Atomistic Insights Into Mechanisms Of Shale Gas Adsorption And Carbon Dioxide Displacement And Storage

Carbon dioxide enhanced shale gas recovery（CO₂-EGR）is very important for clean-energy development and achieving carbon emission peak and carbon neutrality goals.To obtain better prediction and optimize shale gas recovery and CO₂sequestration efficiency,it is necessary to understand the mechanisms of shale gas adsorption,CO₂displacement and CO₂sequestration.Pores in shale reservoir are dominated by nanopores.In this thesis,we focus on investigating the mechanisms of shale gas adsorption,shale gas recovery with CO₂injec-tion and CO₂sequestration in the nanopores by means of molecular simulation methods,including molecular dynamics and grand canonical Monte Carlo.Effects of realistic geo-logical conditions,such as temperature,pressure,pore structure,and the subsurface water,are examined in detail.The kerogen model is built to simulate the shale organic matter.The adsorption properties of single-component and multi-component shale gas in kerogen nanopores are investigated.Heavier hydrocarbons corresponds to higher adsorption capacity.The ad-sorption capacity of C₃H₈is larger than that of C₂H₆,and CH₄showed the smallest ad-sorption capacity.In the competitive adsorption of multi-component shale gas,adsorption of C₃H₈and C₂H₆is enhanced while that of CH₄is weakened.Compared with the traditional pressure drawdown method,the recovery mechanism of single-component and multi-component shale gas by CO₂injection is revealed.The injected CO₂replaces CH₄molecules from the adsorption layer,while during pressure drawdown stage,CH₄molecules are released from the middle of kerogen slit pores.Due to the high proportion of adsorbed gas in nanopores,CO₂injection can promote the shale gas recovery efficiency.For CH₄/C₂H₆/C₃H₈multi-component shale gas,due to the strong interaction between C₃H₈and the kerogen wall,CO₂injection efficiently recovers C₃H₈from 2 nm wide pores while the pressure drawdown method is only efficient for CH₄and₂₆.C₃H₈in the 4 nm wide pores can be recovered by pressure drawdown method but with very low efficiency.On the whole,CO₂injection can greatly improve the recovery efficiency of heavy hydrocarbons.Effect of the formation water on CO₂sequestration mechanism is revealed.When shale nanopores are partially filled with the subsurface water,both the adsorption and dis-solution of CO₂should be considered.The density distribution of adsorbed CO₂hardly changes with the salt concentration.It should be noted that the solubility of CO₂in con-fined water is much higher than that in bulk water,and the solubility decreases as the salt concentration increases.The injected CO₂alters the wettability of kerogen surface from weakly water-wet to CO₂-wet.This thesis unveiled the microscopic mechanisms of the kerogen wettability transformation induced by CO₂.The kerogen is originally at weakly water-wet state be-cause the oxygen-containing functional groups on the surface of kerogen form hydrogen bonds with H₂O.However,the stronger interaction between CO₂and the carbon skeleton of kerogen results in CO₂entering into the water droplet-kerogen interface.As the CO₂pressure rises,the CO₂density on the kerogen surface gradually increases and finally a CO₂film forms at the water-kerogen interface.As a result,the number of hydrogen bonds between the water droplet and kerogen surface decreases and the contact angle of water droplet increases.