The Quantitative Evaluation Of Shale Gas Adsorption Capacity Based On Molecular Simulation

Shale gas is of great protential as its main component is methane.It is mainly reserved as adsorption state and free state.Therefore,the study on the methane in adsorption state and the influence factors matters significantly for the exploration.To make quantitative assessment of the adsorption distribution for the supercritical methane in the nano-scale shale pore,the study has formulated an assessment model of Wang Yin Pu-Guan Yin Tang group in Xiuwu Basin including four parameters by adopting the method of molecular dynamic simulation and Monte Carlo:relative concentration(excess adsorption ratio&real concentration peak),diffusion coefficient,saturated adsorption capacity and isosteric heat.Construct a 6nm cylindrical full-atomic pore model that meets the underground geological conditions.Explore the distribution characteristics and adsorption mechanism of methane in different shale components,burial depth and water saturation based on the Polanyi theory.The critical aperture for adsorption gas occurrence is obtained by simulation.The major mineral components of Wang Yin Pu-Guan Yin Tang group are quartz and clay mineral.The organic carbon average content,kerogen I mainly,is about 7.45%.,the average porosity is about 1.17%,the average pore size is about 5.92nm and the average saturated adsorption is about 2.38/cm~3.The shale matrixes have significant impacts on the adsorption capacity:Kerogen I>Smectite>Chlorite>Illite>Quartz.The kerogen is single-thick-layered(7.75?)with high density.Clay minerals are all narrow multi-layered(4.22~4.92?)with low density,their adsorption capacity is proportional to the proportion of the surface oxygen atom and inversely proportional to the thickness of the single adsorption layer.Quartz does not show any adsoption capacity.The adsorption effect is inhibited as the burial depth increased.At the depth of around 3000~3500m,all kinds of minerals appear to have a local minimum value of adsorption capacity.The water saturation generates great impacts on the adsorption capacity.In the same gas content condition,as the hydrogen bond and the molecular polarity,the water is preferentially adsorbed by the pore wall at the low water saturation(20%~40%).When the water saturation further increases(60%~80%),the pore wall has less adsorption effect due to the excess molecular density.Cylindrical pore with high gas content has the critical aperture.When aperture is relatively large,adsorption layers exist;when aperture is relatively narrow,the adsorption potential is counteracted by the opposite pore wall,methane can overcome the residual adsorption potential and thus exist in free state.The critical aperture of kerogen I is 2.0nm,2.0nm for smectite,1.0nm for chlorite,and 5.0nm for illite.