天然气水合物沉积物分解过程中本构关系研究

天然气水合物具有储量大、分布广泛、清洁燃烧等优点,近年来受到研究人员的广泛关注。为了实现天然气水合物资源的安全高效开采,对其沉积层的力学稳定性进行系统评估是十分必要的。本研究在实验室内重塑了40%孔隙度的天然气水合物沉积物试样,并基于力学实验设备,对其在不同围压条件下分解过程中的力学强度及变形进行了一系列测试,获取了相应的应力应变数据。研究结果表明,水合物分解会造成沉积层强度的降低。此外,基于实验数据,在借鉴土力学邓肯-张本构模型的基础上,考虑了围压及分解时间对沉积物力学特性的影响,本文构建了适用于不同围压条件下天然气水合物沉积物分解过程中的本构模型,研究结果表明,该模型可以较好地模拟沉积物试样在分解过程中的应力应变关系,可为实现天然气水合物的安全开采提供一定的理论依据。     

含水合物沉积物的渗透率实验研究

天然气水合物储层的渗透率是影响水合物开采时气、水运移的关键,也是水合物开采潜力评价、资源评价、开采工艺选择等需要了解的关键参数.目前,国内外学者对天然气水合物储层的渗透率进行了一定的研究并有了初步认识.但是,对于围压、轴向压力、水合物饱和度、赋存模式等对水合物沉积物渗透率的影响机制和机理还不清楚.本文在自主设计的水合物...     

Modeling the mechanical response of gas hydrate reservoirs in triaxial stress space

Exploitation of gas from deep-sea methane hydrate-bearing layers might lead to some geological disasters, including marine landslides and excessive settlement of marine ground. The first offshore gas production tests for methane hydrate-bearing sediments were carried out in eastern Nankai Trough. However, knowledge on mechanical behavior of gas hydrate reservoirs with similar gradation and minerology component to the marine sediment is still insufficient. Consequently, proper modeling of geomechanical properties of methane hydrate-bearing sediments is crucial for reservoir simulation and deep ocean ground stability analysis for long-term gas production in the future. This study conducted a series of triaxial shear tests to examine the shear response of methane hydrate-bearing sediments with a similar grading curve and minerology components to the hydrate-rich sediments in Nankai Trough. The test results demonstrated that the presence of hydrate mass between sand grains altered the stress-strain pattern from strain-hardening to postpeak strain-softening. A simple constitutive model based on several empirical relationships of granular materials is proposed to describe the stress-strain relationship of methane hydrate-bearing sediments under triaxial stress condition. This model can reproduce the enhancement of shear strength, initial stiffness, and dilation behavior of methane hydrate-bearing sediments containing different amounts of fines content with a rise in the methane hydrate saturation at a wide range of effective confining pressures. The numerical results indicate that the parameter A associated with initial stiffness of stress-strain curve and the parameter $\text{α}$ related with dilation properties are jointly governed by the confining pressure, fines content, and hydrate saturation.     

A state-dependent subloading constitutive model with unified hardening function for gas hydrate-bearing sediments

Natural gas hydrate in ocean sediments and permafrost areas may become a significant potential energy resource. Since the hydrate dissociation may affect the stability of production wells and even lead to geological hazards, it is essential to study the mechanical properties of gas hydrate-bearing sediments (GHBS) for the efficient and safe extraction of gas hydrate. This study presents an extended subloading Modified Cam-Clay model for clayey-silty and sandy GHBS. The state-dependent unified hardening function and equivalent skeleton void ratio are newly introduced to consider the coupled effects of stress level, void ratio, dilatancy, and hydrate saturation on the mechanical behavior of GHBS. Based on the theory of hyperelasticity, an elastic constitutive relation accounting for the influences of cementation caused by hydrate formation and sediment structure change caused by hydrate dissociation is established by using a stiffness evolution function related to hydrate saturation. The bonding and debonding law of strength reflecting the hydrate cementation and its degradation are used. The model is applied to simulate different triaxial tests of GHBS, and its performance in predicting the isotropic compression, strain hardening and softening, shear contraction and dilation, and collapse induced by hydrate dissociation is investigated.     

