所占比例略微增大。     

不同开采方法下深海能源土离散元模拟

天然气水合物以胶结及孔隙填充等形式存在于深海能源土中,开采时因其分解会劣化地层力学特性进而引发海底事故,使得人们对能源土开采过程进行中力学特性的变化愈发重视。在前期室内试验的基础上,将一个温度-水压-力学二维微观胶结模型引入离散元商业软件PFC2D中,通过对排气、排水性较好的土体进行升温及降压法开采进行数值模拟,并将模拟结果与相同条件下的室内试验结果对比,验证了该胶结模型的适用性。进一步分析了颗粒接触分布与颗粒平均纯转动率（averaged pure rotation rate, APR）在水合物分解时的变化情况。升温分解过程中随温度升高,颗粒总接触分布各向异性程度增大;胶结接触逐渐减少并始终保持主方向为水平方向,无胶结接触增多并始终保持主方向为竖直方向;APR值逐渐增大且正负值分布逐渐趋于集中。降压分解过程中随反（水）压降低,颗粒总接触由各向同性分布逐渐发展为主方向为竖直方向的各向异性,APR值较小且分布均匀;恢复反压后,试样进一步破坏,颗粒总接触各向异性更加明显,APR值增大且正负值呈集中分布。     

考虑水合物填充和胶结效应的深海能源土弹塑性本构模型

水合物的填充效应和胶结效应增大了能源土的密实性和强度,使能源土呈现出类似于密实砂土或胶结土的性质。在黏土和砂土的统一硬化模型(CSUH模型)框架下,总结了能源土的力学性质,引入压硬性参量描述水合物对能源土填充和胶结双重作用下的等向压缩特性,引入黏聚强度修正屈服函数并构建了黏聚强度的演变规律,利用状态参数调整剪胀方程,反映能源土剪胀、软化等特性对密实度的依赖性,从而建立能够描述能源土强度、刚度、剪胀与软化等特性的弹塑性本构模型。编制了模型的测试程序,把模拟结果与能源土室内试验结果进行对比。结果表明:提出的弹塑性本构模型能够较好地描述能源土的应力-应变关系、剪缩硬化和剪胀软化等力学特性。     

人工胶结砂土力学特性的离散元模拟

采用离散单元法（DEM）对胶结砂土力学特性进行模拟。将基于室内试验测得的理想胶结颗粒接触力学响应引入到开发的二维离散元程序（NS2D）中,模拟胶结砂土颗粒间的胶结作用。对不同胶结强度和围压的胶结砂土进行平面应变双轴压缩试验模拟,并将模拟结果与Wang和Leung[1]提供的人工胶结砂土的试验结果进行比较。最后对数值模拟中胶结试样的微观力学响应（接触力链、胶结点破坏率和位移场）进行分析。结果表明,离散元数值模拟能够有效地反映胶结砂土的主要力学特性,相比同一初始孔隙比的无胶结松散砂土,胶结砂土将具有更高的强度,应力-应变关系呈应变软化,体变为先剪缩后剪胀,且两者的差异随胶结强度的增大和围压的减小而越趋显著。此外,胶结砂土宏观力学响应（应力-应变关系和剪胀性）与其微观力学响应密切相关。     

考虑水合物胶结厚度的深海能源土粒间胶结模型研究

天然气水合物被公认是解决当前能源危机的潜在新型能源而备受关注。含水合物的海底土体称为深海能源土。水合物在能源土中有不同的赋存形式（如填充型水合物和胶结型水合物等）,由于胶结型水合物对整体强度的贡献比其他存在形式更大,尤其是饱和度较低的情况。针对于胶结型水合物的赋存形式进行研究,水合物作为胶结物质存在于土颗粒之间,胶结厚度会在一定范围内变化。为真实地反映此现象,通过对能源土试样的电镜扫描图片整理分析,获得水合物饱和度与粒间胶结厚度的函数关系。基于前期已经完成的不同粒间胶结厚度下胶结力学特性的试验研究成果,为探究胶结厚度变化对能源土体宏观力学特性的影响,建立了考虑水合物胶结厚度的能源土粒间胶结模型,并介绍此模型中相关胶结参数及其确定方法。     

