Supercritical methane adsorption in coal and implications for the occurrence of deep coalbed methane based on dual adsorption modes

The methane occurrence in coal is significant for coalbed methane (CBM) gas-in-place (GIP) resource, production dynamics prediction, and development programming. The high-pressure adsorption isotherms of two coal samples were measured at 303.15 K, 318.15 K, 333.15 K, 353.15 K, and 373.15 K. Based on the methane excess adsorption data at different temperatures and pressures, the fitting accuracy of five supercritical adsorption isothermal models (SL, SDL, SBET, SDR, and SL + SDR model), thermodynamic parameters, threshold values, maximum micropore filling pore size (rm) and nano-confined methane density were calculated. The methane adsorption process and the occurrent mechanism of deep CBM were further discussed, also the methane adsorption amount was calculated under different adsorption mechanisms. The results show that: the data of fitting accuracy, isosteric heat of adsorption (qst), and entropy loss (Delta S) suggest that the SL + SDR model based on dual adsorption mechanisms is more suitable for fitting supercritical methane-coal adsorption isotherms. The adsorption mechanism of supercritical methane in coal is micropore filling and surface adsorption. The adsorption process is described into four stages based on the threshold pressure and pore sizes. Micropore filling adsorption mainly occurs in micropores smaller than 1.1 nm, and surface adsorption in pore size 1-2 nm pores has a concentrate contribution. The ratio of micropore filling adsorption amount to total adsorption amount for the two coal samples at different temperatures ranged from 38.65% to 64.83%, indicating that micropore filling adsorption has a significant role. The nano-confined effect and low water saturation lead to the deep CBM enrichment of free gas. The study's results can help to elaborate on the occurrence behavior of unconventional natural gas, such as CBM and shale gas in the reservoir.