天然气水合物储层力学特性研究进展

自然界中的水合物一般产出于深水海底浅层未固结成岩的松散沉积物中和陆域冻土区岩石裂隙或孔隙中。水合物的分解会导致地层胶结强度、孔隙度、地质结构等发生变化，从而引发地质灾害，严重威胁水合物资源的安全开采。本文在大量调研文献的基础上，结合已有的天然气水合物制样、三轴力学测试研究现状和本构模型研究进展，系统分析影响含水合物沉积物的力学特性的主要因素和本构模型的发展趋势，梳理了获得的共识和存在的问题，提出了下一步研究方向，从而为下一步含水合物沉积物力学强度实验、本构模型开发以及储层稳定性研究等提供参考。     

Shear strength and pore pressure characteristics of methane hydrate-bearing soil under undrained condition

Hydrate exploitation requires a deep understanding on the mechanical behavior of methane hydrate-bearing sediment (MHBS). Due to the low permeability of overlying strata, partial MHBS likely exhibit failure behavior under undrained condition. Therefore, it is essential to understand the undrained shear strength and excess pore pressure behavior of MHBS for facilitating the evaluation of the stability of hydrate-bearing layer during methane hydrate recovery. This study conducted several undrained triaxial compression and hydrate dissociation tests on methane hydrate-bearing sand specimen to analyze the shear strength and excess pore pressure characteristics of MHBS under undrained condition. The experimental result shows that hydrate saturation and initial effective confining pressure significantly affect the undrained mechanical behavior of MHBS. Hydrate saturation increases the shear strength and negative excess pore pressure. High initial effective confining pressure also enhances the shear strength but suppressed the negative excess pore pressure. Hydrate saturation has a minimal effect on the undrained internal friction angle but remarkably enhances the undrained cohesion. The effective internal friction angle and cohesion exhibits an increase with the increase in hydrate saturation. Notably, completely different from the common soil, the effective undrained strength indexes are not equivalent to the drained strength indexes for MHBS, which should be careful in evaluating the stability of methane hydrate-bearing layer. In addition, the hydrate dissociation test by thermal stimulation method concludes that hydrate dissociation induces the positive excess pore pressure, axial compression, and volume expansion under undrained condition. The large deviatoric stress enhances volume expansion of MHBS but hinders the generation in excess pore pressure during hydrate dissociation. These findings significantly contribute to the safe exploitation process of methane hydrate.     

广西沿海硫酸盐反复侵蚀地聚物混凝土力学试验与损伤特性研究

广西沿海地区海水浪溅及频繁降雨均会导致混凝土构件受到干湿交替作用的影响.通过室内试验探究了干湿交替作用下地聚物混凝土的应力—应变关系、抗压强度与耐侵蚀系数等力学性能.试验结果表明,地聚物混凝土的应力—应变曲线可分为线性增长、减速增长、缓慢下降三个阶段,分别对应试件的弹性变形、弹塑性变形及破坏阶段.不同干湿交替次数条件下...     

含水合物沉积物力学-声学联合试验设备的开发和初步应用

为开展天然气水合物沉积物力学、声学特性的试验研究，对高压-温控三轴试验设备的管道线路进行重新规划，并引入GCTS超声波测试系统。为了检验仪器性能，利用福建标准砂和甲烷气体制备不同水合物饱和度的沉积物试样，开展不同有效围压下的三轴排水剪切试验。结果表明：设备能稳定制备含水合物试样并进行力学试验，试验全过程可实时测试试样的压缩波速和剪切波速；波速变化与试样内部密实度、颗粒分布的变化等密切相关，可反映试样体积的变化及剪切初期的剪胀特性；通过固结后的波速计算试样的泊松比，在0.16～0.31之间。     

Compressional wave velocity of hydrate-bearing bentheimer sediments with varying pore fillings

