煤岩孔裂隙结构分形特征及渗透率模型研究

为探究煤岩孔裂隙结构与渗透特性的联动关系,采用扫描电镜、偏光和分形等手段分析煤岩孔裂隙结构分布特征,利用自主研发的出口端正压三轴渗流装置,开展恒定有效应力条件下孔隙压力升高的渗流试验。基于分形理论,考虑煤岩表面孔隙分布情况对煤岩渗透率的影响机理,建立考虑孔裂隙分形特征的煤岩渗透率模型,通过试验验证其合理性,对煤岩孔裂隙下分形维数和渗透率耦合进行定量分析。研究结果表明:①六盘水矿区煤岩表面含有一定数量的孔隙和裂隙,其中四角田7号煤层孔裂隙发育情况最好,具有2条清晰的宽度较大的裂隙,并伴有大量交叉微裂隙及孔隙发育,煤岩结构破坏严重;②通过盒维数法可得煤岩孔裂隙分布具有明显的分形特征,且煤岩孔隙率与分形维数呈正相关关系;③恒定有效应力条件下,煤岩渗透率随孔隙压力升高呈现先急剧降低后趋于平缓的趋势,受孔裂隙结构影响,在相同的孔隙压力下煤岩渗透率存在明显差异。煤岩表面孔裂隙结构越复杂其分形维数越大,有助于瓦斯运移,渗透率呈上升趋势;④考虑孔裂隙分形特征的煤岩渗透率模型计算值与实测值吻合度较高,与前人研究成果相比,无论理论机理的适用性还是对试验点的匹配方面都更加适用,且能较好地反映孔隙压力与渗透率的联动关系。

Study on fractal characteristics of coal pore fissure structure and permeability model

In order to explore the linkage relationship between the pore fracture structure and permeability characteristics of coal and rock,scanning electron microscopy,polarized light and fractal methods were used to analyze the distribution characteristics of coal and rock pore and fracture structure,and the self-developed outlet end positive pressure triaxial seepage device was used to carry out the seepage test of pore pressure increase under constant effective stress conditions.Based on the fractal theory,and considering the influence mechanism of coal surface pore distribution on coal permeability.The coal permeability model considering the fractal characteristics of pores and fractures was established and the rationality of the test was verified by experiments and the fractal dimension and permeability coupling under the fracture of coal pores were quantitatively analyzed.The results show that:①The surface of coal in Liupanshui mining area contains a cer?tain number of pores and fissures.Sijiaotian No.7 coal seam has the best development of pores and cracks,with two clear and large cracks,accompanied by a large number of intersecting micro-cracks.Pores are developed,and the coal and rock structure is seriously damaged.②The distribution of pores and cracks in coal rock has obvious fractal characteristics through box dimension method and the porosity of coal rock is positively correlated with fractal dimension.③Under the condition of constant effective stress,the permeability of coal rock decreases sharply with the increase of pore pressure then tends to be gentle.Affected by the pore and fissure structure,the permeability of coal rock is obviously different under the same pore pressure.The more complicated the pore and fracture structure of coal and rock sur?face,the larger the fractal dimension,which is helpful for gas migration and the permeability is on the rise.④The calculated values of the coal permeability model considering the fractal characteristics of the pores and fractures are in good agreement with the measured values.Compared with the previous research results,both the applicability of the theoretical mechanism and the matching of the test points are more applicable and it reflects the linkage relationship between pore pressure and permeability.