孔隙中水合物含量及赋存模式对土体力学特性的影响机理

Illuminating the evolution rules of mechanical characteristics for gas hydrate-bearing soils (GHBS) under exploitation procedure is an important problem. It is demonstrated through some researches that mechanical properties of GHBS are not only related to hydrate saturation, but also controlled by occurrence morphology of hydrate in pores of soils. Up to data, however, the knowledge, which is about the impact discipline and mechanism of hydrate saturation and occurrence morphology on mechanical characteristics, is lacking, impeding commercial exploitation of gas hydrate. In this project, some samples of GHBS are manufactured with simulating the in situ formation of GHBS in water-rich and gas-rich environment, and then sheared on direct shearing test and tri-axial shearing tests conditions. With the experimental results, the influence mechanism of hydrate saturation and occurrence morphology contributing to mechanical behaviors is analyzed. Meanwhile, with analyzing the measuring phase equilibrium conditions of hydrate（P-T-Sh）in soils with different void ratio, a macro-parameters phase equilibrium formula considering the deformation effect of soil is proposed. Furthermore, a constitutive model with taking chemical phase transition and mechanical deformation into account. It is noted worthily that this constitutive model can capture the influence of hydrate occurrence morphology on mechanical properties by introducing new state concept effective hydrate saturation. In addition, based on the above works, an effective numerical procedure is developed for evaluating and analyzing the recovery-related engineering and geological hazards. This research will provide vital theoretical and technical support for mitigating production-related hazards such as borehole, local and regional slope stability and optimizing the recovery procedures for extracting methane from GHBS.

阐明开采扰动下含天然气水合物土的工程力学特性的变化规律是水合物资源开采中所必须要解决的关键问题。研究表明，含水合物土的力学特性不仅与水合物含量有关，而且还取决于水合物在孔隙中的赋存模式。目前国内外对水合物含量及赋存模式对土体力学特性的影响机理及规律缺乏系统深入的认识，严重制约了对开采扰动下场地稳定性的评价。本项目拟通过分别模拟富水环境和富气环境制备含天然气水合物土试样，开展直剪和三轴剪切试验，研究水合物含量及赋存模式对含水合物土体的力学特性的影响规律；基于系列相平衡条件测试结果，建立考虑体变影响宏观相平衡关系（P-T-Sh）；建立能考虑水合物含量及赋存模式影响和化学相变-力学变形耦合效应的本构模型；在此基础上，提出模拟开采扰动下含水合物土层变形与破坏问题的数值模型和稳定性分析方法。本研究不仅有助于阐明含天然气水合物土的力学特性及变化规律，而且为天然气水合物的有效开采提供重要的理论依据。

天然气水合物是一种潜在的战略性替代能源。阐明开采扰动下含天然气水合物土的工程力学特性的变化规律是水合物资源开采中所必须要解决的关键问题。目前的研究显示含水合物土的力学特性与水合物赋存模式和含量存在重要关系，而目前国内外对水合物含量及赋存模式对土体力学特性的影响机理及规律缺乏系统深入的认识。在此背景下，本项目通过模拟富水环境和富气环境制得了填充模式、持力体模式和胶结模式的含水合物土试样，研究了水合物含量及赋存模式含水合物土体的力学特性的影响规律，在揭示水合物颗粒与土体颗粒相互作用机理的基础上，提出采用有效水合物饱和度来描述水合物赋存模式的影响，并且在临界状态力学框架内建立了能考虑水合物含量及赋存模式影响和化学相变-力学变形耦合效应的本构模型。在模拟中，分析了水合物含量、赋存模式、水饱和度对含水合物沉积物力学特性的影响规律；此外，还分析了水合物分解对沉积物变形的影响情况。基于系列性相平衡测试结果和前人的试验结果，建立了考虑体变影响的宏观参数的相平衡模型。该相平衡模型相对于以前的模型，所有参数均为宏观参数；此外，该模型还能很好的考虑的孔隙比变化对相平衡曲线的影响。通过分析水合物形成对沉积物孔隙结构的影响，基于van Genuchten 模型提出了含水合物沉积物的持水性模型，该模型很好地考虑水合物饱和度对进气值、残余含水量的影响规律。另外，项目还提出了模拟开采扰动下含水合物土层变形与破坏问题的数值模型和稳定性分析方法。以上工作为天然气水合物的有效开采提供重要的理论依据。