一个深海能源土弹塑性本构模型

天然气水合物赋存在低温高压环境中,会在土颗粒间形成胶结从而增大深海能源土抗剪强度。基于损伤力学理论,将结构性砂土本构模型推广应用于深海能源土分析中,模拟计算了三轴固结排水剪切试验,再根据应力-应变曲线关系定量反演初始屈服系数与水合物饱和度之间的函数关系,并修正了原有的结构性砂土破损规律,建立了深海能源土弹塑性本构模型。另外,根据该模型模拟了另外一组深海能源土三轴剪切试验和等向固结压缩试验。计算结果表明：新建立的深海能源土本构模型可以有效模拟深海能源土剪切强度随水合物饱和度之间的增长关系;随着水合物饱和度的增加,三轴压缩试验中深海能源土峰值强度及割线模量(E50)逐渐增加,等向固结压缩试验中屈服强度增加,与试验结果有较好的一致性,表明了该模型的合理性。     

考虑颗粒抗转动的砂土双轴试验离散元模拟

颗粒间抗转动作用是影响砂土力学行为的重要因素。将蒋明镜等（2005年）提出的考虑颗粒抗转动作用的颗粒接触模型植入PFC2D中,对砂土双轴试验进行了模拟,研究颗粒抗转动作用对砂土力学行为的影响。研究显示：颗粒抗转动对砂土力学行为影响显著。抗转作用越强,砂土强度越高;抗转系数为0.4时,松砂亦出现软化及剪胀现象。抗转动作用对孔隙比及配位数变化趋势影响也很显著。     

Study of mechanical behavior and strain localization of methane hydrate bearing sediments with different saturations by a new DEM model

This paper presents a numerical investigation into mechanical behavior and strain localization in methane hydrate (MH) bearing sediments using the distinct element method (DEM). Based on the results of a series of laboratory tests on the bonded granules idealized by two glued aluminum rods and the available experimental data of methane hydrate samples, a pressure and temperature dependent bond contact model was proposed and implemented into a two-dimensional (2D) DEM code. This 2D DEM code was then used to numerically carry out a series of biaxial compression tests on the MH samples with different methane hydrate saturations, whose results were then compared with the experimental data obtained by Masui et al. [9]. In addition, stress, strain, void ratio and velocity fields, the distributions of bond breakage and averaged pure rotation rate (APR) as well as the evolution of strain localization were examined to investigate the relationships between micromechanical variables and macromechanical responses in the DEM MH samples. The numerical results show that: (1) the shear strength increases as methane hydrate saturation S_MH increases, which is in good agreement with the experimental observation; (2) the strain localization in all the DEM MH samples develops with onset of inhomogeneity of void ratio, velocity, strain, APR, and distortion of stress fields and contact force chains; and (3) the methane hydrate saturation affects the type of strain localization, with one shear band developed in the case of 40.9% and 67.8% methane saturation samples, and two shear bands formed for 50.1% methane saturation sample.     

An SMP critical state model for methane hydrate‐bearing sands

Mechanical properties of methane hydrate‐bearing soils are complex. Their behavior undergoes a significant change when hydrates dissociate and become methane gas. On the other hand, methane hydrates are ice‐like compounds and, depending on the hydrate accumulation habits and the degree of hydrate saturation, may cement soil particles into stronger and stiffer soils. A new constitutive model is proposed that is capable of capturing essential characteristics of hydrate‐bearing soils. The core of the model includes the spatial mobilized plane concept; a transformed stress, t_ij; the critical state; and the subloading framework. The proposed model gives soil responses due to stress changes or hydrate saturation changes or both. The performance of the model has been found satisfactory, over a range of hydrate saturation and confining pressures, using triaxial test data from laboratory‐synthesized samples and from field samples extracted from Nankai Trough, Japan. Copyright © 2015 John Wiley & Sons, Ltd.     

