Investigation On Competitive Adsorption Effect Of Multi-Component Gas And Isotope Fractionation Effect In Shale

The adsorption capacity of gas in shale is important for the evaluation of shale gas resource potential.The investigation of shale gas adsorption mainly depends on the adsorption experiment,but the adsorption experiment can only provide the excess adsorption amount,which cannot reveal the adsoption mechanism.In addition,the equipment limits the temperature and pressure situation in the experiment.It is impossible to optimize or establish the adsorption model if the mechanism is unclear,especially for the multi-component gas adsorption,which is more comlex.However,the Grand Canonical Monte Carlo?GCMC?method can not only provide the excess adsoption amount,but also provide the interaction energy,adsorption heat,density field,radial distribution function and gas distribution,which is important to reveal the adsorption mechanism.In addition,the temperature and pressure are almost unlimited in the simulation.The investigation of isotope fractionation effect during shale gas field desorption process shows the isotope fractionation effect is relative to the adsorption amount,so it is hopeful to evaluate the adsorption amount by isotope fractionation effect.However,the quantitative investigation of multi-component gas adsorption and isotope fractionation effect is not enough at present.Aimed at these problems,we investigated the mechanism of adsorption behavior by combining the molecular simulation and adsorption experiment.Because we need to confirm the model and the parameters of force field firstly when performing the simulation,these parameters will affect the simulation results largely.Therefore,the simulation results need to be examined by the experimental measurement to confirm the feasibility of simulation.However,according to the publications at present,there is significant quantitative difference between the experimental measurement and the simulation result.This difference makes us question about the feasibility of molecular simulation methodon investigating the mechanism of shale gas adsorption,even though it is feasible and believable in theory.In this study,we found the basic reason caused the difference by thorough analysis,and established the multi-scale expression method to make the simulation result and the experimental measurement comparable.Inadittion,we optimized the reasonable method to calculate the free volume and the bulk gas density by comparative analysis.The reasonable expression of excess adsorption amount and the reasonable calculate method of free volume and bulk density make the simulation result equivelant to the experimental measurement.Therefore,we can examine the feasibility of simulation by the experimental measurement,which confirmed the reasonability of the adsorption mechanism revealed by the molecular simulation.Based on the multi-scale expression,we simulated the adsorption of CH₄ and CO₂ in montmorillonite,illite,kaolinite and kerogen.We revealed the mechanism of shale gas adsorption in different mineral pores,and compared the difference of adsorption mechanisms among different minerals.We confirmed the adsorption site of CH₄ and CO₂ on the surface of different minerals,and then revealed the basic reason caused the difference on adsorption capacity amoung these minerals from the viewpoint of the capacity of sigle adsorption site,the density of adsorption site and the specific surface area of minerals.In addition,we revealed the micro mechanism of shale gas adsorption behavior,such as the number of adsorption layers,the thickness of adsorption layer,the density of adsorption layer and the ratio of adsorption phase to total gas amount.These micro mechanism is of significant importance for evaluating the adsorption amount under geological condition,which has attracted great attention of explorationist.We simulated the adsorption of CH₄ and CO₂ mixture gas in different minerals.By analyzing the adsorption site and the interaction energy,we revealed that CO₂ has the priority to occupy the adsorption site,which hinders the adsorption of CH₄.It is found that the hinder effect is strong on montmorillonite and illite surface with strong polarity,but the hinder effect is weak on kaolinit or kerogen surface with weak polarity.The mechanism of multi-component gas adsorption revealed by simulation can provide the theoretical direction for improving the recovery of CH₄ by injecting CO₂.To reveal the multi-component and isotope fractionation effect during adsorption and desorption process,we performed the adsorption experiments on multi-component gas,and measured the composition and isotope of gas at each pressure during both the adsorption and desoptionprocess.Itisfoundthattheadsorptioncapacityis:CO₂>CO₂+CH₄>CH₄>CH₄+N₂>N₂,and ¹³CH₄ is preferable in adsorption but lag in desorption.We optimized and established the model to evaluate the isotope fractionation effect quantitatively based on the mechanism revealed by molecular simulation.In this study,we combined the molelcular simulation and the adsorption experiments successfully,and examined the feasibility of molecular simulation by the experimental measurement,making the mechanism revealed by GCMC method more objective and reasonable.The mechanism revealed by GCMC method is helpful to understand the adsorption behavior clearly,and evaluate the isotope fractionation effect quantitatively.The ojective evaluation of isotope fractionation effect laid the solid foundation for evaluating the adsorption gas amount in geological condition by the isotope fractionation effect.