Molecular Insight Into The Enhanced Shale Gas Recovery By Carbon Dioxide

The extraction of unconventional natural gas has captured great attention because of global energy shortage and the increasing demand of energy consumption in the global energy markets.Shale gas,as one of the novel unconventional resources,has been successful exploited and raised a new energy revolution all over the world.Vast majority of shale gas is adsorbed in the nanoscaled pores of shale formation,due to the ultralow permeability of shale reservoir.Therefore,the high-volume hydraulic fracturing technique has been widely used in shale gas extraction,but still caused public concern,because of many issues,such as the waste of water resource and the pollution of the ground water.Recently,a new promising technique,that enhanced shale gas recovery by CO₂（CO₂-EGR）,had attracted much more attention,which not only achieve the shale gas extraction,but also achieve CO₂ capture and storage（CCS）at the same time.Most of the current studies about the enhanced shale gas recovery mainly focused on the macro-level explorations,the investigations on the micro-level are rare.The lack of the key microscopic information actually hinder the development and efficient application of shale gas exploitation techniques.Because of the complex composition of shale and the complicated environment of shale formation,the experimental exploration cannot provide accurate microscopic information,therefore,the molecular simulation method was recognized as feasible way to demonstrate the micro information in the process of shale gas extraction.In this study,the grand canonical Monte Carlo（GCMC）and molecular dynamics（MD）simulation methods,accompanied by the density functional theory（DFT）were used to investigate the detailed micro information of shale gas and the CO₂-EGR process.Based on the interest finding in the above research work,the capture of CO₂ from flue gas using porous materials constructed by some natural organic and inorganic substances which has strong interaction with CO₂ had also been investigated.The main content and innovation about this work are summarised as below:In the first part,the micro behavior of CH₄ and CO₂ in nanopores with typical chemical composition of shale had been investigated.The properties of adsorption and diffusion of CH₄ and CO₂ in nanopores of kerogen,quartz,montmorillonite,calcite and complex composition had been investigated.It is found that the adsorption capacity of CO₂ in nanopores is stronger than CH₄,meantime the self-diffusion of CO₂ is weaker than CH₄.Part of the CH₄ molecules adsorbed in nanopores can diffuse out driven by the pressure gradient,while a great deal of CH₄ still adsorbed in the nanopores,and the amount of the residual gas related to the composition of the nanopores,the temperature and pressure.The main conclusions about this part are summarized as below:1.The CO₂ has stronger adsorption capacity than CH₄ in kerogen nanopores,and the CH₄ and CO₂ molecules prefer to adsorb onto different positions of the kerogen fragments,according to the different interaction between CH₄ and CO₂ molecule with the kerogen fragments.The self-diffusion of gas molecules adsorbed inside the kerogen matrix is the weakest,meanwhile the part adsorbed around the center of the nanopore has the strongest self-diffusion capability.The CH₄ molecules adsorbed inside the kerogen nanopore can be desorbed gradually with the decreasing equilibrium pressure,and about 68%of CH₄ could diffuse out from the kerogen slit nanopores freely when the pressure is decreasing to 5 MPa at 323 K,and majority of the residual CH₄ still adsorbed close onto the surface of the kerogen slit nanopores.2.It was found that in quartz nanopores the CH₄ and CO₂ molecules have different adsorption properties,and would be affected as the pore size changing.The-self-diffusion capability of gases in quartz nanopores get enhanced gradually with the pore size increasing.Approximately 55%of CH₄ could diffuse out from the nanopores as the pressure from 20 MPa decreased to 3 MPa in the nanopore with the diameter of 0.65 nm,whereas about 80%of CH₄ could diffuse out in the nanopore with the diameter of 3nm.The surface of quartz nanopore was hydrophobically modified in different degree,it is found that the CH₄ molecules prefer to adsorb onto the hydrophobic surface,while the CO₂ molecules prefer to adsorb onto the hydrophilic surface,and the capacity of CH₄ diffusing out from the nanopores decreases with the enlarged surface hydrophobicity.3.It was found that the adsorption capacity of CO₂ is stronger than CH₄ in nanopores of montmorillonite and calcite,and the CO₂ has particularly strong interaction with the calcite surface,therefore the CO₂ molecules can adsorb onto the calcite surface rapidly and achieve a saturated adsorption layer.The CH₄ adsorbed inside the nanopores of montmorillonite and calcite can also diffuse out as the environmental pressure decreasing.And about more than 59%and 65%of CH₄ could diffuse out from the nanopores of montmorillonite and calcite,respectively,as the pressure from 20 MPa decreased to 5 MPa at 323 K.4.The micro-behaviors of CH₄ and CO₂ in nanopores with complex composition had been investigated.Graphene was chosen to represent the hydrophobic organic medium,the quartz and calcite were chosen to represent the