| As one of the important basic parameters of reservoir rocks,wettability’s characteristics directly affect the micro and macro distribution characteristics of fluids in rock pores,as well as determine the degree of permeability and displacement efficiency of injected fluids.A large number of research achievements have been made on improving the recovery efficiency of conventional oil and gas reservoirs by adding surfactant and low-salinity water to change the wettability of mineral surface.However,whether this technology is suitable for unconventional shale gas reservoirs is still in the exploration stage.There has been a preliminary understanding of the wettability of shale reservoirs,but there is a lack of in-depth research on the occurrence and migration of shale gas.In view of the above problems,this paper adopts the research method of combining experiment and theory to systematically and deeply study the influence law and mechanism of wettability on the fluid occurrence and migration characteristics of shale gas reservoirs from the macroscopic scale and molecular scale.Firstly,based on the experimental evaluation,XRD diffraction experiment,environmental scanning electron microscope experiment,low-pressure nitrogen adsorption experiment and methane isothermal adsorption experiment were conducted on the samples.Then,the molecular simulation study was carried out to modify the main mineral components of shale(illite,montmorillonite and quartz)with different wettability,to study the adsorption and diffision law of CH4 in minerals modified with different wettability.Finally,the density functional theory was used to explore the adsorption of different fluids on the quartz surface.The research of this paper is funded by the key project of the national natural science foundation of China(5183000045)and Sichuan major scientific and technological project(2019YFG0457).By systematic research,the following conclusions and understandings are obtained:(1)XRD diffraction,environmental scanning electron microscopy,low-pressure nitrogen adsorption and methane isothermal adsorption experiments were carried out on sodium-rich montmorillonite and illite.The results showed that both minerals were of high purity.The pore morphology of the two minerals was mainly parallel slab-like and slit-like pore with all sides open.The adsorption capacity of illite to methane was larger than that of montmorillonite,and the fitting results of Langmuir,Toth and Langmuir-Freundlich adsorption models were better.(2)Methane adsorption and diffusion were simulated in different wetting illite and montmorillonite models.The results showed that the change trend of methane adsorption in each model was consistent with macro-scale.The adsorption capacity of methyl modified model was stronger.Meanwhile,the adsorption capacity of different wettability models was also related to the free volume.The average adsorption heat meant physical adsorption.The diffusion coefficient of methane decreased with the increase of pressure.The density distribution of methane in the Inm model was different,while in the 2nm model was consistent.The tiny change of the interlayer spacing of montmorillonite would lead to the adsorption of a large number of molecules.(3)Different wettability modifications were carried out on the quartz structure,and adsorption and diffusion of methane was simulated.The results showed that the adsorption capacity of the unmodified(100)surface was greater than that of other surfaces.The adsorption amount of methane increased with the increase of pressure in each model,and the methyl modified model was more methanophilic.In the Inm model,methane only formed an adsorption layer on the wall,while five adsorption layers were formed in the 2nm model.The diffusion coefficient of methane decreased with the increase of pressure.(4)The density functional theory of quartz(100)surface was studied,and the results showed that the adsorption of CH4,H2O and CO2 on SiO2(100)surface was physical adsorption.The adsorption capacity of CO2 was the strongest,while that of CH4 was the weakest.The physical structure of adsorbates changed to varying degrees after adsorption.The density of states of adsorbents basically coincided with each other,while CH4,HZO and CO2 all shifted towards the low energy region with different amplitude. |
