Experimental Study On Competitive Wettability Characteristics Of Carbon Dioxide And Water In Different Metamorphic Coal At High Temperature And High Pressure

CO₂ injection into coal bed is an effective technology to enhance methane recovery and carbon dioxide sequestration.However,it's not clear that the influence of CO₂ and coal physicochemical reaction on coal surface wettability which affects immiscible multiphase displacement.In this thesis,by measuring the static contact angles and dynamic contact angles,we studied the influence of different metamorphic coals with gaseous,liquid and supercritical CO₂ at pressures of 1-141 bar and temperatures of 25-60?on the surface wetting characteristics.To explore the effect of CO₂ and coal-based surface reaction on wettability,we compared with the coal surface wetting characteristics after CO₂-H₂O and N₂-H₂O reaction.It is found that the variation of CO₂-H₂O competitive wetting characteristics with coal metamorphic degree is affected by temperature and pressure conditions.When the temperature and pressure are lower than the critical temperature and pressure of carbon dioxide,the contact angles increase with the increase of coal metamorphic degree.However,in the supercritical CO₂ environment,the contact angles of low-rank bituminous coal are close to or greater than that of middle-and high-rank bituminous coal,but lower than that of anthracite.The difference of CO₂-H₂O competitive wetting characteristics of coals with different metamorphic degrees is mainly related to coal composition,surface functional group content,surface morphology and degree of coalification.The influence of temperature and pressure on the competitive wetting characteristics of CO₂-H₂O is related to the phase of CO₂.Reaction with gas CO₂slightly changes the wetting characteristics of coal surface,from water wetting to intermediate wetting,and the static contact angle increases from 60°to 70°-90°.Reaction with liquid and supercritical CO₂ changes the coal surface to intense CO₂wetting with contact angles up to 115-180°.The static and dynamic contact angles increase significantly with the increase of pressure,and the effect of temperature on the contact angle is opposite,and the effect is more significant under supercritical pressure.The change of contact angle with pressure and temperature is strongly positively correlated with the density of CO₂.The competitive wetting characteristics of CO₂-H₂O and N₂-H₂O in coal are different due to the gas adsorption and its reactivity with coal based surface.When both CO₂ and N₂ are gaseous,the contact angle values of the two gases in the same coal sample are close to each other,and both of them show strong water wetting.When CO₂is transformed into supercritical state but N₂ is still gaseous,the contact angle of coal after reaction with Sc CO₂ is significantly higher than that of N₂,increasing from 20°-50°to 35°-55°,and its wettability to water is weakened.The contact angle of the coal increases with time and gradually decreases with the reaction of Sc CO₂,but the contact Angle of the coal with N₂ and gaseous CO₂ does not have this phenomenon.The reaction of Sc CO₂ with coal surface results in the decrease of wettability of coal to water.There are 50 figures,22 tables and 103 references in this paper.