南海水合物黏土沉积物力学特性试验模拟研究

利用自行研制的含水合物沉积物合成、分解与力学性质测量一体化试验设备,以南海水合物区域的海底粉质黏土作为骨架,制备含水合物沉积物样品,并对其进行了三轴压缩试验研究,获得了水合物分解前后的应力应变曲线和抗剪强度特性. 结果表明:在水合物饱和度0%~45% 的范围内,水合物沉积物的应力应变曲线均表现为弹塑性变形,存在明显的应变硬化现象;抗剪强度、内摩擦角和黏聚力随水合物饱和度的增加而增加. 相对而言,内摩擦角随饱和度增加幅度较小,其他参数在水合物饱和度超过25% 时,呈陡然增高趋势;水合物分解后导致抗剪强度最大可降低为初始的1/4,不同初始饱和度条件下水合物完全分解后沉积物的抗剪强度基本相等,并大于同等围压条件下初始不含水合物的沉积物抗剪强度.      

含水合物沉积物三轴剪切试验与损伤统计分析

天然气水合物开采诱发水合物分解,削弱水合物地层强度,可能导致地层滑动和生产平台倒塌等工程地质灾害,对水合物开采安全性构成严重威胁.深入理解含水合物沉积物力学性质并建立合理的本构关系模型是水合物开采安全性评价的前提条件.在自主研发的含水合物沉积物力学性质测试实验装置上,采用饱和水海砂沉积物气体扩散法制备了含水合物沉积物样品,并开展了系列的排水三轴剪切试验,通过时域反射技术实现了样品中水合物饱和度的实时在线测量;基于复合材料的罗伊斯(Reuss)应力串联模型和沃伊特(Voigt)应变并联模型提出了含水合物沉积物等效弹性模量的细观力学混合律模型,结合损伤统计理论和摩尔-库伦破坏准则改进了含水合物沉积物的本构关系模型.结果表明:随着水合物饱和度的增加和有效围压的减小,应力-应变曲线由应变硬化型变为应变软化型,割线模量和峰值强度均随水合物饱和度与有效围压的增加而提高,黏聚力受水合物饱和度影响明显,而内摩擦角基本不变;提出的等效弹性模量细观力学混合律模型与改进的本构关系模型均具有良好的适用性,模型参数少且物理意义明确.      

含水合物黏土的力学性质试验研究

基于含四氢呋喃水合物黏土样品在不同水合物饱和度、围压及水合物分解前后的高压三轴剪切试验和超声波测量数据,分析了含水合物黏土的应力-应变关系和强度特性。试验结果表明:(1)含水合物黏土的应力-应变曲线展现出弹性、塑性变形以及应变硬化三个阶段(在应变低于1.5%时近似为弹性,在应变2%~6%范围内表现为塑性,在大于6%后呈现明显的应变硬化特性),与不含水合物黏土的应力-应变关系有明显不同;(2)含水合物黏土在水合物分解前后的应力-应变关系存在明显的不同,水合物分解后比水合物分解前的不排水强度值降低程度最大为50%;(3)含水合物黏土的不排水抗剪强度随水合物饱和度和围压的增加而增大,并比不含水合物黏土的强度提高1~6倍。上述结果表明水合物的存在增强了黏土颗粒之间的连结或胶结作用。     

有效应力升降过程中不同饱和度水合物沉积物渗透率实验研究

为了研究天然气水合物开采过程中水合物饱和度和有效应力变化对含水合物沉积层渗透率变化规律的影响,利用自主研制的水合物沉积物合成与三轴渗流实验一体化装置,进行了有效应力升降过程中不同饱和度水合物沉积物渗透性实验。结果表明:有效应力升降过程中,不同饱和度水合物沉积物的渗透率与有效应力均呈负指数规律变化,并表现出在低有效应力阶段渗透率变化的幅度大于高有效应力阶段;有效应力升降过程中,试样产生的不可恢复变形使渗透率不能完全恢复,造成渗透率永久损失;有效应力不变时,水合物沉积物的渗透率与水合物饱和度呈负指数规律变化,且曲线的斜率随饱和度增加由大变小;水合物的饱和度越大,最大渗透率损害率越大,渗透率恢复的程度越差。     

