| The adsorption characteristics of CH4 and other hydrocarbons in micropores and mesopores are critical to accurately estimate shale gas reserves and their long-term production,because C2H6 is the second largest hydrocarbon composition in shale gas and may have a significant effect on adsorption of shale.Therefore,understanding the adsorption of CH4 and C2H6 binary mixture in the supercritical state can expand the existing competitive adsorption research of shale gas,and provide an important theoretical basis for revealing the storage,transportation and production mechanism of shale gas.In this paper,we first use the Friend ethane empirical state equation and S-W equation to analyze the changes of density,viscosity and compressibility and other physical properties of ethane under supercritical conditions.Then the three selected groups of shale cores were tested to characterize their physical and chemical properties.The isothermal adsorption experiments of C2H6 single component,CH4 and C2H6 binary mixture have been carried out by volumetric method.The single-component and multi-component adsorption models of shale gas were optimized,and the adsorption characteristics of CH4 and C2H6 single components were compared and analyzed.Finally,the competitive adsorption behavior of CH4 and C2H6 in binary mixture was described.The main conclusions and understandings are as follows:（1）Near the critical temperature and pressure,the density,viscosity and compressi-bility of ethane change dramastically,and its density decreases sharply with the increase of temperature;it increases sharply with the increase of pressure.The trend of viscosity with temperature and pressure is basically consistent with the density.Its compression factor increases with the increase of temperature and decreases with the increase of pressure,and then there is a minimum value.（2）Based on the volumetric method,under the conditions of temperature within 40-80℃and pressure within 0-20MPa,a C2H6 single-component isothermal adsorption experiment is carried out,and various models are used to fit the adsorption data to analyze the effect of temperature and pressure on adsorption.The characteristic of adsorption of CH4 and C2H6 are compared.The results show that:the modified D-A model has the best fitting effect,and its R2 is greater than 0.98;the isosteric adsorption heat of C2H6 is at22.32～39.95KJ/mol.Compared to CH4,under lower pressure conditions,the change of temperature has a more significant effect on C2H6;under higher pressure conditions,C2H6 adsorption reaches saturation first;the difference coefficient of C2H6/CH4 is always greater than 1.（3）Based on the volumetric method,the CH4 and C2H6 binary mixture adsorption experiment was carried out at the temperature of 40℃and 80℃and the pressure of 0-16MPa,using the EL model to fit the adsorption data,and through the composition of ethane in the free phase gas and the C2H6/CH4 selection coefficient,it can describe the competitive adsorption behavior of binary mixture.The results show that the EL model has a poor fitting effect on the binary mixture,and its R2 is less than 0.92;the concentration of C2H6 in the free phase is always lower than that in the source gas.When the pressure increases,it gradually approaches the concentration of ethane in source gas.During the entire adsorption process,the C2H6/CH4 selection coefficient is roughly between 1.2-3.5,having a downward trend with increasing pressure;at higher pressures,the amount of CH4 adsorption has a downward trend,which is significantly affected by the partial pressure of C2H6. |
