Fractal characteristics of pores in non-marine shales from the Huainan coalfield, eastern China

• Pore morphology is dominated by cylindrical and slit shaped type.
• Average pore diameter is between 4.435 and 22.791 nm.
• Surface fractal dimension (D1) is primarily affected by shale constituents.
• Structure fractal dimension (D2) is controlled by pore type and connectivity.
• Fractal dimensions reflect gas accumulation and adsorption capacity.

The joint development of coal-bed methane (CBM) and shale gas at the Huainan coalfield in East China has drawn the attention of industrial and academic communities. In CBM and shale gas systems, a large amount of gas might be stored as adsorbed gas, which is closely related to the pore structure. In this paper, we report on the fractal characteristics of pores of non-marine organic shales from the Huainan coalfield. Measurements of X-ray diffraction, total organic carbon, vitrinite reflectance, and nitrogen adsorption were conducted on 13 shale samples to characterize the fractal dimensions and pore properties. Results indicated that pore morphology is dominated by cylindrical and slit shaped types. A small number of wedge shaped pores can be identified, but bottle neck pores are rare. The average pore diameter is between 4.44 and 22.80 nm, and pores with a diameters less than 50 nm are dominant. The pore surface fractal dimension (D1) and pore structure fractal dimension (D2) can be used to indicate overall fractal characteristics. The D1 values are primarily affected by shale constituents. In terms of immature and low total organic carbon samples, the thermal maturity of theses shales is a cardinal factor governing pore structures, especially for surface area. D2 values are obviously controlled by the type and connectivity of pores, and they are significant in analyzing pore structures, especially in assessing average pore diameter. Similarly to coals, shales with high D1 values are favorable for methane adsorption, while high D2 values go against adsorption capacity.