中低煤阶煤层气储层孔隙结构分段分形特征

Multi-Fractal Characteristics of Pore Structure of Coal Rock in Low and Medium Rank CBM Reservoirs

为明确韩城、保德区块煤岩孔隙结构分形特征,基于火柴棍模型,推导了新的分形特征表征方法,并利用扫描电镜实验和压汞实验数据对新方法进行验证,在此基础上研究了中、低煤岩孔隙结构分形特征。结果表明,火柴棍模型更能精确表征中、低煤阶煤岩的双重孔隙结构特征。在双对数坐标中,进汞饱和度与毛管压力成双线性关系,即以半径1μm为界,中低阶煤岩孔隙结构具有分段分形特征,孔隙和裂缝具有不同的分形区间和分形维数。韩城、保德区块裂缝分形维数和孔隙分形维数分别在2.80~2.98和2.17~2.33,且裂缝分形维数随孔隙分形维数增加而增加,两类分形维数均随平均孔隙半径、孔隙度和渗透率的增加依次降低。表明煤岩分形维数可以作为储层评价的关键指标,分形维数越小,储层物性越好。煤岩分形维数能够表征其孔隙结构的非均质性,分形维数越大,孔隙结构非均质性越强。韩城、保德区块割理、裂缝的分形维数远远大于孔隙分形维数。

In order to clarify the fractal features of the pore structure of coal rock in Hancheng and Baode block, a new fractal characterization method was derived based on matchstick model. The new method was validated by scanning electron microscopy (SEM) and mercury intrusion test data. On this basis, the fractal characteristics of pore structure in medium and low coal rocks were studied. The results show that the matchstick model can more accurately characterize the dual pore structure of middle and low coal rank coal rocks. In the bi - logarithmic coordinate, the mercury saturation is in a bilinear relationship with the capillary pressure, that is, the pore structure of medium-low rank coal rocks has the fractal characteristics of sections, and the pores and cracks have different fractal intervals and fractal dimensions by the dividing line of 1 μm. The pores fractal dimension ranges from 2.80 to 2.98 and the crack fractal dimension ranges from 2.17 to 2.33 for Hancheng and Baode block. The fractal dimension of fracture for coal sample increases with the increase of the fractal dimension of pores, and both fractal dimensions decrease with the increase of average pore radius, porosity and permeability. Fractal dimension of coal rock is the key index of reservoir evaluation and can characterize the heterogeneity of its pore structure. The larger the fractal dimension is, the stronger the heterogeneity of pore structure is. The fractal dimension of cleavage and fracture is much larger than that of pore in Hancheng and Baode Block.