Analysis of shale adsorption characteristics and adsorption abnormal cause under high pressure

In view of molecular,the possible internal cause of shale adsorption trend with pressure change was discussed.And according to simulation data,the causes of shale adsorption abnormal in the late adsorption were analyzed.Results show that methane molecules adsorbed on the surface of the shale is closed related to the interaction of gravitation and repulsion between molecules.Early in the adsorption,shale surface energy is relatively large,the intermolecular force between methane molecules and shale surface molecules is gradually increasing gravitation,which leads shale adsorption to increasing rapidly with pressure increasing.In the middle of adsorption,shale surface energy decreases,the intermolecular force between methane molecules and shale surface molecules transforms from gravitation to repulsion,which leads shale adsorption to increasing slowly to maximum.At this point the adsorption rate and desorption rate are dynamic equilibrium.Under super high pressure in the adsorption late,shale surface energy tends to zero,the intermolecular force between methane molecules and shale surface molecules is rapidly increasing repulsion,which leads shale adsorption to decreasing with pressure increasing even negative.There are two main reasons for shale adsorption abnormal:First,a relatively low shale adsorption and the greater methane molecular diameter than helium result in larger free volume value in the experiment,which leads to the results of "negative absorption";Second,shale methane is in a supercritical adsorption under high pressureand intermolecular has larger repulsion.