煤基质游离瓦斯密度梯度扩散理论及双重孔隙瓦斯流动仿真研究

The law of coal gas flow is theoretical basis of mine gas control and extraction. The single permeability model of coal body and the double porosity model both mismatched the experiments and the field observation results, which weren’t used for quantitative prediction or analysis of gas extraction. On a large number of our previous studies, this project will propose a hypothesis that the mass flux of gas flow in coal matrix is proportional to the free gas density gradient, and then to establish a new theoretical model of gas flow in coal matrix driven by the free gas density gradient, which should be verified by experiments and numerical simulation of gas adsorption and desorption within coal particles. In the project, the characteristics of coal fracture will be studied, and the coupling mechanism between gas flow in coal matrix and gas flow in coal crack is also discussed. A new model of gas coupled flow in double porous medium coal will be established on that the coal matrix gas seepage field is superimposed at any point of the continuous smooth gas seepage field of fracture. Based on the above mentioned, the model of gas coupled flow of mining coal-wall and the model of coal surrounding drainages are established. A software system will be developed independently to forecast accurately gas emission at mining face and to reveal the law of gas drainage attenuation under the influences of negative pressure, borehole size and drainage time. It can provide a theoretical basis and technical means for predicting gas emission underground and designing the borehole gas drainage.

煤体瓦斯流动规律是矿井瓦斯治理及抽采的理论基础。煤体单渗透模型、双重孔隙扩散—渗流模型均与实验和现场观测结果存在较大差异，不能定量预测和分析瓦斯涌出量或抽放量。本项目在课题组大量前期研究的基础上，提出煤基质瓦斯流质量通量与游离瓦斯密度梯度成正比的假说，建立以游离瓦斯密度梯度为动力的煤基质瓦斯流动理论，通过煤粒瓦斯吸附解吸实验及数值模拟等来验证假说；研究煤体孔隙裂隙结构特征，探讨煤基质瓦斯流动与裂隙瓦斯流动的耦合机理，通过在连续光滑的裂隙瓦斯渗流场空间任一点处叠加一个煤基质瓦斯流动场，构建新的双重孔隙介质煤体瓦斯耦合流动模型；在此基础上，建立采动煤体及钻孔周围瓦斯流动的有限体积法计算模型，自主开发解算软件，科学计算不同边界条件下煤体内瓦斯非稳态流动过程，实现采掘面瓦斯涌出的理论预测，揭示负压、孔间距及抽放时间等对钻孔瓦斯抽放量及衰减的影响规律，为井下瓦斯涌出量预测及钻孔抽放设计提供理论依据。

井下煤层瓦斯抽采是预防瓦斯灾害以及预测煤层气产量的重要技术措施。作为一种包含基质和裂隙的双重孔隙介质，煤中基质系统的瓦斯扩散流动机理尚不明晰，这可能会导致对钻孔瓦斯抽采量的预测不准确的现象。本项目首先从传质角度出发，建立了游离瓦斯密度梯度驱动扩散的煤基质瓦斯扩散数值模型，提出了一种由无量纲准则和瓦斯解吸经验公式为数学桥梁的微孔道扩散系数反演新方法，并通过一系列多环境因素下的煤粒瓦斯吸附解吸实验数据全面验证了该模型的可靠性和普适性；在此基础上构建了煤基质游离瓦斯密度梯度扩散和裂隙瓦斯渗流的双重孔隙介质煤体瓦斯耦合流动模型，并自主编制了相应的数值解算软件，衍生出了一种基于现场历史数据匹配法与无量纲准则的煤层瓦斯关键扩散和渗透性能参数反演方法，通过一系列现场瓦斯抽采数据验证了模型和软件的合理性；随后调查了钻孔周围煤层瓦斯压力动态演化规律，评估了抽采负压、原始瓦斯压力、裂隙透气性系数等关键参数对钻孔瓦斯抽采和衰减规律的贡献；根据国家相关标准和规定确定了煤层钻孔有效抽采半径的基本评判指标，并结合软件模拟结果以及实测数据给出了不同煤层原始瓦斯含量区域的有效抽采半径计算式；传统径向流量法测定煤层透气性系数存在一些缺陷，而我们则给出了一种准确合理的煤层透气性系数计算方法；最后针对钻孔瓦斯抽采过程中的漏风负作用，开发了一种煤壁新型喷涂堵漏工艺，能够有效封堵煤壁漏风通道以及明显改善瓦斯抽采浓度。本项目研究内容对煤层瓦斯气体运移建模、煤矿瓦斯抽采量预测以及钻孔瓦斯抽采设计工作具有重要意义。