Thermo-Hydro-Mechanical-Chemical Simulation of Methane Hydrate Dissociation in Porous Media

The vast amount of methane deposits in permafrost and oceanic sediments has significant energy and environmental implications. There are increasing interests in the development of numerical simulation techniques to predict the reservoir responses due to natural gas recovery from methane hydrate dissociation. There has been extensive amount of work on modeling the chemo- thermo- hydro-responses associated with hydrate dissociation. The mechanical responses of hydrate bearing ground, however, have largely been overlooked and are just starting to receive more and more attention. From energy recovery perspective, a comprehensive model that includes the mechanical responses of hydrate disassociation is crucial for predicting the mechanical stability of gas hydrate reservoir and potential geohazards. This paper proposes a thermo-hydro-mechanical-chemical model for simulating the dissociation of methane hydrate. The governing equations for the conservation of energy (thermal), mass (hydraulic) and momentum (mechanical) were derived from the local balance equations. The proposed governing equation system for methane hydrate as a four-phase four-component composite was simplified based on reasonable assumptions to facilitate numerical implementations. The dissociation reaction was considered using chemical kinetics. Auxiliary relationships such as the soil water characteristic curve, constitutive correlations, stress formulation based on the mixture theory were employed to mathematically close the formulation. The mathematical model was implemented using the finite element method. The simulation results were evaluated and compared with those obtained by conventional simulators based on thermo-hydro-chemical models. The mechanical module of this new model was applied to predict the geotechnical responses induced by gas recovery in a typical oceanic reservoir.     

Distinct microbial communities thriving in gas hydrate-associated sediments from the eastern Japan Sea

Marine gas hydrate and cold-seep systems, which maintain a large amount of methane in the seabed, may critically impact the geochemical and ecological characteristics of the deep-sea sedimentary environment. However, it remains unclear whether marine sediments associated with gas hydrate harbor novel microbial communities that are distinct from those from typical marine sediments. In this study, microbial community structures thriving in sediments associated with and without gas hydrate in the eastern Japan Sea were characterized by 16S rRNA gene-based phylogenetic analyses. Uncultivated bacterial lineages of candidate division JS1 and a novel group NT-B2 were dominant in the sediments from gas hydrate-associated sites. Whereas, microbial populations from sites not associated with gas hydrate were mainly composed of Bacteroidetes, Nitrospirales, Chlamydiales, Chlorobiales, and yet-uncultured bacterial lineages of OD1 and TM06. The good correlation between the dominance of JS1 and NT-B2 and the association of gas hydrate could be attributed to the supply of more energetically favorable energy sources in gas-rich fluids from the deep subsurface than refractory organic matter of terrigenous and diatomaceous origin.     

Gas outburst with sediments because of tetrahydrofuran hydrate dissociation

Methane hydrate (MH) is a new energy resource in the 21^st century. But the dissociation of MH from sediments during the MH exploration or oil/gas exploration under a hydrate layer accompanied by the softening of soils and formation of excess pore gas pressure may lead to ground failures and environmental disasters. In this study, experiments on modeling the weakening and failure of the sediment by heat‐induced dissociation of tetrahydrofuran (THF) hydrate were presented. The failure mode of gas outburst was observed. Gas outbursts is a process where gas and soils in hydrate‐dissociation zone burst out after the continuous skeleton of over‐layer is fractured during the expansion of the dissociation zone and the formation of gas zone and excess pore gas pressure. An analytical method is presented by decoupling heat transfer and soil deformation. The geometrical and mechanical similarities for gas outburst are obtained. An empirical criterion for the occurrence of outburst is proposed using the theory of thermal conduction, rigid plastic mechanics, and the experimental data. Copyright © 2015 John Wiley & Sons, Ltd.