Thermodynamic energy change and occurrence mechanism of multiple fluids in coal reservoirs

Systematic understanding of the occurrence mechanism of multiple fluids in coal reservoirs is the basis for efficient development of coalbed methane (CBM). In this study, the adsorption capacity, adsorption potential and adsorption heat data of coal samples of different metamorphic degrees under single/multi components isothermal adsorption were obtained through isothermal adsorption equipment and the atomic force microscopy (AFM). The adsorption potential energy and electric potential energy were innovatively combined to expound the characteristics of the thermodynamic energy change in coal reservoirs, and the occurrence mechanism of multiple fluids in coal reservoirs were discussed. It is found that: (a) the adsorption amount of these two gases decreases first and then increases with the increase of coal rank in both single-phase adsorption and competitive adsorption. (b) CO2 shows superior adsorption in comparison to CH4. The larger the proportion of a certain component in the mixed gases system, the better this component's adsorption performance. (c) The adsorption potential of the two gases is not affected by temperature, and with the increase of coal metamorphism, the adsorption potential shows first a decrease, followed by an increase. (d) The CH4 adsorption heat of coal samples increase with higher adsorption amount, but the CO2 adsorption heat of some coal samples has a negative linear correlation with adsorption amount. •The competitive adsorption behavior of supercritical CO2 and CH4 at high temperature and high pressure conditions were characterized.•The correlation between the thermodynamic energy changes of the system and the maceral properties of coal were innovatively explored.•For the first time, adsorption potential theory and electric potential theory are combined to explore the gas occurrence mechanism in coal reservoir.