高煤阶煤孔隙结构及分形特征
Pore Structures and Fractal Characteristics of High Rank Coals

摘要： 高煤阶煤与中低煤阶煤在孔隙结构特征方面存在明显差异，分形理论为定量描述高煤阶煤储层孔隙特征提供了有效手段。基于扫描电镜、压汞实验和孔渗测试，以华北地区最大镜质体反射率(Ro,max)在19%~295%之间的9个煤样为研究对象，采用分段回归的方法对各样品进行不同孔径段分形维数计算，并讨论了孔隙结构分形维数与孔隙体积百分比、Ro,max、孔隙度和渗透率的关系。结果表明，高煤阶煤微小孔发育，半封闭孔含量较高，孔隙连通性一般，且孔隙结构具有明显的分段分形特征，同一煤样的超大孔(孔隙半径r＞5 μm)、大孔(05 μm＜r＜5 μm)、中孔(005 μm＜r＜05 μm)和微小孔(r＜005 μm)的分形维数依次减小；各煤样超大孔、大孔、中孔分形维数均随Ro,max增加而增加，随对应孔隙体积百分比增加而减小；孔隙度或渗透率与超大孔、大孔和中孔、微小孔分形维数分别呈二次相关、线性正相关、负相关；各分形区间分形维数分布的偏度和峰度与孔隙度或渗透率分别呈高度正相关和负相关，这为高煤阶煤孔隙度、渗透率提供了理想的线性方程(y=ax+b)预测模型。
Abstract: Significant differences exist in pore structures between high rank coals and mediumlow rank coals, and the principle of fractal geometry is an effective tool for quantitatively describing pore characteristics of high rank coal reservoirs. The experiments comprising scanning electron microscopy, mercury intrusion, porosity and permeability testing were performed on nine coal samples (Ro,max from 19% to 295%) from North China. The pore fractal dimensions of samples were calculated using the subsection regression method and the relationships between the pore fractal dimension and different parameters including pore volume percent, coal degree of metamorphism, porosity and permeability were discussed. The results show that coal samples are characterized by abundant microascopores, relatively high semiclosed pore content, general pore connectivity and clearly piecewise fractal dimensions. For each sample, fractal dimensions of supermacropore (pore radius r＞5 μm), macropore (05 μm＜r＜5 μm), mesopore (005 μm＜r＜05 μm) and microascopore (r＜005 μm) decrease in turn. In addition, fractal dimensions of these pores except microascopores increase with the increasing Ro,max and decreasing pore volume percent for all samples. The correlations between coal porosity (or permeability) and fractal dimensions of supermacropore, macropore and mesopore, microascopore present as quadratic,linearly positive and linearly negative curves, respectively. The skewness and kurtosis of fractal dimension distribution for each sample are positively and negatively associated with porosity or permeability respectively. Meanwhile, based on skewness and kurtosis, the prediction models of linear equations (y=ax+b)can be used to predict porosity and permeability of high rank coals