hydrophilic surfaces.It is found that the CH₄ molecule prefers to adsorb onto the graphene surface,while the CO₂ molecule has strongest adsorption interaction with the calcite surface.The gases have different adsorption distance to different pore surfaces,according to the different charge properties and the surface roughness.And it is very interesting to find that CO₂ gas has different adsorption orientation on different pore surfaces.The self-diffusion of CH₄ adsorbed onto the graphene surface is the weakest,while the CO₂ adsorbed onto the calcite surface has the weakest self-diffusion.In the second part,the competitive adsorption properties of CO₂ over CH₄ in various shale nanopores have been investigated,accompanied by the displacement properties of CH₄ by CO₂ in various nanopores.It is found that the competitive adsorption of CO₂ over CH₄ is ubiquitous in shale nanopores,therefore the adsorbed CH₄ in shale nanopores could be displaced effectively.And some differences are existed in the displacement efficiency of different nanopores.The main conclusion about this part are summarised as below:1.The competitive adsorption of CO₂ over CH₄ is obvious in kerogen nanoproes due to the stronger adsorption interaction between the CO₂ molecules with the kerogen fragments comparing with CH₄.The selectivity of CO₂/CH₄（SCO₂/CH₄）decreases first and then increases with the pressure increasing,and the temperature is unfavorable to the competitive adsorption.The residual CH₄ can be displaced by CO₂ gradually with the pressure increasing,and the displacement efficiency could reach 84%at the displacement pressure of 20 MPa and 323 K.Moreover,it is found that a small part of CH₄ molecules is still adsorbed firmly inside the intrinsic pores of the kerogen matrix after the displacement by CO₂,which is hard to be displaced by the injected CO₂.2.The competitive adsorption properties of CO₂ over CH₄ and the displacement efficiency of CH₄ by CO₂ in quartz nanopores with the variation of pore size and surface hydrophilicity have been investigated.The SCO₂/CH₄ gets enhanced as the pore size increasing,and the surface hydrophilicity also has significant contributions to the competitive adsorption.Meantime,the displacement efficiency gets enhanced with the pore size increasing and the enlarged surface hydrophilicity.3.The competitive adsorption of CO₂ over CH₄ is obvious in montmorillonite nanopores,the SCO₂/CH₄ decreases with the enlarged pressures and temperature.Meantime,the displacement efficiency gets enhanced with the pressure increasing,and when the displacement pressure reaches 15 MPa at 323 K,the displacement reaches the extremum value of 83%.4.The competitive adsorption of CO₂ over CH₄ is very strong in calcite nanopore due to the strong adsorption interaction between the CO₂ molecules with the calcite surface.The SCO₂/CH₄ is extremely large at the initial adsorption,and decreases with the pressure increasing,meantime the temperature is beneficial to the competitive adsorption.The displacement efficiency could reach 90%when the displacement pressure is 20 MPa at 323 K.5.The competitive adsorption of CO₂ over CH₄ in nanopores with complex composition is extensively exist.And the competitive adsorption of CO₂ over CH₄ is particularly obvious in the nanopores with the calcite surface,due to the strong interaction between the CO₂ molecules with the calcite surface.In the third part,based on the previous research results,the possibility for capturing CO₂ by using the materials of natural organic（asphaltene）and inorganic mineral（calcite）had been investigated.The properties of adsorption,self-diffusion and the competitive adsorption of CO₂ over N₂ in nanopores of asphaltene and calcite had been demonstrated,and found that both of these two adsorbents have good abilities to capture CO₂ from flue gas,which have potential applications in the CO₂ capture from flue gas.The detailed information are summarized as below:1.The CO₂ molecules have stronger adsorption interaction with the asphaltene comparing with N₂,the adsorption energy of CO₂ adsorbed ono the asphaltene fragments could reach-3.56 kcal/mol,while the adsorption energy of N₂ adsorbed ono the asphaltene fragments is about-2.06 kcal/mol.Therefore,the competitive adsorption of CO₂ over N₂ is obvious in asphaltene nanopores,and which decreases with the enlarged pressure and temperature.2.The CO₂ and N₂ molecules have different adsorption properties in calcite nanopores.And the CO₂ molecules have very strong adsorption interaction in calcite nanopores.Therefore,the competitive adsorption of CO₂ over N₂ is exist,and the competitive adsorption capacity decreases with the enlarged pore size and the increasing pressure.To sum up,the microscopic behaviors of CH₄,CO₂ and N₂ in various nanopores were explored.And the mechanism of the competitive adsorption of CO₂ over CH₄,accompanied by the displacement of CH₄ by CO₂ in nanopores were investigated,for the purpose to give out useful guidance for the efficient performance of CO₂-EGR.Meantime,the CO₂ capture by using the natural organic and inorganic minerals were also investigated,which revealed the possibility of capturing CO₂ from flue gas using porous materials constructed by asphaltene and calcite,and might provide useful guidance in the CO₂ capture from flue gas using natural and low-cost materials.