Effects of temperature and pressure on pore morphology of different rank coals: Implications for CO2 geological storage

Abstract

The understanding of CO2 effects on coal pore morphology is important during CO2 geological storage (CGS). In this study, the CGS was simulated by conducting experiments on four different samples at temperatures and pressures of 32–80 ℃ and 9–20 MPa respectively. Mercury intrusion porosimetry (MIP) and low-pressure nitrogen adsorption (LP-N2GA) were employed to study pore size distributions, pore shapes, roughness and compressibility. The results show that the pore size distributions of high-rank coals are mainly influenced in the micropore range while low-rank in the macropore range. The pores in ScCO2 treated samples become more complicated indicated by two peaks in the MIP curves at 7.2 nm and 13.7 nm, while only one occurs in the untreated samples. The CGS process is able to open, reshape or enlarge existing pores, which improve pore connectivity and porosity. The fractal dimension shows that the surface and structure morphology becomes more complex after the treatment with the increasing temperatures and pressures, leading to increments in the main characteristics, specific surface area and total pore volume. These characteristics of the Sihe coal mine samples, treated under 62.5 ℃ and 15 MPa, increase from 1.3579 m2/g to 3.048 m2/g and 0.001076 cm3/g to 0.001338 cm3/g respectively. The compressibility changes indicate that coal samples are more easily compressed after the treatment, which might negatively affect CGS. Even though no clear tendencies exist among different coal ranks, the compressibility increases with increasing temperatures and pressures. Even though coal swelling accompanying CO2 adsorption may decrease the permeability of the coal as cleats and pores are squeezed, the results of our study do show that the CGS tends to increase the gas adsorption volume, specific surface areas and pore connectivity, which improve the storage capacities and benefit the enhanced coalbed methane recovery.