| CBM is a kind of important unconventional natural gas,which has great potential for development.The progress of its occurrence mechanism directly affects CBM exploration and development.In fact,the top and bottom strata are rich in large amount of CBM,some of which are in free state,while most of the CBM is in the adsorbed state in the rock.In addition,methane is actually the most important component of CBM,so it can reflect the adsorption characteristics of CBM by studying the adsorption characteristics of methane.Clay minerals,as the main mineral component in rock strata,have high micropore volume and large specific surface area,which can provide sufficient space for the adsorption of methane.Many scholars have already discussed the adsorption characteristics of methane in montmorillonite and illite.However,the methane adsorption characteristics of kaolinite are seldom reported.Still,due to the complicated microscopic adsorption mechanism,it is difficult to find the adsorption characteristics of kaolinite effectively by ordinary macroscopic experiments.However,molecular simulation is another kind of research that is both experimental and theoretical means providing a relatively low-cost,efficient means for people to study kaolinite methane adsorption properties,which provides an effective way.In this paper,we use the Materials Studio software to construct the kaolinite super cell model,and combine the molecular mechanics and Monte Carlo simulation to calculate the adsorption capacity,adsorption energy and interaction energy of methane on kaolinite surface under different water contents(0%,2%,4%,6%,8% and 10%)and different pore pressures(1MPa,2MPa,···,19 MPa,20MPa)on the adsorption characteristics of methane on kaolinite surface.On the elucidation of the microscopic mechanism of kaolinite adsorption of methane,the main conclusions are as follows:(1)With the increase of pore pressure,the adsorption amount of methane in kaolinite first increases and then becomes stable,showing a Langmuir law,indicating that pressure is a favorable factor for kaolinite to adsorb methane,but when the pressure exceeds a certain value,the pressure increases a little impact on kaolinite adsorption of methane;(2)With the increase of pore pressure,the methane adsorption of kaolinite can first increase and then stabilize,showing a Langmuir law,and the change tendency of adsorption energy and adsorption capacity are the same.(3)When kaolinite adsorbs methane,the methane molecules adsorbed on the surface of the silica tetrahedron are more than the methane molecules on the surface of the aluminum octahedron.This is because the adsorption force between the methane molecule and the silicon tetrahedron is the hydrogenbonding force,The adsorption between methane molecules and aluminum octahedra is van der Waal’s strength,and the hydrogen bonding force is much stronger than van der Waals forces.(4)With the increase of water content,the adsorption amount of methane and adsorption energy of kaolinite decrease linearly,the presence of water molecules occupy the adsorption position of methane,and water is the unfavorable factor for kaolinite to adsorb methane.(5)The change of hydrogen bond energy of kaolinite adsorbed by water molecules is larger than that of van der Waals energy.After adsorption of methane molecules,the change of van der Waals energy mainly shows that methane is the typical physical adsorption by kaolinite and the adsorbed water is chemical Adsorption,accompanied by physical adsorption;(6)The adsorption of methane by kaolinite and the distance between methane molecules and kaolinite molecules show a negative exponential relationship.The pressure increases,the distance between molecules decreases,the adsorption force increases,the adsorption capacity and adsorption capacity increase;water content increases,the distance increases,the adsorption force decreases,the adsorption capacity and adsorption energy decreases,and vice versa. |
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