A potential alternative energy resource to meet energy demands is the vast amount of gas stored in hydrate reserves. However, major challenges in terms of exploration and production surround profitable and effective exploitation of these reserves. The measurement of acoustic velocity is a useful method for exploration of gas hydrate reserves and can be an efficient method to characterize the hydrate-bearing sediments. In this study, the compressional wave velocity (P-wave velocity) of consolidated sediments (Bentheimer) with and without tetrahydrofuran hydrate-bearing pore fillings were measured using the pulse transmission method. The study has found that the P-wave velocity of consolidated sediments increase with increasing hydrate formation and confining pressure. Of the two samples tested, the increase in wave velocity of the dry and hydrate-bearing samples amounted to 27.6% and 31.9%, respectively. Interestingly, at the initial stage of hydrate formation, there was no change in P-wave velocity, which was followed by a steady increase as the hydrate crystals began to agglomerate and then it increased rapidly to a constant value, suggesting that the test solution had converted to a hydrate solid.     

考虑临界状态的堆石料广义塑性本构模型

基于大型静力三轴剪切试验结果,构造了考虑颗粒破碎的塑性模量表达式和考虑临界状态的剪胀方程,建立了适用于堆石料的广义塑性本构模型,并利用本构模型对堆石料三轴试验进行预测。结果表明,考虑和不考虑临界状态的本构模型均能较好地模拟堆石料在高围压下的剪缩性、在低围压下的剪胀性以及由于颗粒破碎引起峰值和剪胀应力比的非线性特性;考虑临界状态的本构模型能模拟应力——应变曲线逐渐逼近临界状态的规律,但其对三轴试验的预测精度略低于不考虑临界状态本构模型。     

“CH4-CO2”置换法开采天然气水合物

天然气水合物作为一种储量巨大的清洁能源,其开采价值已引起越来越多的关注。当前天然气水合物开采技术仍不足以商业化应用,还需进一步的实验研究进行理论支撑。二氧化碳置换开采天然气水合物被认为是能同时实现天然气水合物开采与二氧化碳减排、封存的双赢技术,近年来得到了较广泛的研究。置换过程中,混合天然气水合物的热力学和结构性质是预测含水合物沉积物中的热流和水合物解离所需的热量以及评估水合物储层的CO₂储存能力的关键因素。本综述在调研国内外天然气水合物开采技术研究现状的基础上,围绕CO₂置换开采天然气水合物的热力学特性、微观机理与置换效率等,总结了各研究取得的成果,针对置换研究中存在的问题进行了分析,认为当前置换法开采天然气水合物最关键难点在于提高置换效率,而解决该问题的根本在于从热力学和动力学角度弄清楚置换反应的微观机理及控制性因素,明确置换机理,从而在未来的研究有的放矢。     

海洋天然气水合物开采的固相控制策略*

天然气水合物是一种潜在的洁净能源资源,我国南海有丰富的储量,被认为是后石油时代的重要战略资源之一,然而目前大部分的海洋天然气水合物开采都受到出砂影响。针对深水天然气水合物开采过程中易出砂的现象,通过前期的出砂实验、理论和数值模拟,提出了海洋天然气水合物开采过程中固相（砂和水合物）控制方案。总结了天然气水合物开采过程中出砂特性和防砂案例,提出大颗粒水合物对泥质粉砂具有挡砂作用,进而影响防砂设计精度。据此,根据开发角度的天然气水合物藏6类细分,提出了考虑水合物颗粒本身及其分解作用的固相控制方法。结合南海天然气水合物储层公开资料,设计出相应的固相控制精度,以期最终形成出砂/防砂/井筒携砂/水合物二次生成预防为一体的固相控制体系,为南海天然气水合物安全高效地商业开发提供参考。     