石英砂粒径对水合物沉积物力学性质的影响

利用自主研发的水合物沉积物原位合成与力学性质测试的高压低温三轴仪,通过多级加荷的试验方法,以不同粒径的砂粒作为沉积物骨架进行三轴压缩试验,得到了剪切过程的应力-应变关系曲线,以及不同粒径尺寸沉积物的强度,还有剪切过程中的体积变化关系。结果表明:含水合物沉积物强度随着沉积物粒径尺寸的增大而增强;在降压剪切过程中,所有粒径的水合物沉积物式样均有明显的剪缩现象。     

基于瞬态压力脉冲法的含水合物沉积物渗透性实验研究

水合物藏的渗透特性是判断工业开采可行性的重要参数。为了研究以粉细砂为主构成的多孔介质中水合物对其渗透率的影响,基于瞬态压力脉冲法的原理及求解方法,研制了一套适用于测量含水合物沉积物渗透率的实验装置。利用该装置对含不同饱和度二氧化碳水合物的粉细砂样品进行了渗透率测量实验。结果表明,随着粉细砂样品中二氧化碳水合物饱和度的增加,其渗透率呈指数衰减趋势;将实验数据与渗透率模型进行对比,发现实验数据与水合物占据毛细管中心的平行毛细管模型最为相近。据此推断,水合物在细砂样品中的赋存状态以悬浮模式或接触模式为主。     

辽西风积土动力特性实验研究

通过一系列动态三轴实验,研究了原状饱和辽西风积土在不同固结压力条件下的动态强度、动态弹性模量和阻尼比。结果表明,辽西风积土的破坏动态强度随着固结围压和固结应力比的增大而增大,而动态剪切强度明显区别于静态剪强度;其动态弹模量随着固结应力比和固结围压的增加而增加,而阻尼比随着固结应力比和固结围压的增大而减小。同时,分别拟合得到最大动态弹模量和最大阻尼比与有效固结应力的关系式,并且对动态弹模量和阻尼比进行了归一化处理。     

含水合物沉积物的弹塑性本构模型

土的密度对其力学特性具有明显影响.水合物以一种固相赋存于沉积物的孔隙中,使得水合物的含量和其赋存形式都会影响含水合物沉积物(GHBS)的密度,因此在研究和描述含水合物沉积物的力学性质时应考虑水合物含量和赋存形式对其密度的影响.本文基于黏土和砂土统一的本构模型(CSUH模型),首先建立水合物体积分数与压硬性参量的关系式来反映水合物对沉积物压缩规律的影响.其次,为了合理考虑水合物含量和赋存形式对沉积物密度的影响,建立了可以描述有效初始孔隙的计算式,并将其引入到状态参量中来描述水合物对沉积物剪胀性和峰值强度的影响.最后,结合CSUH模型中水滴形屈服面,建立了一个含水合物沉积物的弹塑性本构模型.通过与室内试验结果比较,验证了该模型不仅能够合理地描述不同赋存形式、不同水合物含量下含水合物沉积物的应力应变关系,而且在描述具有相同赋存形式含水合物沉积物的力学特性时,不同的水合物含量只需采用一组参数.     

含水合物沉积物的弹塑性本构模型

土的密度对其力学特性具有明显影响.水合物以一种固相赋存于沉积物的孔隙中,使得水合物的含量和其赋存形式都会影响含水合物沉积物(GHBS)的密度,因此在研究和描述含水合物沉积物的力学性质时应考虑水合物含量和赋存形式对其密度的影响.本文基于黏土和砂土统一的本构模型(CSUH模型),首先建立水合物体积分数与压硬性参量的关系式来反映水合物对沉积物压缩规律的影响.其次,为了合理考虑水合物含量和赋存形式对沉积物密度的影响,建立了可以描述有效初始孔隙的计算式,并将其引入到状态参量中来描述水合物对沉积物剪胀性和峰值强度的影响.最后,结合CSUH模型中水滴形屈服面,建立了一个含水合物沉积物的弹塑性本构模型.通过与室内试验结果比较,验证了该模型不仅能够合理地描述不同赋存形式、不同水合物含量下含水合物沉积物的应力应变关系,而且在描述具有相同赋存形式含水合物沉积物的力学特性时,不同的水合物含量只需采用一组参数.      

