页岩纳米孔隙表面分形特征及其影响因素

Surface Fractal Characteristics and Their Influence on Shale Nanopores

明确页岩纳米孔隙表面分形特征有助于深化对页岩吸附机理与吸附模型的认识。以五峰组/龙马溪组、延长组页岩为研究对象,基于氮气吸附法孔隙结构参数,计算了页岩纳米孔隙表面分形维数,并与泥岩、致密砂岩分形维数对比,揭示了有机质、黏土矿物等对孔隙表面分形特征的影响。研究表明,五峰组/龙马溪组页岩纳米孔隙表面分维值为2.7~2.9,延长组页岩为2.3~2.7,其分维值取决于10 nm以下孔隙累计表面积大小;页岩分维值与有机质成熟演化阶段正相关,而与有机质含量关系复杂;有机质孔与黏土矿物晶间孔表面均具有明显的分形特征,该孔隙发育是页岩具有表面分形特征的根本性原因,但前者的分维值显著大于后者有机质与黏土矿物表面呈现高度粗糙性,严重偏离光滑平整特性,因此,甲烷在页岩纳米孔内的吸附属于典型的非均匀固体上的吸附。

To further understand shale adsorption mechanism and adsorption model, the surface fractal characteristics of shale nanopores must be clarified. Using the shales from the Wufeng group/Longmaxi group and the Yanchang group as study objects,the surface fractal dimension of the shale nanopores was calculated based on the pore structure parameters obtained from the nitrogen adsorption method. It is then compared with the fractal dimension of mudstone and dense sandstone to reveal the effect of organic matter, clay mineral, and other factors on the fractal characteristics of the pore surface. The results show that the surface fractal dimension of shale nanopores in the Wufeng group/Longmaxi group varies between 2.7~2.9, and that for the Yanchang group shale varies between 2.3~2.7, and the fractal dimension depends on the cumulative surface area of pores below 10 nm. The value of the fractal dimensions of the shale has a positive correlation with the developmental stages of the maturation of organic matter. Its relation with organic matter content is however complicated. The surfaces of pores having organic matter and clay mineral crystal contents have obvious fractal characteristics. The development of these pores is the fundamental reason behind the shale having surface fractal characteristics; however, the fractal dimension value of the former is significantly greater than that of the latter. The surface of organic matter and clay minerals is highly rough and seriously deviates from smoothness. Therefore, the adsorption of methane in shale nanopores is a typical heterogeneous solid adsorption process.