Influences of pressures and temperatures on pore structures of different rank coals during CO2 geological storage process

Pores structure changes in coal under different burial temperatures and pressures are very important in understanding the CO2 geological storage process. To address this problem, the CO2 geological storage process of four different rank coal samples were replicated in a high pressure supercritical CO2 (ScCO2) geochemical reactor. Coal grains of size ranging from 4 to 8 mm were exposed to ScCO2 and water under different simulated burial depths (1000 m, 1500 m and 2000 m) for 240 h. Helium pycnometer, mercury intrusion porosimetry and low temperature liquid nitrogen adsorption analysis were employed to measure the densities, porosities, pore volumes and specific surface areas of coal before and after the ScCO2 treatment. The results show that the influences of CO2 geological storage on high rank coal are mainly in micropore range, whereas on low rank coal are mainly in macropore range. After exposure to the ScCO2 and H2O, the true densities of coal samples slightly decrease under different burial temperatures and pressures, while the total intrusion volume of mercury and all porosities increase with the burial depth. The increments of pore volumes and specific surface areas are not linear with the increase in temperatures and pressures. The significant increments for all samples occur when the burial depth is 1500 m. The increasing temperatures and pressures do not always correspond to an increase in the coal porosities and total pore volumes, both of which are very important parameters in evaluating the CO2 storage capacity. Thus, the burial depth of CO2 geological storage should be taken into consideration.