Experimental Study On Chemical Structure And Wettability Of Shale After Carbon Dioxide Action

When CO2 is injected into shale gas reservoirs instead of water to enhance shale gas exploitation,it can realize the permanent storage of CO2 in shale reservoirs.After CO2 is injected into shale reservoir,under high temperature and high pressure environment,the CO2-shale interaction changes the mechanical and physicochemical properties of shale,thus affecting the exploitation effect of shale gas and the stability of CO2 storage.At present,there are related researches on the mechanical properties and pore structure of shale after the action of CO2,while the research on chemical properties such as chemical structure of shale after the action of CO2 is still in the initial stage,and its influencing rules and mechanism are still unclear.Therefore,it is of certain guiding significance to carry out research on shale chemical characteristics after CO2 effect in reservoir environment for enhancing efficient exploitation of shale gas and integrating geological storage with CO2.In this paper,longmaxi outcropping shale is taken as the research object.CO2 immersion shale experiments with different temperature,pressure and water content are conducted.FTIR infrared spectrum analysis,XRD analysis and SEM-EDS analysis of the obtained samples are carried out to obtain the following results:Based on FTIR infrared spectrum analysis,reveals the change rule of chemical structure of shale after CO2 action.After CO2 treatment,branched chain index and aromatic number of shale organic components decrease,aromatic ring opening,fat chain length increases,and organic compounds undergo cracking or extraction.The characteristic peaks of carbonate are divided,the characteristic peaks of silicate overlap,the dissolution of carbonate occurs and the formation of new species of carbonate.The increase of temperature and pressure can promote the effect degree of CO2-shale,and the law of influence is consistent,but the effect degree of pressure is more significant.The sample treated with CO2+H2O has a stronger and wider characteristic peak of adsorbed water,and the absorption peak intensity of the carbonate absorption peak(around 1420cm-1)is shown as follows: untreated >CO2 treated with >CO2+H2O treatment,the dissolution degree of CO2+H2O is the most severe.By XRD and SEM-EDS analysis,the effect of CO2 on shale mineral composition and mineral microstructure is obtained.After CO2 treatment,the number of diffraction peaks in dry and water-bearing shale increases with the pressure of CO2 treatment,and there is electrical interaction between CO2 and quartz and calcite mineral particles.Compared with the dry condition,the surface dissolution reaction of shale in water bearing condition is more severe.The degree of dissolution reaction of minerals changes slightly with the increase of CO2 pressure.At 4MPa,the dissolution reaction is basically completed after CO2 treatment.EDS and binary maps show that the white large particles on the shale surface are mainly dolomite and calcite.After CO2 treatment,the shale surface structure becomes looser,the content of Ca,Mg,C and O decreases,and significant corrosion holes appear,and the dissolution of shale is more obvious after CO2+ water treatment.the wetting characteristics of shale under different phase CO2 and water cut conditions are clarified.Under drying conditions,the measurement results of contact Angle under different phase states show that: gas < liquid < supercritical state,the content of carbonate,the hydrophobic mineral component of shale,decreases with the increase of CO2 treatment pressure,while the hydrophobic shale increases.Water conditions,shale wetting Angle by surface micro porous capillary force and hydrophilic mineral content dual control,when shale gas and liquid surface morphology change is small,the supercritical CO2 solubility increases,there will be more hydrogen ions in the solution,and continue to happen dissolution reaction,the state of the water contact Angle size is generally less than the dry sample of the same processing conditions for CO2,embodied in: supercritical gas < liquid.In supercritical state,the contact Angle of shale surface tends to increase with the increase of CO2 treatment pressure.