Experimental study on methane adsorbed-phase density, adsorption capacity and thermodynamic mechanism of anthracite coal under deep temperature and pressure conditions

The adsorption capacity of coal is the main factor affecting the production of coalbed methane (CBM) wells. High-pressure isothermal adsorption experiments at different temperatures were carried out by applying four anthracite coal samples. The adsorbed phase density, adsorption capacity, and their influencing factors and thermodynamic mechanisms of anthracite coal were investigated. The results show that the excess adsorption of anthracite coal under temperature and pressure conditions at depth increases sharply at pressures from 0 to 4 MPa, slows down the growth rate at pressures from about 4–8 MPa, and decreases with increasing pressure at pressures greater than 8 MPa. Using the excess adsorption data obtained from high-pressure isothermal adsorption experiments, the methane adsorbed phase density was obtained by removing the highest point and applying the data calculation of the descending section after the highest point, and the data-fitting method for the calculation of the adsorbed phase density was established, which realized the conversion of excess adsorption-absolute adsorption. The adsorbed phase density decreases with increasing temperature and is less than 0.423 g/cm 3 . The absolute adsorption of each sample decreased with increasing temperature, increased with increasing pressure, and slowly increased greater than 8 MPa until saturation. The adsorption capacity of anthracite coal increases with the increase of vitrinite reflectance and decreases with the increase of the moisture-ash content. Further analysis of the thermodynamic mechanisms of coal adsorption. The effect of temperature and pressure on adsorption potential, adsorption volume, surface free energy and equivalent heat of adsorption was revealed. • High-pressure isothermal adsorption experiments in anthracite coal under deep temperature and pressure were carried out. • Establishment of a method for calculating methane adsorbed phase density in coal, data-fitting method. • The effects of vitrinite reflectance, moisture content and ash yield on the adsorption properties of coal were analyzed. • Uncovering methane adsorption properties and their thermodynamic mechanisms in anthracite coal.