Isothermal adsorption and desorption of coalbed methane underdifferent water saturation and negative pressure

This paper presents results of an experimental study on the influence of water saturation and negative pressure on isothermal adsorption and desorption of coalbed methane.The conventional isothermal adsorption and desorption processes and isothermal desorption process under negative pressure were simulated using large quantity coalbed methane adsorption/desorption simulation equipment.The coal samples used in the simulation were collected from the northeastern Ordos Basin.Using different treatment technique, three coal samples which have different water saturation were obtained, namely dry coal sample, balanced water coal sample, and saturated water coal sample.The pressure and adsorption volume under different water saturation and different negative pressure were measured and then studied by regression analysis.The influence of water on isothermal adsorption process was analyzed and interpreted from the perspective of intermolecular forces.The results show that the desorption process and adsorption process of coal sample are irreversible, with desorption hysteresis.Water has advantage in competitive adsorption of water and methane because the interaction between water molecules and coal molecules is greater than that between methane molecules and coal molecules.It is also found that higher water saturation of coal sample is associated with lower methane adsorbing ability.Within a certain range, the effect of water on depressurized desorption process is not significant.However, above a certain level of water saturation, the external water inhibits the depressurized desorption process.The inhabitation might be related to compositions of coal samples and molecular structure of coal.When fracture fluid loss is severe, gas content of the coal seam decreases due to desorption of methane replaced by water, resulting in longer drainage time and reduced cumulative gas production.The result demonstrates that the fracture fluid loss should be controlled strictly.Desorption volume caused by the unit pressure drop is larger in negative pressure desorption stage, which indicates that the negative pressure drainage and production measures has potential to enhance CBM recovery.