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。     

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.     

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.     

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...     

Mechanical experiments and constitutive model of natural gas hydrate reservoirs

Natural gas hydrate is a new type of green energy resources and has great development prospects, and it has attracted worldwide attentions. The exploitation of natural gas hydrate may result in a series of geological disasters. Therefore, the constitutive model of natural gas hydrate bearing sediments needs to be established to reveal deformation laws of the reservoir sediments and accurately evaluate mechanical properties of hydrate reservoirs. This is the basic guarantees for the effective exploitation of natural gas hydrate resources. The triaxial compressive tests were conducted on samples of natural gas hydrate sediment. Furthermore, the Duncan-Chang hyperbolic model was modified by considering the influences of hydrate saturation based on the test results to obtain the constitutive model according with the deformation characteristics of natural gas hydrate reservoirs. The results show that the stress-strain curves of natural gas hydrate reservoirs show unobvious compaction stage and peak strength, short elastic stage, long yield stage, and significant strain hardening characteristics. After applying loads on natural gas hydrate bearing sediments, the internal solid particles were dislocated and slid. When the loads were small, the sediments demonstrated elastic deformation. With the increase of loads, plastic flows appeared in the interior, and the hydrate crystals were re-orientated, thus the sediments showing plastic deformation. Initial tangent elastic modulus increased with the effective confining pressures, which had little correlations with hydrate saturation. Furthermore, the damage ratio increases with the increase of effective confining pressures, while slightly decreases with the increase of natural gas hydrate saturation. The predicted results of stress-strain curves of sediments with different hydrate saturations well coincide with the results of triaxial compressive tests, indicting the feasibility and rationality of this model.     

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。     

Analysis of physical properties of gas hydrate-bearing unconsolidated sediment samples from the ultra-deepwater area in the South China Sea

Marine natural gas hydrate has recently attracted global attention as a potential new clean energy source. Laboratory measurements of various physical properties of gas hydrate-bearing marine sediments can provide valuable information for developing efficient and safe extraction technology of natural gas hydrates. This study presents comprehensive measurement results and analysis of drilled hydrate-bearing sediments samples recovered from Qiongdongnan Basin in the South China Sea. The results show that the gas hydrate in the core samples is mainly methane hydrate with a methane content of approximately 95%, and the other components are ethane and carbon dioxide. The saturation of the samples fluctuates from 2%–60%, the porosity is approximately 38%–43%, and the water content is approximately 30%–50%, which indicate that high water saturation means that timely drainage should be paid attention to during hydrate extraction. In addition, the median diameter of the sediment samples is mainly distributed in the range of 15 to 34 μm, and attention should be paid to the prevention and control of sand production in the mining process. Moreover, the thermal conductivity is distributed in the range of 0.75 to 0.96 W/(m∙K) as measured by the flat plate heat source method. The relatively low thermal conductivity of hydrates at this study site indicates that a combined approach is encouraged for natural gas production technologies. It is also found that clay flakes and fine particles are attached to the surface of large particles in large numbers. Such characteristics will lead to insufficient permeability during the production process.     

甲烷水合物及其沉积层导热特性的研究现状

总结了近年来国内外甲烷水合物及其沉积层导热特性的研究现状,从实验测试和模拟研究两方面分析了甲烷水合物及其沉积层的导热影响因素、相关规律和导热机理。研究结果表明：纯质水合物的导热性能与外界温度、压力、客体分子数量、多孔介质、笼形结构等因素有关,其值大小主要由主体水分子形成的笼形结构决定;而水合物沉积层导热系数的大小主要依赖于实验样品的组成成分、初始水饱和度、各组分分布,与温度、压力以及垂直有效应力关系不大。最后,指出了现存研究方法中一些值得改进的地方,并对今后的研究工作进行了展望。     

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。     

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.     

运聚体系—天然气水合物不均匀性分布的关键控制因素初探

海域天然气水合物的聚集和分布呈现出明显的不均匀性。本文运用“天然气水合物油气系统”的理论和方法,从墨西哥湾、布莱克海台、水合物脊、南海海槽等国际典型水合物赋存区的稳定条件、气体组分和来源、流体运移、沉积条件四个方面解剖各区水合物的成藏控制因素。通过分析和对比认识到水合物赋存区的范围相对较小,海底温度和压力可视为均一条件,热成因气和生物成因气均可作为水合物的气体来源,同一个区域内的气体组分相对稳定,但有利沉积体和为含气流体运移提供通道的运移条件,也即“天然气水合物运聚体系”是控制水合物分布的关键因素。南海北部陆坡神狐海域水合物储集于颗粒相对较粗、孔隙空间相对较大沉积体中,而下部的气烟囱和断层构成了神狐海域的含气流体运移通道,这种有利的运聚体系有机结合可能是天然气水合物富集的关键因素。因此,针对水合物储集体的精细沉积学解释和流体运移通道的解剖,可能是南海北部陆坡天然气水合物勘探中需要引起重视的一个方向。     

