Quantitative experimental investigation of CO2 enhancement of the desorption rate of adsorbed CH4 in coal

Abstract

The injection of CO2 into a coal seam can effectively improve the recovery efficiency of coalbed methane. Existing CO2-enhanced coalbed methane(ECBM) is mainly focused on improving the physical recovery efficiency of free CH4. In macro experiments, it is difficult to simultaneously separate adsorbed CH4 and free CH4; therefore, there are few studies on using CO2 to improve the recovery efficiency of coal seams. In this study, the LF-NMR method was used to evaluate the transformation law of adsorbed and free CH4 during the CO2 injection process, and the enhancement of adsorbed CH4 recovery efficiency using CO2 was quantified. The results showed that adsorbed CH4 and free CH4 formed separate T2peaks, allowing them to be distinguished. The T2spectra of adsorbed CH4 (adsorbed on pore surfaces) were distributed between 0.01 and 2.247806 ms, whereas the T2spectrum of free CH4 (existing between the coal particles and the inner walls of sample cavities) were distributed between 15.702901 and 714.942899 ms. CO2 injection increased the total desorption efficiency of adsorbed CH4 by 19.89%, the desorption efficiency of adsorbed CH4 per unit time by 14.108% and the amount of CH4 desorption per unit mass of coal by 35.23%. The results of the study demonstrate that CO2 injection into a coal seam can significantly improve the recovery efficiency of adsorbed CH4 and shorten the CH4 desorption time; findings that are highly significant in relation to enhancing coalbed methane exploitation and CO2 geological storage.

Keywords

• Coalbed methane
• LF-NMR
• CO2 injection
• Adsorption state CH4
• Gas recovery efficiency