Study On Interfacial Characteristics Of CO₂/CH₄/Water System

In recent years,reducing CO₂ emissions has become the world’s unanimous appeal,and CO₂ geological storage technology is an effective way to solve carbon emissions problems.In the exhaust gas produced in production and life,in addition to CO₂,other gases such as CH₄,Ar,and H₂S are often mixed.At present,CO₂ is usually captured in the exhaust gas before geological storage,which will greatly increase the cost of CO₂ geological storage.Therefore,direct geological storage of exhaust gas will be the key to significantly reduce the cost of CO₂geological storage.It has been found that while CO₂ geological storage is in progress,the use of captured CO₂ for oil and gas（natural gas,coalbed methane）recovery can improve the extraction efficiency of oil fields and gas fields.Supercritical carbon dioxide（ScCO₂）fracturing technology,which is used for shale gas extraction,with its green,environmentally,friendly and efficient,is gradually replacing the hydraulic fracturing technology.In the first part of this dissertation,the influences of adding CH₄,Ar and H₂S to CO₂ on the interfacial tension（IFT）and wettability（contact angles,CA）of the CO₂/water/silica system have been investigated using molecular dynamics simulation methods at 20 MPa and 318 K when the molar concentration of impurity gas was fixed to 20%.For the conditions studied:（1）CH₄ has no significant effect;（2）Ar leads to higher IFT,larger CA on silica surface with high hydroxyl density,and smaller CA on silica surface with low hydroxyl density;（3）H₂S causes decrease of IFT and increase of CA.Capillary pressure and gas storage capacity were predicted using IFT and CA data and the variation of IFT and CA were explained based on density profiles normal to gas/water and gas/silica interfaces.These findings may be helpful to better understand the effect of impurities on CCS.In the second part of this dissertation,based on the background of using ScCO₂ fracturing technology to extract shale gas,the interfacial tension of（CH₄+CO₂）/water system was simulated.The results show that the addition of CO₂ can effectively reduce the interfacial tension between CH₄ and water,thereby reducing the capillary pressure during fracturing,increasing the fracture permeability,and effectively preventing CO₂ leakage during the subsequent CO₂ sequestration process.At the same time,it was found that the interfacial tension of（CO₂+CH₄）/water mixed at a molar ratio of 1:1 is equal to the average of the interfacial tension of CO₂/water and CH₄/water under the same conditions.This finding is instructive for exploring the microscopic mechanism of CO₂ in enhancing shale gas extraction.It is instructive for exploring the microscopic mechanism of CO₂ in enhancing shale gas extraction.