富浅层气砂质海床的气侵流化机理与麻坑效应研究

Shallow gas is an important marine engineering geological disaster. Attributing to the fact that fluidization mechanism of loose seabed sediments caused by shallow gas is unclear in the marine, the formation mechanism of gas invasion leading to sediments fluidization and the gas-liquid-particle-flow coupling interaction of surface pockmark effects under conditons of complex ocean environment load and enrich shallow gas bearing seabed are to be investigated intensively in order to meet the urgent demand for geological disaster assessments and safty evaluations of offshore engineerings. Based on tests for the engineering properties and dynamical responses of unsaturated gassy sand, the hydraulic respose characteristics of shallow gas bearing seabed under the wave disturbance are to be studied, so as to analyze the evolution law of excess pore pressure gradient and pore fluid upwelling. By verification tests of X-ray computed-tomography imaging of gas migration in sediments, and with discrete element analytical methods coupling the multiphase fluid flow and sediment mechanics theory, a grain-scale mechanistic model of preferential gas invasion mode is established to explore the micro mechanism of seafloor gas seepage caused by gas invasion. Through the large-scale flume physical simulation experiments and numercial simulations considering gas-liquid-solid interaction coupled multi-physical field, critical conditons for seabed seepage leading to fluidization are proposed, with which to preliminarily reveal the surface pockmark effects formation mechanism of gas bearing sediments. The research is hoping to develop the gas-liquid-particle-flow coupling analytical method and testing technology applicated in study on marine gas seepage and shallow gas geogolical disasters, and may provide a scientific reference for the fine assessment of marine geology and the disaster prevention and reduction of ocean engineerings.

海底浅层气是一种重要的海洋工程地质灾害。针对海洋环境中浅层气诱发海床松散沉积物流化失稳成因机制认知不足的现状，面向我国东海能源开发利用工程地质灾害评估和安全性评价需求，深入研究海洋复杂环境荷载与浅层气富集海床条件下海底沉积物的气侵流化机理与麻坑效应的气液固流耦合作用机制。基于非饱和态含气砂土的工程特性与动力响应试验，研究波浪扰动下含气海床的水动力响应特征，探讨底床超孔压梯度与孔隙上升流演化规律；借助微观气侵CT扫描验证试验及耦合多相流和沉积力学理论的离散元分析方法，建立颗粒尺度气侵模式优选细观模型，解析海底浅层气气侵渗漏的微观物理机制；通过大型水槽物理模拟实验和实现气液固流作用耦合多场数值模拟仿真，给出砂床流化失稳临界条件，初步揭示含气海床麻坑地貌效应的成因机制。以期发展海洋气体渗漏和浅层气地质灾害形成的气液固流耦合分析方法与测试技术，为我国海洋地质的精细化评估和海洋工程防减灾提供依据。

本项目以我国东海典型区域的海底浅层气灾害地质和麻坑地貌为研究背景，从麻坑区域的地质特征和浅层气成因出发，探讨了海底浅层气的赋存形态、麻坑内外土层特性差异，以及海底浅层气渗漏与麻坑形成的关联性。借助样品扰动性分析，提出基于剪切波速的海底取样样品质量等级评价方法；探索了定量可控的含气土样室内人工制备技术，给出了含气样品的均匀性量化评价方法。开发弯曲元-共振柱联合测试试验装置，探讨了砂土小应变动力特性；基于循环荷载下含气砂土的液化特性试验，建立描述海底高饱和度砂质含气沉积物动力响应特性的动孔压模型与亚塑性边界面模型，并推导了波浪作用下含气海床动力响应的解析解。基于波浪作用下砂质含气海床的大型水槽物理模型试验，揭示了波浪扰动下含气砂床的动力响应及海床孔隙上升流规律，指出富含浅层气更易引起海床的瞬态液化响应，并给出瞬态液化的失稳准则。基于气侵CT扫描试验，验证了砂质海床中浅层气宏观气侵行为的细观模式，建立毛细入侵和劈裂条件判据，揭示了气侵优选模式与沉积物粒度间的关联；基于离散元DEM与格子玻尔兹曼LBM耦合方法，建立颗粒尺度细观力学模型，初步实现海底含气沉积物的气液固流全耦合分析，揭示砂质海床浅层气气侵与流化失稳的细观力学机制，阐释了海底麻坑地貌形成的物理机制。上述研究，深化了对海洋气体渗漏与麻坑效应的认识，可为海洋地质灾害评估和海洋工程防灾减灾提供理论依据。