Molecular Simulations Of Clay Nanopore Surfaces-Fuild Interactions

Clay minerals,a family of phyllosilicate minerals that are widely distributed in nature.The majority of geological processes and biogeochemical processes in the lithosphere involve the participation of clay minerals.The 2:1 type clay minerals with one-dimensional and two-dimensional pore structure are ideal raw materials in industry.Whether in the one-dimensional or two-dimensional channels of clay minerals,confined water is common.In material sciences,it has been found that confined water have different hydrogen bond network and ion solvation behavior from the bulk.Therefore,research on confined water in clay minerals has been receiving significant attention.Due to the limitations of experimental methods,it is difficult to obtain fibrous minerals of high purity and control high temperature and high pressure.Therefore,the studies on confined water in one-dimensional pores and in two-dimensional pores under basin conditions are still very rare.Furthermore,most studies on the interaction between clay minerals and water are concentrated in two-dimensional tunnel under normal temperature and pressure.Due to the strong interaction between water molecules and two-dimensional pores of clay minerals,the entry of non-polar molecules such as methane is not favored.However,the accumulation of shale gas is closely related to the clay surfaces.Therefore,competitive adsorption of methane and water in the pores of clay minerals has been the focus.It has recently been found that the higher the content of illite in shale gas reservoirs,the better the quality of gas reservoirs,which indicates that the illite surface is very important for the adsorption of methane.In this thesis,molecular simulation method is used to investigate the hydration of one-dimensional and two-dimensional pores in clay minerals,the interaction between clay surface and water molecules,the influence of high temperature and high pressure on the hydration in two-dimensional pores and the competitive adsorption of methane-water molecules.(1)Hydration in one-dimensional pores of fibrous clay mineralsFor sepiolite,it was found that the zeolitic water in the pores can be lost only at a very low relative humidity(<5%),and the structural water was very hard to get away.This thesis derived a new distribution model for zeolitic water,which is different from previous experiments.In this model,a new zeolitic water location was constructed.It was also found that the confined water in the one-dimensional channel has much lower mobility than in the two-dimensional channelIn the study of palygorskite,three models were selected:electrical-neutral with pure trioctahedron,electrical-neutral with two coexisting octahedrons,and charged palygorskite.The difference in the octahedron in palygorskite did not affect the adsorption amount of water,but the cations in the charged palygorskite channel decreased the water adsorption amount.The distribution patterns of zeolitic water in the two neutral palygorskites are similar and consistent with the experiments:they have two zeolitic water locations.The presence of cations has a very obvious effect on the distribution of zeolitic water in the channels.The mobility of water molecules in the pores is affected by both octahedrons and cations.(2)Hydration in two-dimensional pores of I/S mixed clay minerals:rectoriteThe hydration of rectorite was compared with that of montmorillonite.The similar effects'for them could be found:with the increase of the water content,the monolayer and the bilayer hydrate states are sequentially formed,and the bilayer state is the most stable state.In rectorite,since the surface charge of the illite layer is higher than that of the montmorillonite layer,the interaction between the Na ion and the illite layer is stronger,so Na tends to be closer to the surface of illite layer,exhibiting an asymmetric distribution.This is different from the symmetrical distribution in montmorillonite.(3)Hydration in two-dimensional pores of montmorillonite under basin conditionsThis thesis studied the effect of temperature and pressure on the distribution and mobility of confined water in the montmorillonite interlayer.It is found that the water content of the bilayer hydrate in Na-montmorillonite decreases slightly as temperature and pressure increases.The monolayer hydrate is more stable than the bilayer hydrate at a lithostatic pressures of 7 km.These both mean natural dehydrtions of montmorillonite.The increase of the depth also increase the mobilities of species in the pores.(4)Competitive adsorption of methane and water molecules in two-dimensional pores of illite under basin conditionsThis thesis systematically studied competitive adsorption of methane and water molecules in two-dimensional nanopores formed by illite layer under basin conditions.The relative humidity was found to be a key factor for whether methane molecules can enter clay pores.The increase in buried depth can increase the adsorption amount of methane.The water adsorbed is mainly distributed on the clay surface and coordinated with the cations.The methane molecules are mainly distributed in the middle of the pores.Meanwhile,the increase in pore size and depth can increase the mobility of the species confined in the pores.In summary,the one-dimensional pores and two-dimensional pores of clay minerals have obvious confinement effects on the molecules and ions in the pores,and the confinement effects ofone-dimensional pores are stronger than the two-dimensional pores,which could be used as theoretical basis in the design of the clay mineral materials.This thesis finds that dehydration events would happen in the two-dimensional pores of clay minerals with the depth increases,which can be one explaination of the sources of water in basin.This thesis also finds that low relative humidity is a necessary conditon for the large amount of methane molecules to enter the two-dimensional pores,which provides theoretical guidance for finding shale gas reservoirs.