超前工作面采动煤体损伤与孔裂隙结构演化特征研究

Study on coal damage and pore-fracture evolution ahead of the mining face under the disturbance of coal mining

  • 摘要: 我国大型盆地中赋存丰富的煤与煤层气资源,煤层开采扰动原始应力场、裂隙场,影响煤层气运移分布及工程抽采。利用研制的低场核磁共振真三轴试验系统,开展煤层开采扰动煤体应力−裂隙−损伤演化试验,监测载荷过程中T2谱、MRI信号变化及瓦斯抽采状况,分析不同区域煤体孔隙结构强度、损伤程度、孔隙裂隙分布与连通性的载荷响应规律。研究表明:煤体孔隙、裂隙由吸附孔(0~0.1 μm,60%~74%)、渗流孔(0.1~1 μm,26%~29%)和运移孔(>1 μm,0%~13%)构成;载荷煤体经历了弹性、弹−塑性、塑性、损伤阶段,弹性—塑性—损伤阶段煤体孔隙度分别为4.9%—2.62%—3.1%(原岩应力区域)、2.46%—1.45%—1.8%(峰值前区域)、2.57%—1.03%—1.95%(工作面区域),煤体损伤前吸附孔向渗流孔、运移孔转变,煤体损伤过程吸附孔和运移孔向渗流孔转变,孔裂隙结构连通性增强;原岩应力区域至工作面区域煤体吸附孔、渗流孔的非均匀性、各向异性增强,运移孔的连通性与均匀性增强,孔隙度载荷响应逐渐增加,孔隙扩张、节理裂隙等显著发育,吸附孔、渗流孔及渗流孔−运移孔主导了原岩区、峰值区及工作面区煤体孔隙、裂隙变化;原岩区域煤体孔裂隙发生压缩与扩张变化形成吸附型瓦斯解析−渗流通道,峰值区域煤体剪切裂隙、拉伸裂隙充分发育形成吸附−渗流−运移型瓦斯分布通道,工作面区域煤体出现大尺度剪切裂隙且拉伸裂隙间连通性加强,形成瓦斯渗流−运移通道。瓦斯抽采过程中超前工作面瓦斯抽采经历了稳定期(原岩区域吸附−解析型瓦斯)与高峰期(峰值破损区域解析−运移型瓦斯),峰值区域煤体的破损对井下瓦斯抽采体积分数与流量产生重要影响。研究成果为煤与伴生瓦斯资源安全高效开采提供理论支撑。

     

    Abstract: Abundant coal and coalbed methane resources are endowed in the vast basins of China. Mining disturbance alters the primary stress and fracture fields, thereby impacting the transportation and distribution of coalbed methane as well as gas extraction. This study conducted a true triaxial stress loading experiment on coal samples to investigate the evolution of the stress-fracture-damage field induced by mining stress, utilizing a self-developed low-field nuclear magnetic resonance (NMR) true triaxial test system. The porosity, compressibility, connectivity, damage characteristics, and spatial distribution of pore-fracture structure were quantitatively characterized in different regions through T2 distribution curves and MRIs. Subsequently, the relationship between dynamic pore-fracture and gas extraction was further discussed, combined with practical engineering. The results show coal’s pore-fracture was identified as adsorption pore (0-0.1 μm, 60%-74%), seepage pore (0.1-1 μm, 26%-29%), and migration pore (>1 μm, 0%-13%). The coal experienced elastic, elastic-plastic, plastic, and damage stages. Corresponding to elastic, plastic, and damage stages, the porosity of 4.9%, 2.62%, and 3.1%, the porosity of 2.46%, 1.45% and1.8%, and porosity of 2.57%, 1.03% and 1.95% occurred in in-situ stress zone, in peak-stress zone, and working face, respectively. Before damage, the adsorption pore gradually transforms into seepage and migration pores, in contrast, the adsorption and migration pores gradually transform into seepage pores during damage, which results in enhanced pore connectivity. The heterogeneity of the adsorption and migration pores enhanced while presenting an opposite trend in the seepage pore, accompanied by increased compressibility and significantly developed pore-fracture during the range from the in-situ stress zone to the working face. Adsorption pore, seepage pore, and seepage-migration pores dominate pore-fracture evolution within the in-situ stress zone, peak stress zone, and working face, respectively. In the in-situ stress zone, compression and expansion of pore-fracture form adsorption-type gas desorption-seepage channels, while developed shear and tensile fractures in the peak zone generate adsorption-permeation-migration type gas distribution channels, with gas seepage-migration channels in the working face due to large-scale shear fissures and enhanced connectivity between tension fractures. During the gas extraction, two distinct stages were identified: a stable stage characterized by gas adsorption-desorption in the in-situ stress zone and a peak stage involving gas desorption-migration in the damage zone. The damage of coal in the peak zone significantly impacts the underground gas extraction efficiency. These findings provide theoretical support for the safe and efficient exploitation of coal and associated gas resources.

     

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