Molecular Simulation Of Supercritical Carbon Dioxide Fluid Adsorption Behavior In Rock

In this paper, molecular dynamics (MD) method is put in the structure research ofsingle phase of supercritical carbon dioxide, and combined with liquid acoustic theorysimulated several physical parameters include density, self-diffusion coefficients, viscosityand surface tension. The results show that: there is density fluctuation phenomena inside thesupercritical carbon dioxide bulk, the instantaneous dipole moment produced by molecularbond angle deviate from180°is the primary cause for this phenomenon； super-criticalstate is condition that carbon dioxide transition from liquid to gas, has clear liquidcharacteristics, so liquid acoustic theory can be used in the research of supercritical carbondioxide.; the simulation results of self-diffusion coefficients, viscosity and surface tension.are consistent with the data of the National Bureau of Standards (NIST).According the geological conditions molecular dynamics simulations is use to studythe structure of the carbon dioxide molecules adsorbed in the quartz slit under thesupercritical state, the characteristics of the adsorbed structure and the causes of formed isbeing analyzed, and the affect of slit width on the adsorption is being studied. The resultsshow that: in the quartz slit which is width is30, supercritical carbon dioxide moleculesarranged into two layer-the adsorption layer and the free layer, in which the adsorption layerarranged into the first and the second layer, the first layer has the maximum density,molecular aggregation phenomenon is obvious, the free layer as the minimum density, thesecond adsorption layer has a density in the middle, adsorption layer has a betterarrangement and same as the arrangement of the hydroxyl on quartz surface; Vdw force andhydrogen bonds is the is the primary cause for this phenomenon, in which dispersion forceand induced force are the main force and hydrogen bonds are conducive to the stabilizationof the adsorption structure; the adsorption structure formed a relatively stable electric fieldwithin the system, this made structure more stability; carbon dioxide molecules of eachlayer presents different characteristics in regularity and migration ability, The firstadsorption layer presents order the best order, and which In turn reduce in the second adsorption layer and the free layer, but the migration ability is on the contrary, the molecularin each layer all have a better migration ability in horizontal direction than in normaldirection; the slit width reduction will causes a decline in the adsorption layer structure,cause the order of each layer increase but the migration ability decline, and ultimatelyenhance the adsorption stability.In this paper, the molecular dynamics simulation method is used to evaluate the effectof the main environmental factors of the stratum (slit size, pressure, temperature, wettabilityand formation water) on adsorption of supercritical carbon dioxide in slit rock, and made abrief exposition on the influence mechanism. The results show that: in the quartz slit whichis width is smaller then30, the adsorption capacity of Supercritical carbon dioxide in theslit is increase with the width, but when the slit with below10it begin to reduced again.Pressure has promoting effect on adsorption of supercritical carbon dioxide in the rock,pressurizing made the density of carbon dioxide increase play a major role in this process,the adsorption density gathered cased by slit with supercritical carbon dioxide moleculesassume a inverse proportion with the pressure. temperature is not conducive to adsorption ofsupercritical carbon dioxide in the rock, that’s because physical absorption is a exothermicprocess temperature increment Is not conducive to the process; the effect of rock wettabilityon supercritical carbon dioxide adsorption capacity in rocks assumed a polarizationcharacteristics, adsorption capacity varies with rock from water-wet to oil-wet fully showsthe tendency of decrease after the first increase, hydrogen bonding and similar compatibleaction play a major in this transformation; when formation water is relatively small itconducive to the adsorption of supercritical carbon dioxide in rocks, but too much waterwill cover rock surface and occupy the adsorption of carbon dioxide molecules and madeadsorption reduce.