降压开采模拟试验的水合物分解阵面演化过程

水合物分解阵面演化过程与开采安全性和产气效率密切相关,是开采原位监测的重要组成部分。在玻璃砂样品中进行了甲烷水合物降压开采模拟试验,探讨了水合物饱和度对渗流阵面和水合物分解阵面演化过程的影响,结合已有理论模型,分析了水合物分解阵面传播速度的关键影响因素。结果表明:渗流阵面和水合物分解阵面的传播距离均与时间平方根呈近似线性关系,传播速度均随水合物饱和度的增加而减小;水合物分解阵面的传播速度随多孔介质的有效渗透率和降压幅度的增加而变快,随孔隙率的增加而变慢,粗砂质地层更有利于水合物降压分解阵面的传播。     

Discrete element analysis of the mechanical properties of deep-sea methane hydrate-bearing soils considering interparticle bond thickness

Due to increasing global energy demands, research is being conducted on the mechanical properties of methane hydrate-bearing soils (MHBSs), from which methane hydrate (MH) will be explored. This paper presents a numerical approach to study the mechanical properties of MHBSs. The relationship between the level of MH saturation and the interparticle bond thickness is first obtained by analyzing the scanning electron microscope images of MHBS samples, in which is the bridge connecting the micromechanical behavior captured by the DEM with the macroscopic properties of MHBSs. A simplified thermal-hydromechanical (THM) bond model that considers the different bond thicknesses is then proposed to describe the contact behavior between the soil particles and those incorporated into the discrete element method (DEM). Finally, a series of biaxial compression tests are carried out with different MH saturations under different effective confining pressures to analyze the mechanical properties of deep-sea MHBSs. The results of the DEM numerical simulation are also compared with the findings from triaxial compression tests. The results show that the macromechanical properties of deep-sea MHBSs can be qualitatively captured by the proposed DEM. The shear strength, cohesion, and volumetric contraction of deep-sea MHBSs increase with increasing MH saturation, although its influence on the internal friction angle is obscure. The shear strength and volumetric contraction increase with increasing effective confining pressure. The peak shear strength and the dilation of MHBSs increase as the critical bond thickness increases, while the residual deviator stress largely remains the same at a larger axial strain. With increasing the axial strain, the percentage of broken bonds increases, along with the expansion of the shear band.     

Thermodynamic Conditions of Formation of CO<Subscript>2</Subscript> Hydrate in Carbon Dioxide Injection into a Methane Hydrate Reservoir

The mechanism of replacement of methane by carbon dioxide in the hydrate in the process of CO₂ injection into a reservoir with formation of fronts of methane hydrate dissociation and carbon dioxide hydrate generation is investigated. It is found that such a replacement regime can be implemented in both low- and high-permeability reservoirs. It is shown that in the highintensity injection regime the heat flux from the well does not affect propagation of the fronts of methane hydrate dissociation and carbon dioxide hydrate generation. In this case the replacement regime is maintained by only the heat released at formation of carbon dioxide hydrate. An increase in the injection pressure may lead to suppression of methane hydrate dissociation and termination of the replacement reaction. The critical diagrams of existence of the regime of conversion of methane hydrate to carbon dioxide hydrate are constructed.     

