A Study On Methane Adsorption Behaviors In Saturated-unsaturated Shales

Water plays a significant role in the surface structure and strength of shales,and participates in the occurrence and migration of gas in shales with low permeability.This study investigates the effect of moisture on rock properties and the adsorption and transport of methane.The samples for experiments were collected from the core columns at different depths in the Chaiye 2 drilling well in the Qaidam Basin.The compositions were analyzed using carbon and sulfur analysis,X-ray diffraction and Fluorescence microscope.The volume and specific surface of the pores were analyzed with mercury injection,low temperature N₂ adsorption,and CO₂ adsorption techniques.In order to study the moisture position,the change of porous structure under dry/saturated conditions was determined,and its effect on methane adsorption was studied.The static manometric method was used to determine the adsorption capacity of methane on shales when the relative humidity gradually increased to 96%.The adsorption capacity model considering the density distribution of adsorption phase was proposed,which could accurately estimate the amount of adsorption and obtain a series of optimization parameters.Experiments and models indicate that the occurrence of methane in the micro pores of water-bearing shales relies on dynamic gas-liquid-solid equilibrium.For liquid-solid interactions that reach an equilibrium state firstly,water will interact with most of the pores and change its surface structure,but the interaction is not stable.Adsorbed water will only form a relatively stable liquid-solid interaction in the micropores and gradually change from the adsorbed state to the condensed state varying with pore scale.For methane,the effective adsorption region exhibits a relatively narrow range?0.7 nm to 7 nm?.In pores smaller than 0.7 nm,the effect of water on the adsorption of methane is limited,because these pores are ineffective for methane even without water.With increasing pore diameters,the adsorbed water blocks the methane-surface interaction on effective adsorption sites.Once the pore diameters increase to a certain region,the effect of condensed water on the adsorption of methane is mainly confined,leaving the influence to the migration of methane in the shale pore system.In quartz-based samples,the moisture effect on the adsorption of methane is more pronounced,and the increase in clay and organic matter would“relaxes”this effect.This study provides a dynamic analysis of the state of existence and interaction of water-methane-shale during the change from dry shale to saturated shale.