静三轴试验中固化疏浚泥的力学特性研究

为正确理解固化疏浚泥的力学性质,通过对固化疏浚泥进行固结不排水静三轴压缩试验,研究了围压和固化剂掺量对固化疏浚泥的破坏形式、应力—应变关系及抗剪强度参数的影响。试验结果表明,随着固化剂掺量的增加,固化疏浚泥的应力—应变关系由应变硬化型变为应变软化型;破坏形式由塑性破坏向脆性破坏过渡,而围压对其破坏形式无影响;固化疏浚泥的强度和抗变形能力均随着围压和掺量的增加而增大,但当围压小于土体内结构体的屈服强度时,围压对抗变形能力的影响可忽略;粘聚力和内摩擦角随固化剂掺量分别呈对数型和线性规律增长。     

某区域岩石力学参数试验研究

通过对松辽盆地龙西地区代表性岩样进行力学性质试验研究.得到岩石的弹性模量、泊松比、抗压强度、内聚力及内摩擦角等主要力学参数.试验结果显示,最初围压不断增大,岩石的强度极限也明显增大.但随着围压的增加,强度增量幅度变得越来越小,虽然岩石弹性模量有增大趋势,但不完全呈正比关系,而泊松比的变化规律却不明显.分析单轴与三轴抗压强度与弹性模量的线性关系,以及围压对岩石三轴抗压强度、弹性模量的影响,发现单轴和三轴抗压强度随弹性模量的增加而增大,且三轴抗压强度随弹性模量的变化比单轴的变化幅度大.试验可得出不同井的弹性模量、泊松比及抗压强度的比例关系,从而根据不同围压下三轴压缩强度试验,得出龙西地区几种代表性岩心的内聚力及内摩擦角.为后续的地应力测试实验提供初期数据,并为射孔完井得相关参数优化提供参考.     

基于电-力-声多物理场耦合数值模型的含水合物多孔介质声速和衰减特性研究

在物理模拟实验中对水合物微观赋存模式和饱和度进行准确控制和评价尚存在技术困难,仅依赖实验技术研究含水合物沉积物声学特性、建立储层参数解释模型存在局限性。采用基于有限元的数字岩石物理技术,针对悬浮、接触和胶结三种典型的水合物微观赋存模式分别建立多孔介质的三维电-力-声多物理场耦合模型,考察了微观赋存模式和水合物饱和度对多孔介质声速和衰减的影响规律,对比了声速数值模拟与理论模型计算结果,建立了声波衰减参数与水合物饱和度之间的关系式。研究结果表明：（1）对于三种水合物赋存模式,由于水合物相比孔隙水具有更高的弹性模量,多孔介质的声速随着水合物饱和度的增大而增大;水合物的存在导致声波在传播过程中遇到更多不连续的声阻抗界面,声衰减随着水合物饱和度的增大而近似线性增大;（2）悬浮和接触赋存模式条件下,水合物饱和度对多孔介质的声速和衰减影响规律基本一致;对于相同的水合物饱和度,胶结模式条件下含水合物多孔介质具有更高的声速和更小的声衰减;（3）通过合理选择参数值,利用权重方程与Lee改进的Biot-Gassmann Theory(BGTL)模型估算的含悬浮和接触模式水合物多孔介质的声速较为准确;通过等效...     

Experimental study on dynamic acoustic characteristics of natural gas hydrate sediments at different depths

Acoustic properties are important geophysical parameters, compared to resistivity, pore water and other parameters that need to be obtained through drilling, the acoustic wave velocity of hydrate deposits is much easier to obtain. Therefore, the study of acoustic characteristics of hydrate reservoir is the basis for geophysical exploration and resource evaluation for hydrates. In this study, an experimental apparatus was developed to measure dynamic P- and S-wave velocity of gas hydrate bearing sediment. The effect of sensitivity factors including hydrate saturation, confining pressure, and reservoir solid phase particle size on the acoustic characteristics of hydrate reservoir were explored. The experimental results showed that the longitudinal and transverse wave velocities of natural gas hydrate rock samples correlated positively with hydrate saturation and confining pressure and the particle size of the solid phase exhibited little effect on the vertical and horizontal wave velocity of the gas hydrate reservoir. The results of this study indicated that the compression factor of different rock samples is the main factor affecting the vertical and horizontal wave velocity of the reservoir through mechanical experiments. At the same time, acoustic experiments and computed tomography results revealed the existence of different contact modes between natural gas hydrate and sediment particles under different saturations. Furthermore, the possibility of detecting the microscopic distribution of hydrates in sediments by acoustic waves was verified. The data can be used in well logging to determine hydrate saturation and other properties of hydrate bearing formations.     