水合物沉积物三轴试验存在的关键问题分析

三轴剪切试验是获得含水合物沉积物强度参数最有效的途径之一。目前国内研究者从不同的角度进行了大量研究,在某些关键科学问题上形成了共识,但仍存在诸多科学问题需要业内同仁共同探讨。本文在充分调研国内外含水合物沉积物三轴力学测试成果的基础上指出：由于缺乏统一的测试标准和规范,目前国内含水合物沉积物三轴剪切试验仍存在不同研究成果横向可比较性差、经验模型通用性不足、测试结果与现场实际差距大等关键科学问题;制样方法、粒度分布规律、沉积物本身固结程度、试验温度、剪切速率、气体类型、饱和度测量方法、试样尺寸等的差异是造成上述问题的关键因素。为了增强研究成果的指导意义,笔者建议各研究者在发表研究成果的同时公布详细的测试方法和测试条件,通过建立统一的行业标准来促进含水合物沉积物三轴剪切试验的发展,从而为水合物资源的安全高效开采提供科学依据。     

含甲烷水合物沉积物电–声响应特性联合探测：装置开发与实验研究

为了满足开展天然气水合物模拟实验、测试电学和声学特性参数以及建立特性参数与含水合物饱和度之间关系的需求，开发了一套电–声响应特性联合探测实验装置，以海沙模拟松散沉积物开展了甲烷水合物生成和分解模拟实验，联合探测了电学和声学参数，分析了被测体系的电学和声学响应特性。研究表明：（1）通过电–声响应联合探测实验装置能够同步获取宽频率范围电学阻抗谱和超声波接收信号波形；（2）通过设计电声复合传感器及其阵列式排布方式和“分时轮流”工作模式能够获取电学和声学参数的空间分布信息；（3）基于所提出的波动百分比和相关系数指标对不同频率阻抗模值进行评价所获得的100 kHz阻抗模值随含水合物饱和度的增加先减小后增大，经过有效压力校正后的超声波接收信号波动幅度值随含水合物饱和度的增加而升高；（4）上述两者分别可以作为分析和建立与含水合物饱和度之间关系的电学和声学有效特征参数。所开发的实验装置为将来开展含水合物复杂沉积物的模拟实验与测试工作提供了必要条件，所得到的电学和声学有效特征参数为含水合物饱和度计算模型的建立奠定了基础。     

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...     

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。     

南海北部天然气水合物沉积环境中自生矿物特征

南海北部陆坡发育的自生岩石矿物主要有碳酸盐岩、黄铁矿、石膏等,主要分布在天然气水合物最有潜力的区域,如西沙海槽、神狐海域、东沙西南和东北海域,以东沙东北部海域为自生矿物发育最密集区域。自生碳酸盐岩主要呈结核状、结壳状、烟囱状、块状等产出,神狐海域和东沙西南海域以白云石为主,西沙海槽以文石为主,东沙东北部主要以高镁方解石为主,具有较轻的碳同位素值和较重的氧同位素值;自生黄铁矿主要呈长条状,由草莓状黄铁矿组成,具有较大的硫同位素值,分布较为广泛;自生石膏主要呈球状或者块状,透明自形晶结构,分布在神狐及东沙海域部分站位。南海北部自生矿物的这些特点可为我国水合物勘探提供自生矿物方面的证据。     

西沙海槽晚更新世冰期以来原地微生物成因水合物储库的变化

利用基于热力学理论的CSMHYD程序,计算现今及晚更新世冰期琼东南盆地西沙海槽天然气水合物的稳定带厚度及资源量,讨论晚更新世冰期以来海平面、底水温度和沉积速率变化对西沙海槽天然气水合物储库变化的影响。结果表明：①研究区水深超过600 m的海域具备天然气水合物赋存的温压条件。天然气水合物稳定带最大厚度约300 m,位于研究区的中东部和东南部。②晚更新世冰期以来,底水温度的升高抵消了海平面上升对天然气水合物稳定性的影响。研究区冰期和间冰期旋回中沉积速率发生显著变化,沉积速率大的区域,原地微生物成因天然气水合物的浓度也相应较大;导致冰期-间冰期旋回过程中原地微生物成因水合物储库变化的关键因素是甲烷供给及沉积速率,而不是温压条件的变化。③西沙海槽天然气水合物现今的储库比晚更新世冰期的减少了0.78 × 10¹² m³的甲烷气。     

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。     

含水合物地层渗流特性实验研究进展及瞬态压力脉冲法适用性分析

调研了含水合物地层渗流特性实验研究进展,归纳总结了现有的缺点与不足;概述了瞬态压力脉冲法的基本原理及其应用现状,分析讨论了该方法应用于含水合物地层渗流特性实验研究的适用性。主要结论如下：保持稳定渗流难、测量耗时长和水合物易扰动是现有实验研究存在的主要不足;瞬态压力脉冲法能够解决上述不足,且测量精度高,具有良好的适用性,特别适用于我国南海含水合物地层。建议开展我国南海水合物成藏与开采过程中地层渗流特性模拟实验研究工作。     

海底天然气水合物地层钻井潜在风险及控制措施

天然气水合物是一种清洁高效能源,其广泛分布于大陆永久冻土带和海洋环境中,其中海洋环境又以海洋大陆斜坡和深海盆地为主。对海底天然气水合物地层进行钻井是研究海底天然气水合物及获得海底地层油气资源最直接的手段之一,但海底天然气水合物地层地质环境复杂,钻井面临诸多风险。本文通过综合分析海底天然气水合物地层的储层特性,探讨了海底地层中天然气水合物分解与再形成对钻井工程的影响、可能诱发的地质灾害及环境效应,并提出了相应的应对措施,以期为我国海底天然气水合物地层钻井提供参考。