Microscale Simulation Studies Of Shale Gas Adsorption

With the development of commercial production on shale gas in China,how to improve the production of shale gas has become a hot spot.Compared with conventional oil and gas reservoirs,shale gas stay in the porous and fractured rocks with free state,besides,it is also adsorbed on the surface of clay mineral and organic matter with adsorption state.And the content of the adsorption state of shale gas is between 20%⁸5%of total gas.Some studies have shown that the pores in shale are mainly less than 50nm,so it is necessary to study the law of shale gas adsorption-desorption at the micro-scale in order to enhance the shale gas production.In this paper,molecular simulation was used to investigate the law of shale gas?CH₄?adsorption-desorption on the surface of clay minerals and organic matter,and the adsorption behavior of CH₄ on the surface of organic matter in the case of injecting carbon dioxide.Based on the simulation,the main content and contribution are summarized as follows:?1?The adsorption behavior of shale gas on the surface of organic matter?3 layers graphite?slit pore was simulated under different pressures different temperatures and different pore structures by using Grand canonical Monte Carlo.The results indicate bilayer absorption occurred when shale gas adsorbed on the surface of organic matter,and the adsorption thickness is 0.7nm,which is independent of temperature and pressure.When the pore size is less than 1.4nm,the shale gas stay in the pores with the adsorption state.When the pores are larger than 1.4nm,the shale gas is stored with coexistence systems of free state and adsorption state.The quantitative equation between the amount of shale gas adsorption and temperature and pressure is obtained by fitting average density of the shale gas adsorption on the surface of organic matter using Langmuir adsorption theory.?2?According to the composition of shale mineral,three clay mineral models were established to simulate the adsorption behavior of shale gas.The results showed that the adsorption capacity of three clay mineral models is kaolinite>montmorillonite>illite,which was different from experiment result-montmorillonite>kaolinite>illite,the reason is that the interlayer spacing of montmorillonite will become bigger and the shale gas can be adsorbed into interlayer when the water exists.The adsorption thickness of shale gas on the surface of the three clay minerals is 0.4nm,and the adsorption state can be regarded as single-layer adsorption.The quantitative equation between the amount of shale gas adsorption and temperature and pressure is obtained by fitting average density of the shale gas adsorption on the surface of three clay minerals using Langmuir adsorption theory.?3?Based on the principle that competitive adsorption can promote desorption of shale gas,the adsorption-desorption laws of shale gas on the surface of organic matter are simulated when injecting carbon dioxide.The results turn out that high pressure and high temperature are not conducive to the adsorption of carbon dioxide on the surface of organic matter.And the replacing efficiency of carbon dioxide is the highest when the mole ratio of the mixed gas is 1:1.