Coalbed methane adsorption behavior and its energy variation features under supercritical pressure and temperature conditions

A majority of coalbed methane (CBM) in coal reservoirs is in an adsorbed state. Under geological pressure and temperature conditions (beyond the critical region of methane), supercritical adsorption occurs between methane (CH4) molecules and coal surface. In order to better characterize CBM supercritical adsorption behaviors, isothermal adsorption tests were conducted to simulate the adsorption under different reservoir pressures and temperatures. It is observed that the temperature influences adsorption behaviors little under low-pressure conditions but weakens the sorption capacity of coals under high-pressure conditions. Since the adsorption volume obtained from isothermal adsorption tests is Gibbs excess adsorption volume, it was transformed to the absolute adsorption volume which is essential for reliable gas-in-place estimates. A hypothesis was proposed that a maximum adsorption volume for a certain adsorbent exist in the limiting case of supercritical conditions. Based on this, the limiting adsorption model was built in reference to the Dubinin-Astakhov (DA) adsorption model and Henry dissolution law. Additionally, different energy varying trends (adsorption potential, adsorption space, reduction of surface free energy and isosteric heat of adsorption) over pressure and temperature indicate that the adsorption of CH4 by coals is actually the attraction between CH4 molecules and coal molecules and also a spontaneous process to reduce surface free energy of the system. The pressure and temperature exert an impact on sorption behaviors by controlling attraction forces or kinetic energy of adsorbate and adsorbent molecules.