Molecular Dynamics Investigation On The Adsorption Behaviors Of Gases On Wetting Rock Surface

Natural gas is one of the most efficient and high-quality energy across the world.In recent years,the gas reservoir exploration have made a great progress in Sichuan,Shanxi,Xinjiang provinces and so on in our country.During the fracturing stimulation and transformation process of gas wells,there are many gas components presence on the reservoir rock surface,such as N2,CO2,CH4,and H2 O and so on.Limestone and chalk provide a substantial portion of the world's hydrocarbon reservoirs,so fuller appreciation of calcite surface interactions with gas and water will result in increased gas recovery.Although researchers have been studying calcite-fluid interactions for at least five decades,it is usually very difficult for traditional experimental methods to study the adsorption of those gas mixtures on rock surface at high temperature and pressure of reservoir conditions.Hence,accurately predicting the interfacial physical properties of rock surface at reservoirs conditions is very challenging.In this work,we performed MD simulations to illustrate the direct interaction between the calcite(110)surface with pure gas molecules.Our calculations results can not only give a microscopic insight into the wettability of gas reservoirs but also provide theoretical instructions for hydrate risk evaluation of enhanced gas recovery.Firstly,at 373 K temperature and 20 MPa pressure conditions,by analyzing the binding energy and Helmholtz free energy,we find that the preferential adsorption order of these four gases is: H2O>CO2>CH4>N2.Moreover,we have also calculated the density profiles and radial distribution function profiles of the system.It shows that the H2 O molecules will form two compact adsorbed layers on the surface,which denotes the calcite(110)surface is hydrophilic.The CO2 molecules can also form a adsorbed layer on the surface,while there are no significant features indicating that the CH4 and N2 molecules were apparently adsorbed on calcite(110)surface.Secondly,we designed to explore the behavior on calcite surface in contact with ethanol solutions.The MD simulations show that at the near surface,each ethanol hydroxyl is oriented toward the surface with its O pointing toward a calcium ion and its H binding to an oxygen from a carbonate.The CH3 ends of ethanol point away from the surface,making a hydrophobic termination for this first adsorbed layer and alter the wettability of calcite surface.Through the binding energy and Helmholtz free energy calculations,we find that the ethanol molecules have a stronger interaction with calcite surface than water molecules.At last,we find that once ethanol attaches to calcite,it will continue to control surface behavior.We calculated the adsorption behavior of water,CO2 and CH4 gases on ethanol-controlled calcite surface.From the results,we find that the water adsorption turns to monolayer and CO2 molecules and CH4 molecules become evenly distributed in the system.Therefore,ethanol solvent can make a breakthrough on water lock effect and enhanced gas recovery.