| In the hydrocarbon trap,clay shale is an excellent cap rock for oil and gas resources,and plays an important role in the process of hydrocarbon accumulation.In addition,oil shale is a source rock of petroleum in nature,and storage space for shale oil and shale gas,which is a kind of unconventional oil and gas resource.In order to exploit the conventional resource effectively and the unconventional resource reasonably,it is crucial to realize the safety drilling in the mud shale stratum,and reduce the drilling cost reasonably.However,safety drilling in the mud shale stratum is a serious challenge in drilling engineering for a long period,and have not been solved completely.In fact,free water in the wellbore may infiltrate into the layer,and mud shale have suction effect upon most occasions,leading to the borehole and the surrounding rock inflate disastrously.Severe inflation changes the mechanics properties of surrounding rock and the resulting to the borehole instability.Therefore,Mud shale hydration and expansion will reduce the drilling speed and increase the costs of exploration significantly.The crystal structure of clay minerals consists of an octahedral alumina sheet(O-sheet)and n-tetrahedral silica sheets(T-sheet).When n = 1,O-sheet and T-sheet appear alternately in the manner of 1:1,called T-0 type clay mineral,such as kaolinite.When n = 2,O-sheet and T-sheet appear alternately in the manner of 1:2,and the later cover the upper and lower surfaces of the layer,called T-O-T type clay mineral,such as MMT and illite.In addition,there is another structure with the composition of T-O-T...O,which similar with T-O-T type clay mineral,but contains a brucite sheet in the interlayer space,such as chlorite.Because of isomorphic substitution in the T-O-T type clay minerals,the replacement of higher-charge metal ion(Al3+ or Si4+ inside the layer)by a lower-charge metal ion(Fe2+ or Al3+),the layer is negatively charged and has a strong tendency to attract a cation,namely counterion.Li+,Na+,K+,Ca2+,NH4+,etc.,are the common counterions found in T-O-T type clay minerals.The counterions usually sit on the inner surface.On one hand,the counterions favor to stabilize the clay structure by eliminating the electrostatic repulsion between adjacent layers.On the other hand,the counterions tend to adsorb water molecules,enlarging the interlayer spacing and destroying the laminar structures.The adsorption,accumulation and diffusion of water molecules inside clay are the main reason of clay swelling and splitting.These studies are focus on the interactions between water and series of T-O-T type clay minerals.First,the atomic coordinates of MMT were taken from the X-ray diffraction experiment by Viani et al.The lattice parameters of unit cell are a = 5.18 A,b = 8.98 A,c = 15.00 A,? =? = ?=90.00°.A supercell of 2a X b X c was created in our calculations.Isomorphic substitution of Al3+/Mg2+ occurs in the alumina layer of the supper cell.Counterions like Li+?Na+ or K+,then move onto the surface to balance the negative charge caused by isomorphic substitution.Our tests of dispersion correction showing considerable effect on the interaction energy of water with MMT surface,which is underestimated by calculations without dispersion correction.Both the surface and vacuum desorption models were studied in our calculations.In the former model,water molecule leaves from the counterion while keeping interaction with the MMT surface.In the latter,water molecule leaves from the counterion and enters into vacuum.The surface desorption has lower energy barriers than the vacuum desorption.It is the MMT surface that helps stabilize the intermediate structure of water-counterion-MMT.Moreover,the energy barrier of water desorption from Li-MMT is the highest,followed by those from Na-MMT and K-MMT,resulting from the shortest radius of Li+ among the three counterions,which leads to the strongest binding with water.The energy barrier of vacuum desorption corresponds to the water adsorption energy in magnitude.The temperature dependence of water adsorption and desorption rates were analyzed based on the computed activation energies.Our calculations reveal that the water desorption and adsorption had different paths and energetics.It is therefore important to understand the desorption behavior of water from MMT surface for giving a full picture of water diffusion in MMT.Second,the interlayer behaviors of water interact with MMT and illite were studied based on molecular dynamic simulations(MD)primarily.The extent of hydration reaction was obtained through compare the inner energy of water in bulk phase and corresponding interlayer.The reaction intensity are in the order of:Na-illite>Na-MMT>K-illite>K-MMT.Analysis suggests that,attractive interactions of larger radius counterion K+ to clay surface is higher,and weaker for interact with water.It is difficult for K+ to form outer sphere complexes,and shown as inhibition effect in the interlayer space of clay.For MMT and illite,isomorphic substitution occurs in different sheet.Isomorphic substitution of Al3+/Mg2+ occurs in the alumina sheet for MMT and Si4+/Al3+ occurs in silica sheets for illite,which made higher surface charge density for illite.Secondly,based on the difference between counterion Na+ and K+,competitive adsorption of both counterion on inner surface of illite was studied.The result of self-diffusion coefficient shows that interaction was exist among water,inner surface of clay,and interlayer counterion.On one hand,attraction of counterion and surface to water do good to increase hydration degree of clay.On the other hand,hydrogen bonding between water molecules made adsorption water tend to enter the bulk phase.In surface hydration process,it is feasible to substitute interlayer cations with the one which is stronger interacted with clay surface.Third,calculated the mechanical properties of transition state of swelling,and obtained the inflation pressure and elasticity modulus under different hydration status.Third,equilibrium molecular dynamics(EMD)simulations have been performed to study the effect of temperature,particle volume concentration,and size on the rheological properties of MMT nanofluids.At a fixed temperature,the viscosity of MMT nanofluids increases with their volume concentrations and the increase becomes greater at higher concentrations.At a fixed volume concentration,the viscosity decreases with increasing temperature,and the variation in viscosity is more significant at higher concentrations.The relative viscosity,which is defined as the viscosity ratio of a nanofluid versus base liquid,is almost temperature independent and has a terraced increase with volume concentration.At a fixed temperature and a fixed volume concentration,the viscosity decreases with particle size.Analyzing the viscosity variations of MMT nanofluids,we explored the relationship of viscosity with the particle-particle,particle-water,and water-water interaction.It was found that the viscosity enhancement of nanofluids mainly results from the strong interaction between particles and water molecules.Moreover,an expression was proposed to relate viscosity with temperature,and volume concentration by fitting the computed viscosity values into the named Arrhenius equation.Two parameters were defined in the expression.One is a function of particle-water interaction energy,while the other is a constant.Our proposed expression has similar form with the experimentally concluded one,but we found a new physical meaning for the defined parameters.Our study reveals the relationship of viscosity of MMT nanofluids with temperature and particle concentration,providing a theoretical basis for interpreting the related experimental observations. |
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