Ni Xiaoming, Zhao Zheng, Wang Baoyu, Li Zongyuan
School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China.
Collaborative Innovation Center of Coalbed Methane and Shale Gas for Central Plains Economic Region, Jiaozuo 454000, China.
ACS Omega. 2020 Dec 16;5(51):33225-33234. doi: 10.1021/acsomega.0c04907. eCollection 2020 Dec 29.
Differences in content, distribution, and connectivity of pores and fractures with different sizes in coal lead to different modes of gas migration. An accurate classification of pore-fracture combination types in coal can lay a foundation for studying gas migration. High-pressure mercury intrusion and nuclear magnetic resonance (NMR) experiments were conducted on coal samples collected from the Changping coal mine in Jincheng City, Shanxi Province, and Pingdingshan no. 4 mine in Pingdingshan City, Henan Province, China. The fractal dimensions of pores with different sizes were calculated using the Menger model. By combining the data with T spectra obtained by NMR, critical values for distinguishing diffusion pores from seepage pores-microfractures were determined. In addition, the main parameters affecting development of diffusion pores and seepage pores-microfractures and pore-fracture connectivity were analyzed, and a comprehensive evaluation index system for pores and fractures was established by selecting eight indices. Based on the method combining the analytical hierarchy process with multiparameter superposition, a method for determining critical values, establishing the evaluation index system, and classifying pore-fracture combination types was formed. The pore-fracture combination types in the test coal samples were classified according to the experimental data. The results indicate that the critical values for distinguishing diffusion pores from seepage pores-microfractures based on fractal dimensions obtained through mercury intrusion porosimetry and T spectra obtained by NMR are 72 nm and 2.5 ms, respectively. The studied coal samples can be classified into three combination types, separately characterized by high diffusivity and permeability and poor pore-fracture connectivity; low diffusivity, high permeability, and good pore-fracture connectivity; and low diffusivity and permeability and good pore-fracture connectivity. In the coal samples from the Changping coal mine, diffusion pores and seepage pores-microfractures are developed, while the connectivity between pores and fractures is poor. The coal samples from Pingdingshan no. 4 mine have undeveloped diffusion pores and seepage pores-microfractures but good connectivity between pores and fractures. The research results provide a method for classifying pore-fracture combination types in coal samples taken from different regions.
煤中不同尺寸的孔隙和裂隙在含量、分布及连通性上的差异导致了瓦斯运移方式的不同。对煤中孔隙 - 裂隙组合类型进行准确分类可为研究瓦斯运移奠定基础。对采自中国山西省晋城市长平煤矿和河南省平顶山市平顶山四矿的煤样进行了高压压汞和核磁共振（NMR）实验。利用门格尔模型计算了不同尺寸孔隙的分形维数。结合核磁共振得到的 ( T_2 ) 谱数据，确定了区分扩散孔隙与渗流孔隙 - 微裂隙的临界值。此外，分析了影响扩散孔隙和渗流孔隙 - 微裂隙发育及孔隙 - 裂隙连通性的主要参数，并通过选取八个指标建立了孔隙和裂隙综合评价指标体系。基于层次分析法与多参数叠加相结合的方法，形成了一种确定临界值、建立评价指标体系及对孔隙 - 裂隙组合类型进行分类的方法。根据实验数据对测试煤样的孔隙 - 裂隙组合类型进行了分类。结果表明，基于压汞法获得的分形维数和核磁共振得到的 ( T_2 ) 谱区分扩散孔隙与渗流孔隙 - 微裂隙的临界值分别为72 nm和2.5 ms。所研究的煤样可分为三种组合类型，分别具有高扩散率和渗透率但孔隙 - 裂隙连通性差；低扩散率、高渗透率且孔隙 - 裂隙连通性好；低扩散率和渗透率且孔隙 - 裂隙连通性好的特征。在长平煤矿的煤样中，扩散孔隙和渗流孔隙 - 微裂隙发育，但孔隙与裂隙之间的连通性较差。平顶山四矿的煤样中扩散孔隙和渗流孔隙 - 微裂隙不发育，但孔隙与裂隙之间的连通性良好。研究结果为对不同地区采集的煤样中的孔隙 - 裂隙组合类型进行分类提供了一种方法。
