Molecular Dynamics Simulation On Clay Minerals-Fluid Interfaces

Clay minerals are abundantly present in earth surface system and are the most widely used non-metallic mineral materials.The interaction between clay minerals and geological fluid significantly affect various geochemical processes and geological engineering applications.Smectites are often used as adsorbing and preservative materials for geological disposal of radioactive waste and toxic waste.The swelling capacity of smectites directly dominate the applicability of such engineered barriers.Furthermore,smectites swelling also determines the safety of oil drilling operations.The interfical property of clay minerals critically impacts the formation,migration and accumulation of both conventional and unconventional petroleum.In addition,it also has an influence on the efficiency of CO₂ flooding and security of geologial sequestration of CO₂.Based on comprehensive molecular dynamics?MD?simulations of nine?Na,Ca?-montmorillonites,the swelling behavior,mobility of interlayer species,and interlayer structures have been well disclosed in this study.The results indicate that:?1?Montmorillonites with different Na⁺/Ca²⁺ ratios present very similar swelling behaviors,and have analogous hydration energy curves and immersion energy curves.?2?The immersion energy reveals that bilayer hydrated state is the most thermodynamically stable state for all nine montmorillonites.The swelling of Na-montmorillonite from monolayer hydrated state to bilayer hydrated state needs to in a manner overcome the energy barrier,whereas such energy barrier disappears for Ca-montmorillonite.?3?As water content increases,the hydration structures of Na⁺ and Ca²⁺located in the interlayer of montmorillonites transfrom from firstly falling into the surface silicate-oxygen six-membered rings,then to forming inner-sphere complexes,and finally to forming outer-sphere complexes.The transition from inner-sphere complexes to outer-sphere complexes occurs approximately at the water content of 170 mgwater/gclay and corresponds to the swelling porcess of montmorillonites from monolayer hydrated state to bilayer hydrated state.The coordination number of the outer-sphere hydration shell of Na⁺ and Ca²⁺ is 6.0 and 8.0,respectively.Moreover,the stability of the outer-sphere hydration shell is much higher than that of Na⁺.?4?The mobility of interlayer Na⁺,Ca²⁺ and H2O is dramatically restrained by the adsorption and confining effect of montmorillonite surfaces.And the mobility of Ca²⁺ is much lower than that of Na⁺.It is clear that the movement of Na⁺ and H2O is spatially constrained by the the hydration shell of Ca²⁺.Surface wettability is an essential material proterty of clay minerals.In this study,the interactions between four clay minerals?i.e.,pyrophyllite,montmorillonite,illite and kaolinite?and alkanes/brine fluid are revealed.And this study also provides information about the influences of the amount and location of layer charge on the clay surface wettability.The results show that:?1?The amount and location of layer charge obviously affect the clay surface wettability.As the amount of layer charge increases,the basal surface grdually transform from completely alkane-wet?uncharged pyrophyllite surface?,to mainly alkane-wet?slightly charged montmorillonite surface?,to mainly water-wet?moderately charged montmorillonite surface?,and finally to completely water-wet?highly charged montmorillonite and illite surface?.?2?The salts in the fluid have different influences on the wettability of uncharged pyrophyllite and kaolinite surfaces.The basal surface of pyrophyllite is completely alkane-wet independent of salts.The hydroxylated surface of kaolinite is completely water-wet independent of salts.And the siloxane surface of kaolinite is both alkane-wet and water-wet at the absence of salts.However,the presence of salts makes the siloxane surface of kaolinite completely water-wet.?3?The completely hydrophilic surface of montmorillonite remarkably inhibits the ovement of the confined water molecules.And the hydroxylated surface of kaolinite more intensely restrains the mobility of water molecules near it than the siloxane surface.?4?In the slit pores,alkane molecules tend to aggregate together and transport as a cluster.It is helpful for understanding the efficient primary migration of hydrocarbon in clayey source rocks and the effective geological sealing of clayey cap rocks due to the difficulty for alkanes cluster entering into nanopores.The interactions between clay minerals and complex fluids are closely related to their surface properties.In this study,the interactions between six clay minerals?i.e.,pyrophyllite,montmorillonite,illite,kaolinite,serpentine and talc?surfaces and CO₂/brine fluid are investigated.The results reveal that:?1?The layer charge of clay minerals significantly affects the adsorbing capacity of CO₂ onto clay surfaces.highly charged montmorillonite and illite surfaces have extremely low CO₂ adsorbing capacity,while uncharged pyrophylite and talc surface has greatly high CO₂ adsorbing capacity.Such high CO₂ adsorbing capacity of pyrophyllite and talc surfaces is attributed to their strong hydrophobicity,and it does not mean strong adsorbing ability.Analogically,the competele hydrophilicity of montmorillonite and illite surfaces results in their low CO₂ adsorbing capacity.?2?Comparing to the siloxane surface,the hydroxylated surface has much lower CO₂ adsorbing capacity.CO₂ molecules adsorbed onto the hydraxylated surface present as two patterns?i.e.,sloping configuration and flat configuration?,while CO₂ molecules adsorbed onto the siloxane surface mainly display as flat configuration.?3?The surface wettability of clay minerals dramatically the morphology and mobility of CO₂ molecules confined in the nanopores.When the surfaces of the nanopores are hydrophilic,CO₂ molecules tend to aggregate together and stay away from the surfaces,and the mobility is very low.When the surfaces of the nanopores are hydrophobic,CO₂ molecules tend to spread on the surfaces,and the mobility is relatively high.These findings are helpful for evaluating the availability of geological sequestration of CO₂.Within nanopores having hydrophilic surfaces,CO₂ molecules aggregate into clusters and have low mobility,hence the geological sequestration of CO₂ by shale and mud rocks is more effective.Within nanopores having hydrophobic surfaces,CO₂ molecules display as gas strings and have very high mobility,therefore the geological sequestration of CO₂ by shale and mud rocks is less effective and insecure.