基于宽频复电阻率的含黏土沉积物中水合物饱和度计算方法

天然气水合物沉积物中的黏土成分显著影响沉积物的电学特性以及水合物饱和度的计算模型。以自主设计开发的复电阻率参数测量装置为实验平台,在20 Hz ~ 100 kHz频率范围内测试了黏土条件下含水合物海沙体系的复电阻率,分析了复电阻率的频散特性、黏土影响以及主导的电极化机制,利用泥质修正Archie公式建立了基于宽频复电阻率的水合物饱和度计算模型。研究结果表明：（1）含水合物海沙体系的复电阻率呈现出显著的频散特性,双电层极化和界面极化分别是20 Hz ~ 1 kHz和1 ~ 100 kHz频段主导的极化机制;（2）黏土颗粒表面的双电层发生形变增强了含水合物沉积物双电层极化作用,提高了复电阻率相角和虚部的绝对值;（3）在20 Hz ~ 100 kHz频率范围内泥质修正Archie公式岩性系数较为稳定,而胶结指数和饱和度指数与测试频率之间的关系在20 Hz ~ 1 kHz和1 ~ 100 kHz频段差异显著,因此应分频段建立水合物饱和度计算模型。本研究为探讨含水合物沉积物的低频电学特性和建立含黏土沉积物中水合物饱和度计算模型提供了新的途径。     

天然气水合物开采技术研究进展

天然气水合物以其储量大、能量密度大、分布广的特点被认为是一种非常具有潜力的替代能源。勘测数据表明迄今已至少在全球116个地区发现了天然气水合物。天然气水合物广泛存在于冻土区和海底沉积物中。但目前实现实际试开采的区域仅有四处,分别位于：美国的阿拉斯加北坡地区、俄罗斯的西伯利亚玛索亚哈气田、加拿大西北部的麦肯齐三角洲及日本的南海海槽。目前主要的开采技术研究主要集中于实验室模拟阶段。美国、德国、日本、中国分别建立了自己的水合物模拟开采实验装置并且进行了相关研究。提出了不同的水合物开采方案,并且对水合物开采过程中的关键技术问题进行了研究。     

基于TDR双参数的沉积物中水合物饱和度测量

针对沉积物中水合物饱和度的测量问题,立足于时域反射技术（TDR）能够同时获得含水合物沉积物表观介电常数和电导率的优点,提出了一种基于介电常数/电导率双参数的水合物饱和度评价新方法。在分析介电常数和电导率测量原理的基础上,设计了四氢呋喃（THF）水合物模拟实验与参数测量系统以及实验方案;基于TDR测量响应分析了孔隙水电导率对水饱和的以及含水合物的模拟沉积物介电常数和电导率的影响;利用TDR获取的不同水合物饱和度条件下的介电常数测量数据对比分析了经典介电常数模型的性能,并以Lichteneker-Rother（LR）模型为原型建立了水合物饱和度与介电常数之间的关系模型,以阿尔奇公式为原型建立了基于TDR测量电导率的水合物饱和度计算模型。研究结果表明：采用LR模型和Maxwell-DeLoor模型能够较准确地描述TDR测量表观介电常数与水合物饱和度之间的关系;基于表观介电常数/电导率双参数的水合物饱和度联合评价方法为提高评价结果的准确度和可靠性提供了新途径。将来需要结合水合物储层实际特征开展模拟实验并进一步完善饱和度计算模型,将基于TDR双参数的饱和度联合评价方法推广应用到含天然气水合物沉积物。