页岩纳米孔隙分形特征及其对甲烷吸附性能的影响

为了更好地了解页岩纳米孔隙特征及其对甲烷吸附性能的影响,对四川盆地上三叠统须五段的6个页岩样品进行了分形分析。通过对氮气吸附/解吸等温线的分析表明,页岩在相对压力为0~0.5和0.5~1时具有不同的吸附特征。利用Frenkel-Halsey-Hill(FHH)方程计算得到两个分形维数D_1和D_2。甲烷的吸附性能随着D_1和D_2的增加而增强,其中D_1对吸附有着更显著的影响。进一步研究表明,D_1代表由于页岩表面不规则性产生的孔隙表面分形特征;而D_2代表的是孔隙结构分形特征,其主要受页岩组分(有机碳含量、石英、黏土矿物等)和孔隙参数(平均孔径、微孔含量等)控制。更高的分形维数D_1对应更不规则的孔隙表面,为甲烷吸附提供更多的空间。而更高的分形维数D_2代表更复杂的孔隙结构以及孔隙表面更强烈的毛细凝聚作用,进而增强甲烷的吸附能力。因此,页岩孔隙表面越不规则,孔隙结构越复杂,甲烷吸附能力越强。

Fractal characteristics of nanopores and Their Effect on Methane Adsorption Capacity in Shales

To better understand the characteristics of nanopores and their influence on methane adsorption capacity of shales, we have conducted fractal analysis for 6 fresh shale samples from the Sichuan Basin in southwestern China. Isotherms of nitrogen gas adsorption/desorption analyses indicate that shales have distinct adsorption characteristics at relative pressure of 0-0.5 and 0.5-1. Two fractal dimensions D1 and D2 were obtained using the fractal Frenkel-Halsey-Hill (FHH) method. The two fractal dimensions, D1 and D2, have different correlations with methane adsorption capacity of shales. Methane adsorption capacity increases as D1 and D2 increase, and D1 has greater impact on the adsorption. Further investigation indicates that D1 represents fractals from pore surface area generated by surface irregularity of shales, while D2 characterizes fractals related to pore structures that are controlled by the composition (e.g., TOC, clay minerals, quartz) and pore parameter (e.g., pore diameter, micropores content) of shales. Higher fractal dimension D1 corresponds to more irregular surfaces that provide more space for methane adsorption. Higher fractal dimension D2 represents higher heterogeneity of pore structure and higher capillary condensation on pore surfaces that increase methane adsorption capacity. Therefore, more irregular shale surface and more homogeneous pore structure result in higher methane adsorption capacity of shales.