Study on kinetics of methane adsorption and desorption in coal based on low-field nuclear magnetic resonance in situ monitoring
[Objective]Adsorbed methane dominates the micro-and nanopore spaces of deep coal reservoirs.The diffusion characteristics during adsorption and desorption are crucial for predicting and implementing coalbed methane development.This study employed an in situ low-field nuclear magnetic resonance(NMR)test system to investigate the kinetic characteristics of adsorption and desorption in anthracite samples from the Qingshui Basin,enabling dynamic monitoring of methane adsorption during diffusion.[Methods]The coal core sample was placed in a core holder at a constant temperature of 35℃,with a confining pressure applied to maintain a constant effective stress of 2 MPa during methane gas adsorption.The initial gas pressures were set at 1.69,2.99,5.08,8.7,11.41,and 13.50 MPa.The reference tank was then connected to the sample tank to conduct adsorption kinetics experiments.High-precision pressure sensors were used to track the pressure decay process of the adsorption system,while low-field NMR imaging continuously monitored the nuclear magnetic signal of methane in the coal core to generate T2 relaxation spectra.[Results]The results indicate that gas diffusion during the adsorption phase can be divided into two stages as the micropores are filled.When the pressure is less than approximately 6 MPa,gas molecules are captured by adsorption sites on the micropore surfaces under strong confinement.Despite weak van der Waals force,molecules in the micropore centers can still transfer mass via molecular and Knudsen diffusions.Intermolecular collisions become more frequent,and increased pressure leads to a reduced Knudsen number,thereby decreasing the collision frequency between molecules and microporous surfaces and reducing the diffusion coefficient of adsorbed methane.When the gas pressure exceeded approximately 6 MPa,surface diffusion became dominant,and as the molecular surface coverage increased,the gas diffusion coefficient also increased.Because of the strong confinement in the micropores,the free energy required for the desorption of gas molecules is much greater than the heat energy released during adsorption,making spontaneous desorption difficult under pressure drops.Consequently,the effective diffusion coefficient desorption was significantly greater than that during adsorption.Weakly adsorbed gas molecules diffuse first,and as the pressure decreases,the proportion of gas molecules at weak adsorption sites gradually diminishes,whereas it increases at strong adsorption sites.Therefore,the diffusion coefficient of methane in the adsorbed phase gradually decreased.[Conclusions]The experimental design provides important methods and data for coalbed methane development.This method holds significant value for teaching NMR analysis techniques in laboratories and for upgrading and reconstructing equipment tailored to specific scientific challenges.
coalbed methaneadsorbed methanenuclear magnetic resonanceadsorption-desorptiondiffusion coefficient
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