Molecular Dynamics Study On The Interfacial Properties And Flow Features Of CO₂-H₂O-rock Mutiphase System

CO₂ geological storage is an effective measure to reduce the emission of greenhouse gas CO₂,while deep saline aquifer is one of its main reservoir.During the CO₂ storage process,the wetting properties of the reservoir rock surface determine the safety of the storage.CO₂ and water form a multiphase flow in the pores of rocks when CO₂ leakage occurs.Therefore,it is important to efficiently store CO₂ by studying the fluid-solid interface properties and the multiphase flow characteristics.Molecular dynamics simulation?MD simulation?is an important tool for the study of the mechanism of physical and chemical processes.It is observable for the microscopic structure and characteristics which are hard to be measured in experiments.On the basis of this,MD simulation is applied here to study the impact of microcosmic characteristics of the rock surface?degree of hydroxylation?on characteristics of the wetting behaviour and multi-phase flows in nano-porous.In this paper,the effect of hydroxylation degree of rock surface on the wetting characteristics of rock under CO₂ storage conditions?330K,20MPa?is firstly studied by using MD simulation method.In the CO₂ atmosphere,we calculate and analyze the droplet shape of H₂O formed on the surface of the rock with different degrees of hydroxylation,and use the contact angle to reflect the wetting characteristics.The results show that the degree of hydroxylation increases from 0%to 50%and the contact angle decreases from 110.24 ° to 14.56 °,when the degree of hydroxylation increases from 50%to 75%,the contact angle increases slightly of 3.22 °;when the degree of hydroxylation continues to rise to 100%,the contact angle and a slight reduction of 6.06 °.Through the analysis of intermolecular force,the effect of the degree of hydroxylation of the rock surface on wetting characteristic is the co-effect of the coulombic effect and the hydrogen bond.The hydrogen bond not only includes the hydrogen bond between the hydroxyl group and the water molecule,but also includes the hydrogen bond between the hydroxyl groups on the rock surface.The former plays a role in promoting the surface wettability,and the latter is the inhibitory effect.Secondly,the MD simulation of CO₂ and H₂O distribution in the pores of nano-rock under 330K and 20MPa is carried out in this paper.The changes of the parameters such as the density of CO₂ and H₂O,the average orientation angle of interface molecules and the radial distribution of interface were analyzed.The effects of hydroxylation degree on the distribution of CO₂ and H₂O in the pores were investigated.The results showed that CO₂ and H₂O were stratified in the form of body phase,and only a small amount of carbon dioxide was dissolved in water.Only CO₂ exists on the rock surface pores where is in the absence of hydroxyl.On the surface of the rock with hydroxyl groups in the pores,there is an aqueous layer formed by water molecules.The thickness of the water layer is affected by the degree of hydroxylation.The higher the degree of hydroxylation,the thicker the water layer.In the body phase of CO₂ and H₂O,the molecules are randomly distributed.The CO₂ tends to be arranged in parallel at the interface of CO₂-H₂O,and the H₂O dipole moment tends to point to the CO₂ side.At the CO₂-rock interface,CO₂ tends to be inclined.At the H₂O-rock interface,near the surface with a degree of hydroxylation of 25%,the H₂O dipole moment tends to point to the water side.At the other interface,the water molecules tend to point to the surface of the rock and the more close to the rock surface,the more obvious the tendency.In the last part of paper,the flow characteristics of CO₂-H₂O in the nano-scale pores with the same degree of hydroxylation in the surface is studied.The characteristics of the single-phase flow and CO₂-H₂O multiphase flow in the nanopores are studied by means of MD simulation.The results show that there is a linear relationship between the average velocity and the driving pressure in the single-phase or multi-phase flow,consistenting with Darcy's law,and the CO₂ flow velocity under the same conditions is higher than H₂O.The flow velocity is symmetrical in the form of parabola along the height of the pore.That is to say,the velocity of water molecules close to the rock wall is low,while the velocity at the center is high.When the CO₂-H₂O multiphase flow,the velocity distribution of CO₂ and H₂O is similar to that od single-phase H₂O.The lower the distance from the wall,the lower the velocity,the CO₂ in the middle of the pore is negligible by the influence of the wall,only by the attraction of nearby water molecules,whose velocity is in line with the law of the single-phase flow of Poise leaves.H₂O molecules are not only attracted by the rock surface,but also by the middle of the CO₂ molecules to,so that water in the CO₂-H₂O interface near the high speed.