Influence of reservoir properties on the adsorption capacity and fractal features of shales from Qinshui coalfield

To investigate the adsorption capacity and fractal features of coal-measure shales and understand the influence of shale properties on the adsorption capacity and fractal features, a series of experiments were conducted on fifteen shale samples from Shanxi-Taiyuan Formations in the Yushe-Wuxiang Block, South-Central Qinshui Basin. The results indicate that organic and clay minerals are the two adsorption carriers for coal-measure shales, Langmuir volume (V-L) of shales increases as the total organic matter (TOC) content or clay minerals content increases. Additionally, there is a negative correlation between V-L and quartz content. As vitrinite reflectance (R-o%) increases, V-L has an evolutionary path of first increasing and then decreasing, the maximum value is reached at near R-o = 2.1%. Furthermore, V-L increases as specific surface area (SSA) increases or average pore diameter (APD) decreases. The coal-measure shales exhibit double fractal features. The fractal dimension of pore surface (D-1) and fractal dimension of pore structure (D-2) vary from 2.4899 to 2.6272 and 2.7318-2.840, respectively. The fractal dimensions show an increasing trend as SSA increases. As pore volume increases, D-1 presents a slight increasing trend while D-2 shows a decreasing trend. And fractal dimensions increase as APD decreases. The relationships of TOC with D-1 and D-2 are opposite. Clay minerals are the determining factor of the fractal dimensions as the fractal dimensions (D-1 and D-2) show an increasing trend as clay minerals content increases. An increase in quartz content or organic matter maturity will greatly reduce the heterogeneity of shales. As for non-clay minerals, the fractal dimensions (D(1)and D-2) show an increasing trend as pyrite content increases, while D-1 increases and D-2 decreases as carbonate mineral content increases. Regarding the composition of clay minerals, D-1 increases as the mixed layers of illite/smectite (I/S) and illite contents increase or kaolinite and chlorite contents increase, while D-2 is opposite. Shales with higher fractal dimensions develop stronger adsorption capacity while shales with higher D-1 and lower D-2 develop lower Langmuir pressure (P-L).