Quantitative analysis of carbon dioxide replacement of adsorbed methane in different coal ranks using low-field NMR technique

Due to differences in geological origin and pore and fissure structure, the effects of carbon dioxide (CO2) displacement of methane (CH4) in coal with different metamorphic degrees differ. The CO2-enhanced coalbed methane (ECBM) process involves the transformation of adsorbed CH4 into free state CH4 under CO2/CH4 competitive adsorption. However, traditional experiments cannot be used to simultaneously analyse the content of the free and adsorbed states of CH4. Therefore, a low-field nuclear magnetic resonance (LF-NMR) technique was used to conduct CO2 replacement of CH4 in Anthracite (WYM), non-caking coals (BNM) and coking coals (JM) and to establish a quantitative model of CO2 replacement of adsorbed CH4. Additionally, the adsorption efficiency and desorption capacity were calculated while discussing the mechanism of displacement desorption affected by pore and fissure structures. The results showed that WYM were mainly slit holes, BNM were mainly ink-bottle holes and JM were mainly cylindrical holes and slit holes. The micropore specific surface area and pore volume of WYM are the largest. After CO2 injection, the P1 peak of T2 spectrum decreased significantly, while the P2 and P3 peaks increased, indicating that CO2 injection effectively promoted the desorption of CH4.The NMR conversion coefficients of adsorbed CH4 for the three coal samples were determined. WYM had the largest initial adsorption capacity of CH4 (0.0156 mol) and total desorption capacity of CO2 replacement of CH4 (0.046 mol), and JM had the largest displacement desorption efficiency (78.885%), indicating that JM favours gas injection to increase CBM production and that WYM and BNM benefit the long-term storage of CO2.