Pore structure and diffusion characteristics of intact and tectonic coals: Implications for selection of CO2 geological sequestration site

With the increasing amount of CO2 in the atmosphere, geologic sequestration becomes a promising measure to reduce the rising concentration of CO2. During the process of CO2 injection into coal seam to store CO2, it is critical to select the appropriate coal seam. In order to study whether the intact or tectonic coal is more suitable for CO2 geological sequestration, the pore structure characteristics and fractal dimensions of these coal samples were analyzed by mercury intrusion porosimetry, N-2 (77 K)/CO2 (273 K) adsorption methods, and methane adsorption and diffusion properties of these coal samples were obtained. The results show that tectonic coals have greater pore volume and specific surface area of micropore, mesopore and macropore, while the fractal dimensions of the tectonic coals are smaller (that is, simpler pore structure, smaller pore surface roughness) than that of the intact coals, indicating that tectonic coal has more favorable pore structure to provide more adsorption sites for CO2 storage. Furthermore, better connectivity as well as the adsorption and diffusion capacity of tectonic coals have been significantly improved compared to intact coals, which indicates that CO2 is easier to migrate into the internal pore system of tectonic coal in a short time, reducing the time consumed by CO2 injection into the coal seam, thus saving the cost of CO2 storage. Interestingly, a fractal conceptual model is proposed to account for the evolution of original long and complex pores were converted into short and simple pores, and closed pores transformed into connectivity pores during the formation of tectonic coals. Therefore, the results of this study contribute to understand the advantages of tectonic coal reservoirs as target sites for CO2 geological sequestration.