Experimental study of effect of temperature and pressure on the desorption rate of coal bed methane

Abstract

Desorption rates have an important influence on coal seam gas emergence patterns, the time to gas and the service life of coal bed methane wells. As the depth of the coal seam increases temperature and pressure have a significant control on methane adsorption, but relatively little attention has been paid to the effect of temperature and pressure on desorption rates. To investigate the effect of temperature and pressure on the desorption rate of coalbed methane, isothermal adsorption/desorption experiments were carried out with the same sample at different temperature and pressure conditions to obtain adsorption amount-time data at different equilibrium pressures. It is shown that the desorption rate at different pressures decreases exponentially with time. The desorption rate constant, the relative initial desorption rate and the median desorption time can reflect the change of desorption rate from different perspectives, where the desorption rate constant and the relative initial desorption rate increase with increasing temperature and pressure, and the median desorption time decreases with increasing temperature and pressure. Combined with the temperature/pressure gradient of the Baode Block in the Ordos Basin, it can be seen that the desorption rate is controlled by both temperature and pressure at shallower depths of 1000 m, and increases with depth; the pressure effect gradually decreases with depth between 1 000 and 1 735 m, while the temperature effect remains obvious.

Keywords

• coalbed methane
• desorption rate
• adsorption kinetics
• bangham model
• desorption rate constant
• relative initial desorption rate
• median desorption volume time