废弃矿井瓦斯-水-煤岩相互作用及其两相流动力学

Coal is the dominant fossil energy for China. During the past decades of high intensity mining thousands of coal mines have been closed because of the coal exhaustion or national policy. One ubiquitous environmental issue ongoing with the closed coal mine is the potential for gas leakage from the underground sites. Concerns about leakage linked to increases in the close of coal mine require a better understanding of how to detect and quantify composition, gas content and leaking performance of gas-in-place accurately which is a significant obstacle to the implementation of gas recovery, greenhouse gas control and groundwater contamination management of the abandoned coal mine. The gas in abandoned coal mine mainly include CH4, CO2, H2S and CO, existing in the adsorbed, dissolved and free species. Since the recovery of water level after the close of coal mine, hydrogeochemistry conditions as well as the gas flow performance in the flooding mine have varied dramatically, which pose significant factors controlling the gas existing species and lead to complex gas-water-rock interactions in the mine pool. Gas ebullition, diffusion, and migration along with the groundwater in the leakage scenario of the mine pool is a typical reactive two phase flow in porous media. The goal of this project is to reveal the mechanism of gas-water-coal and rock interaction and dynamics of two phase flow in the abandoned coal mine. In order to determine the composition, existence state and hydrochemical controlling factors of gas-in-place, physiochemical analysis and environmental isotope test of both groundwater and gas are proposed in the study. Furthermore, this project will present experimental studies on gas adsorption, gas-water-coal interaction and gas flow at different temperatures, pressure, aquatic chemical conditions with multi component gases for addressing gas adsorption and dissolving behaviour and impacts by different hydrogeochemical conditions. Whilst impacts of dissolved gas on physiochemistry of groundwater and coal are detected, mechanism on the interactions of gas, groundwater and coal will be revealed and the hydrogeochemical reaction model will be concreted. A coupled two phase flow model assembling gas dissolving, gas diffusion and two phase flow in the porous media will be established mathematically based on the analysis of gas migration and leakage potential in the abandoned coal mine. The special modelling geometry and hydrogeological parameters of the abandoned mine will be quantitively characterized by the simulating experiment and field investigation results. A COMSOL Multiphysics simulator of the multiscale reactive two phase flow will be developed to visualize the migration of gas flow at abandoned coal mine. The migration rules and dynamics of gas leakage at abandoned coal mine will be concluded based on the numerical simulation.

井工煤矿关闭后矿井地下水位回弹和地应力场重新调整，导致水文地球化学场、瓦斯渗流场显著改变。由此，废弃矿井瓦斯一方面可能缓慢逸出而成为温室气体，甚至通过煤－水－气相互作用而污染地下水系统，但另一方面可通过工程抽出而变成宝贵的洁净能源。本项目针对这一领域中的瓦斯-水-煤岩相互作用及其两相流问题，研究废弃矿井瓦斯重新分布与赋存特性，阐释水文地球化学条件变化及其控制因素对残留瓦斯赋存、析出和迁移的影响机理。在此基础上，通过不同温压、不同水化学条件、多元气体吸附模拟实验、气-水-煤反应模拟实验和瓦斯渗流实验模拟，进一步研究废弃矿井瓦斯吸附和析出规律，探讨废弃矿井瓦斯-水两相流运移特点，分析残留瓦斯-地下水-煤相互作用机理和气体组分与数量的分配规律，构建适合于废弃矿井瓦斯的多孔介质气-水反应性两相流数学模型，从多尺度上揭示废弃矿井瓦斯-水反应性两相流动力学机制。

含瓦斯井工煤矿在关闭后仍残留瓦斯，并伴随地下水位回升在圈闭环境中形成了闭坑矿井水-煤岩-气三相多组分体系，三相反应与气水运移可发生瓦斯泄露及地下水污染风险。本项目通过理论研究、现场调研、模拟实验和数值模拟，（1）研究了关闭矿井瓦斯重新分布与地下水污染模式，阐释了关闭瓦斯矿井的水文地球化学特征，高瓦斯矿井水以酸性矿井水居多，矿化度及硫酸盐含量相对较高，水化学类型以SO4-Ca•Mg型较多；（2）研究了长时序多温压模拟实验条件下酸性、高铁、高矿化度矿井水-CH4/CO2-煤岩三相反应特性，获取了煤岩特征矿物溶蚀与沉淀机理以及煤中矿物和微量元素迁移特性。煤岩中的碳酸盐（方解石、白云石）溶蚀形成新晶面；长石向伊利石演化，原生伊利石向高岭石演化；黄铁矿表面和菱铁矿表面形成了新的混合碳酸盐矿物和石膏矿物，可堵塞微孔；碳酸盐和黏土矿物的溶解使得矿物元素发生迁移，闭坑矿井水中K、Na、Ca、Mg、Al、Si元素含量增加，高CO2环境下闭坑矿井水中常量元素含量较高，且Mn和Ni等微量元素含量超过地下水Ⅲ类水质量标准；（3）研究了三相反应下不同煤级煤中孔隙重新分布和吸附性能变化，各煤级煤中低于100nm孔隙和大于30μm孔隙数量显著增加，高阶煤大孔孔容变化幅度最大为473.38%，对应煤的吸附性能成下降趋势；（4）水岩气三相反应中煤渗透率的变化增量由0.0013mD增长至0.0076mD，变化率从3.23%增长至16.03%，揭示了裂隙溶蚀扩展效应；（5）基于PHREEQC开发了闭坑煤矿地下水-岩石矿物-气反应数值模拟平台，揭示了地下水pH、Eh、离子组成和矿物相以及气体分布规律；（6）基于COMSOL模拟了关闭矿井中水、气及污染物的迁移规律，构建了关闭煤矿地下水污染与气体运移数值模拟平台。上述成果揭示了关闭矿井瓦斯-地下水-煤岩相互作用机理和水文地球化学模式，可预测瓦斯对关闭煤矿地下水的污染潜势，为矿区地下水污染防控和瓦斯次生富集评价提供科学依据和技术支持。