Experimental Investigation Into Simultaneous Adsorption Process Of Water Vapor And Methane Onto Shales

The occurrence of water in rock is diverse,which leads to the serious influence of water on the adsorption and migration of gas in the matrix.Methane usually coexists with water in shale in the natural environment.Therefore,it is very important to explore the simultaneous adsorption of water vapor and methane onto shale for revealing the occurrence of fluids in shale.In this study,adsorption experiments were carried,in which the sample was taken from the Carboniferous in the eastern Qaidam Basin,China.The difference between the adsorption amount of water vapor and methane was compared,and the influence of water vapor and pre-adsorbed water on the methane adsorption process was analyzed.Before the adsorption experiment,the mineral composition,organic chemical characteristics and pore size distribution of the six samples were measured.On the basis,three samples were selected for humidification in an environment with relative humidity of 22%,75%,and 89%,and then to complete the experiment that methane adsorption onto pre-adsorbed water shales(MAWS).The remaining three samples were selected to complete the experiment that the simultaneous adsorption of water vapor and methane onto shale(SAWM).According to the experimental data,the iterative correction method is used to calculate the adsorption amount of water vapor and methane.The t₅₀,t₈₀ and t₁₀₀ were defined as the time for the cumulative adsorption amount to reach 50%,80%and 100%of the equilibrium adsorption amount,respectively.On this basis,the adsorption process of water vapor or methane in shale could be divided into three stages:gas flow control stage(t<t₅₀),transition stage(t₅₀<t<t₈₀)and interface interaction stage(t₈₀<t<t₁₀₀).At the same time,a dynamic adsorption model was established to describe the adsorption process of water vapor and methane accurately.According to the parameters obtained from the model,the adsorption rate,diffusion rate and competitive adsorption intensity of water vapor and methane were compared and analyzed.In the MAWS,the pre-adsorbed water had an inhibitory effect on methane adsorption,causing the methane adsorption amount to decrease by 33.3?70.6%,and the inhibitory effect gradually weakened with the equilibrium degree(the ratio of the adsorption amount of methane at a certain moment to the equilibrium adsorption amount).Comparing the difference of the inhibitory effect in each stage,it was found that the diffusion in clay was greatly affected by the pre-adsorbed water,and the interface interaction was less affected,while the pattern was diametrically opposite in the organic matrix.In the SAWM,the adsorption amount of water vapor was significantly higher than that of methane,and the time for water vapor to reach adsorption equilibrium was longer than that of methane.With the increase of the pressure,the equilibrium adsorption amount of water vapor decreased,while that of methane increased.By analyzing the influence of water vapor on the methane adsorption process,it was found that the existence of water vapor leaded to the extension of the time for methane to reach the adsorption equilibrium,and the decrease of adsorption amount by 10?59%.The relationship between adsorption rate of methane and the adsorption amount of water vapor fitted the exponential₈)=(80)⁽-(9)well.In the adsorption process of methane,there was a critical equilibrium degree.Before that,the decay of methane adsorption rate is accelerated due to the existence of water vapor,and then the decay is slowed down.It was inferred that the effect of water vapor on methane adsorption would transition from inhibition to promotion with the increase of equilibrium degree.By comparing and analyzing the difference of methane adsorption process between MAWS and SAWM,it was found that the methane adsorption capacity was higher and the adsorption rate was faster of MAWS.