In most of natural montmorillonites, Na + and Ca 2+ ions commonly coexist in the interlayer space as compensation ions. Molecular dynamics simulations have been performed to investigate the swelling properties, hydration behaviors, and mobility of the interlayer species of (Na x , Ca y )-montmorillonites with different water contents. Nine montmorillonites with different Na + /Ca 2+ ratio were selected as model clay frameworks, and the content of interlayer water was set within a range from 0 to 486 mg water /g clay . The results show that the montmorillonites with coexisting of Na + and Ca 2+ present slightly different swelling curves, hydration energies, and immersion energies from Na-or Camontmorillonite. The double-layered hydrates are the thermodynamically stable states for all montmorillonites in the regime of crystalline swelling. A total of 170 mg water /g clay is found as the threshold water content for the complexing modes of interlayer Ca 2+ and Na + ions switching from inner-sphere complexes to outer-sphere ones. The self-confusion coefficient of interlayer species obviously reveals the confining effects of clay surfaces. In all montmorillonites, the mobility of Na + is always much greater than that of Ca 2+ due to their different hydration shells. According to the water residence time in typical Na + and Ca 2+ hydration complexes, Ca 2+ hydration complexes is pronounced more stable than those of Na + , and in montmorillonites with high Ca 2+ /Na + ratio, the inhibitory effects of Ca 2+ hydration complexes on the mobility of Na + is clearly revealed.

Understanding the wettability of clay mineral surfaces is crucial for enhancing oil recovery, investigating primary migration of hydrocarbon, and evaluating the performance of sealing rocks in a petroleum system. On the basis of molecular dynamics simulations, we investigated the interactions between four typical clay minerals (i.e., pyrophyllite, montmorillonite, illite, and kaolinite) and confined pore fluids (i.e., water/alkane/salts). The influences of surface group, layer charge, and salts on the wettability of clay surfaces were revealed. As the layer charge increases, the hydrophilicity of the montmorillonite basal surface gradually increases. The basal surface of 2:1-type pyrophyllite is completely alkane-wet independent of salts. However, for 1:1-type kaolinite, the presence of salts makes the siloxane surface completely water-wet, whereas it is partially alkane-wet at the absence of salts. In general, the salt ions adsorbed onto clay surfaces promote the surface hydrophilicity. By using nonequilibrium molecular dynamics, we explored the hydrodynamics of the water/alkane/salts fluid confined in slit nanopores with pore walls made up of montmorillonite and kaolinite. Both montmorillonite and kaolinite surfaces remarkably restrain the movement of the water confined in nanopores. Decane molecules tend to aggregate together and transport as a cluster. Moreover, the migration of the decane cluster is faster than that of water molecules. These findings are helpful for understanding the primary migration of hydrocarbon in clayey source rocks and the geological sealing of oil by clayey cap rocks in petroleum systems.

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The value of crude oil accommodated in shale has been recognized and has attracted increasing attention from the academic and industrial society. The occurrence and mobility of crude oil in clay pores, therefore, become essential issues for evaluation and recovery of shale oil. The distribution, structure, and transport of the oil−brine mixture confined in a slit-shaped montmorillonite mesopore with different water amounts have been investigated using equilibrium molecular dynamics and nonequilibrium molecular dynamics (NEMD) simulations. A mimic model of crude oil, a mixture of 19 organic molecules, was employed, and thus the behavior of different organic molecules could be characterized in detail. A temperature of 410 K and a pressure of 300 atm corresponding to a buried depth of 3 km were employed. The simulations indicate that the water amount determines the distribution of crude oil. Water and metal ions prefer to cover on hydrophilic montmorillonite surfaces, while nonpolar hydrocarbons tend to be far away from clay surfaces. As the water amount is too low to completely cover the clay surfaces, some polar organic molecules will come into contact with the uncovered clay surface. Abundant organic acid molecules adsorb onto montmorillonite surfaces mainly through participating in the inner-sphere complexes of Na + ions closely located at montmorillonite surfaces (i.e., Na + cation bridge) and forming hydrogen bonds with water molecules in the vicinity. Carbazole molecules tend to aggregate together due to π−π stacking, while thioether molecules mix within alkane molecules and exhibit no characteristic distributions. The mobility of all oil components decreases with the decrease of the water amount, and the mobility of polar components (i.e., organic acid and carbazole) is relatively lower than that of nonpolar hydrocarbons. NEMD simulations clearly indicate that the transport velocity of crude oil markedly increases with the water amount under a specific pressure gradient. The brine covering on clay surfaces significantly weakens oil−clay interfacial interactions. Polar components, especially organic acid, exhibit relatively low transport velocity compared with nonpolar hydrocarbons. These findings highlight the understanding of physical−chemical behaviors of shale oil and provide atomistic information for technology development for enhancing oil recovery.

CO2-switchable surfactants are of great potential in wide industrial applications related to their ability to stabilize and destabilize the emulsions upon command. Molecular dynamics simulations have been performed to reveal...