为研究低渗透煤层微观孔隙结构与气体解吸规律的关系,运用直接观测法、核磁共振法、液氮吸附法等实验手段,对鄂尔多斯盆地伊陕斜坡8号煤层煤样的含气量、孔隙度、渗透率、孔径分布等进行测定。结果显示:煤层气解吸特征与煤的孔径分布、孔的形态、孔的连通性以及比表面积、煤岩成分等因素有关;研究区相隔小于10 m的2套煤层,其孔隙特征、气体解吸规律差异均较大。其中,亮煤的微孔形态以一端开口的圆筒形孔和墨水瓶孔为主,微孔及吸附孔占比为39.2%,煤体微裂缝相对发育,但微孔与介孔的连通性较差,暗淡煤的微孔形态以一端开口的圆筒形孔为主,微孔及吸附孔占比为33.2%,微裂缝不发育,不同孔间连通性差。亮煤气体解吸初期产量大,后期衰减快,气体产出具有明显的阵发式特征;暗淡煤气体解吸初期产量小,约为亮煤的1/2,后期产量递减相对平缓。低渗煤层的开发对策应充分考虑煤层的孔隙结构、孔型、渗透性,应力敏感性、裂缝充填物种类等因素,在精细表征孔隙结构的基础上,根据煤层气不同解吸期特征,结合地质、开发条件制订合理的开发方案。该研究对认识煤层气产气机理及其控制因素,提高煤层气开采效率具有重要的指导意义。
In order to study the relationship between microscopic pore structure and gas desorption law in low-permeability coal seams, the gas content, porosity, permeability, and pore size distribution of coal samples from No.8 coal seam of Yishang slope in the Ordos Basin were determined by using experimental methods such as direct observation method, nuclear magnetic resonance (NMR) method, and liquid nitrogen adsorption method. The results show that the coalbed methane desorption characteristics are related to the pore size distribution, pore morphology, pore connectivity, specific surface area of the coal, and coal rock composition, etc; the pore characteristics and gas desorption laws of the 2 sets of coal seams less than 10 m apart in the study area have large differences. Among them, the microporous morphology of the glance coal was dominated by cylindrical pores open at one end and ink-bottle pores, with the percentage of micropores and adsorbed pores being 39.2%, and the microcracks in the coal body were relatively developed, but the connectivity between the micropores and mesopores was poor, and the microporous morphology of the dull coal was dominated by cylindrical pores open at one end, with the percentage of micropores and adsorbed pores being 33.2%, the microcracks were undeveloped, and the connectivity between different pores was poor. The gas desorption of glance coal has a large output in the early stage but decays quickly in the late stage, and the gas output has obvious paroxysmal characteristics; the gas desorption of dull coal has a small output in the early stage, which is about 1/2 of that of the glance coal, and the decrease of the output in the late stage is relatively flat. The pore structure, pore type, permeability, stress sensitivity, and types of fracture fillers shall be fully considered in the development strategy for low-permeability coal seams. On the basis of fine characterization of the pore structure, a reasonable development plan should be formulated in accordance with the characteristics of different desorption periods of coalbed methane and in conjunction with geological and development conditions. This study has important guiding significance for understanding the mechanism of coalbed methane gas production and its controlling factors, and improving the mining efficiency of coalbed methane.
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