Nanoscale Pore Fractal Characteristics of Permian Shale and Its Impact on Methane-Bearing Capacity: A Case Study from Southern North China Basin, Central China

Fractal dimension is closely related to the nanoscale pore structure of shale, and it also has an important influence on the gas content of shale. To investigate the correlation between the fractal dimension and the methane (CH4) bearing features of shale, seven Permian shale samples were analyzed with field emission scanning electron microscopy (FE-SEM), low temperature nitrogen (N-2), carbon dioxide (CO2) and CH4 adsorption and on-site gas desorption experiments. Based on the N-2 adsorption and desorption data, we proposed a new method to better determine the gas adsorption stage at different relative pressure (P/P-0) points in the multilayer adsorption or capillary condensation stage. On this basis, two fractal dimensions, D-1 (representing the surface roughness) and D-2 (representing pore irregularity), were obtained. By correlating the fractal dimensions and nanoscale pore structure parameters, we found that D-1 does not correlate with the pore structure parameters except for the micropore volume. Influenced by the aggregation of porous and nonporous materials, D-2 has a positive linear relationship with the specific surface area (SSA) and micropore volume but has a negative linear correlation with the average diameter of pores. D-1 is negatively correlated with water saturation and positively correlated with free CH4 content. The CH4 adsorption content is positively correlated with D-2. By fitting the on-site desorption data, the positive correlation between the total desorbed CH4 content and the desorbed CH4 content in stage 2 and D-2 was also confirmed. D-2 better reflects the CH4 adsorption capacity of organic-rich shale than D-1. However, D-1 can be used to reflect the influence of shale surface properties on water saturation and to indirectly reflect the free CH4 content in shale. The fractal dimension (D-1 and D-2) is a clear indicator of the total free and adsorbed CH4 content, but cannot indicate the desorbed CH4 content at different stages.