Exploring the undrained cyclic behavior of methane-hydrate-bearing sediments using CFD–DEM

Based on the mechanical experimental results of methane hydrate (MH), a bond contact model considering the rate-dependency of MH is proposed. A CFD–DEM scheme considering fluid compressibility is used to simulate a series of undrained cyclic shear tests of numerical methane-hydrate-bearing sediment (MHBS) samples. The dynamic behavior, including stress–strain relationship, dynamic shear modulus, and damping ratio, is investigated. In addition, the force chains, contact fabric and averaged pure rotation rate (APR) are examined to investigate the relationships between micromechanical variables and macromechanical responses in the DEM MH samples. The effects of temperature, confining pressure and MH saturation are also analyzed. Due to the micro-structural strengthening by the MH bonds, no obvious change in microscopic quantities is observed, and the samples remain at the elastic stage under the applied low-shear stress level. When confining pressure and MH saturation increase, the dynamic elastic modulus increases, while the damping ratio decreases. An increasing temperature (leading to weakening of MH bonds) can lower the dynamic elastic modulus, but has almost no impact on the damping ratio. On the contrary, an increasing cyclic shear stress level lowers the damping ratio, but has almost no effect on the dynamic elastic modulus.     

Modeling of Gas Hydrate Formation Upon Injection of Carbon Dioxide Gas into A Natural Reservoir

This paper presents the results of numerical modeling of gas hydrate formation upon injection of carbon dioxide into a finite-length reservoir saturated with methane and water. It is shown that at different stages, hydrate formation can occur on both the frontal surface and in a reservoir region of finite length. The effects of pressure at the reservoir boundaries and the effects of the permeability and initial water saturation of the reservoir on the hydrate formation process were studied. The dependences of the time of the complete conversion of water into gas hydrate in the entire reservoir on the injection pressure and reservoir permeability were obtained.     

Experimental study on mechanical properties of methane-hydrate-bearing sediments

Mechanical properties of methane hydrate-bearing-sediments (MHBS) are basic parameters for safety analysis of hydrate exploration and exploitation. Young’s modulus, cohesion, and internal friction angle of hydrate-bearing sediments synthesized in laboratory, are investigated using tri-axial tests. Stress-strain curves and strength parameters are obtained and discussed for different compositions and different hydrate saturation, followed by empirical expressions related to the cohesion, internal friction angle, and modulus of MHBS. Almost all tested MHBS samples exhibit plastic failure. With the increase of total saturation of ice and methane hydrate (MH), the specimens’ internal friction angle decreases while the cohesion increases.     

Recovery of Methane From Gas Hydrates in a Porous Medium by Injection of Carbon Dioxide

This paper presents a mathematical model for methane hydrate–carbon dioxide replacement by injection of carbon dioxide gas into a porous medium rich in methane and its gas hydrate. Numerical solutions describing the pressure and temperature variation in a reservoir of finite length are obtained. It is shown that the replacement process is accompanied by a decrease in pressure and an increase in temperature of the porous medium. It is established that during the time of complete replacement of methane from a reservoir decreases with increasing permeability of the porous medium and the pressure of the injected gas.     

Methane Hydrate and Two-Dimensional Fluid Transport Model: Comparison with Blake Ridge Chlorinity Measurements

The potential benefits and risks of natural methane hydrate occurrences have raised the need to understand the processes governing hydrate formation and dissociation over the last decades, and models of increasing complexity have been developed for this purpose. We propose a formulation of a multi-dimensional methane hydrate model that couples the established chemistry of hydrate formation to the fluid flow in marine sediments undergoing compaction. The numerical model applies the Finite Volume Method to construct a segregated solver for the coupled system. The solution of the sequence of individual processes is stabilised with an adaptive Picard iteration in each time step. We implement the model based on the OpenFOAM library and extend the functionality by generalising the diffusion term to take into account variable porosity of the sediment matrix. The advantage of this robust and efficient scheme is that the formulation is conservative by construction, allowing an accurate solution of mass transport when strong concentration gradients develop in the vicinity of phase boundaries. We validate the model using data from the Blake Ridge hydrate province. Good agreement is found for the pore water chlorinity, a proxy for hydrate formation, except for regions where heterogeneous hydrate formation results in highly variable measurements.