【摘要】 多期次载荷作用下的煤体,其孔隙结构会发生复杂变化,渗透率也随之改变。然而,不同加卸载速率与循环周期决定着煤体渗透率变化路径,影响其应力敏感性,开展循环载荷控制下煤体渗透率演化规律研究,对于解释复杂应力场下煤层渗透率的各向异性特征有理论支撑作用。借助于煤层渗透率应力敏感模型分析,研究了影响煤体渗透率变化的关键表征参数及其函数关系;为验证关键参数对煤体渗透率影响,采用预定轴压和气压、加卸载围压的方式开展煤体三轴循环变载气体渗流实验,分析在不同围压(2.0～12.0 MPa)下煤体渗透率和体应变的演化规律;为研究煤体孔隙结构变化对渗透率的影响,通过低温氮气吸附实验和荧光显微镜煤样观测统计,完成了循环载荷加卸载前后煤体孔隙结构变化对比。研究结果表明,煤体加载/卸载过程中渗透率变化趋势与围压变化负相关,总体可以分为线性段、指数段和稳定段等3个阶段;随循环加载次数的增加煤体应变逐步增大,而渗透率却随之降低;相同条件下,煤体渗透率随体应变增加而升高,增幅在16.79%以上,而渗透率恢复率逐步降低,且与围压变化负相关;3次循环加卸载实验导致煤体孔隙结构发生了显著变化,微孔体积提高71.79%,比表面积增加52.19%,而平均孔径降低32.06%,但循环载荷没有改变煤体的最可几孔径;孔隙结构变化的数据表明,微孔体积增加是煤体渗透率劣化的重要标志之一。对比循环载荷作用前后的孔隙结构实验数据发现,影响气体吸附-解吸的孔隙结构变化,决定了"迟滞环"面积,而决定"迟滞环"形状的关键因素是由煤体最可几孔径控制的突变压力。另外,煤体应变包括裂隙体积变化和孔隙体积变化两部分,其中裂隙影响重要度指标(χ)反映了裂隙体积变化在煤体应变中的权重关系,χ变化随围压升高而降低。更多还原

【Abstract】 The pore structure and the permeability of coal matrix can be changed complexly under the multi-stage loading.However,the evolution of coal permeability and its stress sensitivity are determined by the different loading/unloading rates and the cyclic cycles.It is of theoretical significance for interpreting the anisotropic characteristics of coal seam permeability under complex stress field by researching the evolution law of coal permeability under cyclic loading.With the aid of the stress sensitive model of coal seam permeability,the key characterization parameters and their functional relationships affecting permeability development were proposed in this study.As for the purpose of verifying the control effect of the key parameters on coal permeability,a triaxial cyclic variable-load gas seepage experiment was carried out by preset axial pressure,gas pressure and loading-unloading confining pressure,and the evolution laws of coal permeability and coal volume strain under different confining pressures(2.0-12.0 MPa) were analyzed.As for the influence of pore structure on permeability,the comparison of pore structure change before and after cyclic loading and unloading was completed by the low temperature nitrogen adsorption experiment and the fluorescence microscope observation.The results showed that the change trend of permeability was negatively correlated with the confining pressure in the process of loading/unloading of coal and it was divided into three stages for the whole process:linear stage,exponential stage and stable stage.With the increase of cyclic loading times,the volumetric strain of coal increased gradually,but the permeability decreased.Under the same experimental conditions,the permeability of coal increased with the increase of volumetric strain,increasing by more than 16.79%,while the permeability recovery rate decreased gradually,and was negatively correlated with the confining pressure.After three cyclic loading and unloading experiments,the pore structure of coal matrix was changed significantly,with the micro-pore volume increased by 71.79%,the specific surface area increased by 52.19%,and the average pore size decreased by 32.06%,but the most probable aperture of coal matrix was not affected by the cyclic stress.The data of pore structure change showed that the increase of micro pore volume was one of the key indicators of coal permeability deterioration.Comparing the experimental data of pore structure before and after cyclic loading,it was found that the change degree of pore structure affected gas adsorption-desorption,determined the area of "hysteresis loop",and the key to determine the shape of "hysteresis loop" was the sudden change pressure controlled by the most probable pore size of coal.In addition,the strain of coal body included the change of fracture volume and pore volume,the important index of fracture influence(χ) showed the weight relationship of the change of fracture volume in the strain of coal body,and decreased with the increase of confining pressure.更多还原