Study On The Displacement And Transportation Mechanism Of CO₂/CH₄ In Organic Nanopores Of Shale

Nanopores are widely distributed in unconventional tight reservoir rocks.Accurate cognition and quantitative description of CH₄ adsorption,desorption properties,CO₂/CH₄ displacement mechanism and transportation ability in nanopores of tight reservoir rocks are crucial for the implementation of CO₂ fracturing technology.However,unconventional tight reservoir usually shows a large burial depth and complex geological conditions.In particular,the process of CH₄ adsorption,desorption,CO₂/CH₄ displacement and transportation in the nanopores are concealed and dynamic change course.No matter in situ test or laboratory tests,the existing technical methods and equipments are difficult to accurately detect and quantitatively characterize these processes.Numerical simulation has become an effective means to break through the bottleneck of detection technology,to understand and quantitatively describe the dynamic process and mechanism of CO₂/CH₄ displacement and transportation in nano-pores.However,due to the lack of understanding of the physical mechanism of CH₄adsorption,desorption,CO₂/CH₄ displacement and migration in the complex pore structure,as well as the limitations of the simulation calculation method,there is still no satisfactory result for this problem.Aiming at the above problem,this paper takes shale organic matter with widely distributed nanopore structure as the research object.The competitive adsorption and desorption characteristics,displacement mechanism and transportation of CO₂/CH₄ in the nanopores of kerogen were quantitatively analyzed by laboratory test and molecular simulation method.Based on the experimental measurements,a molecular model of shale kerogen and nanopore with a rough surface controlled by fractal function were constructed.According to the kerogen structure,a gas diffusive tortuosity model describing the molecular-nanoscale pore structure was established.The effects of nano-pore surface morphology and structural characteristics on the adsorption,displacement mechanism and transport characteristics of CO₂/CH₄ were simulated and revealed.The innovative results of this study are summarized as follows:?1?Quantification of the CH4 adsorption performance in kerogen-rich shale reservoirs.The composition and pore networks of shale were analyzed comprehensively by means of shale thermal stability evaluation,TOC analysis,inorganic matters identification and nitrogen adsorption test.Based on the programmed heating method,chemical separation and adsorption experiments,the adsorption capacities of CH₄ in the pores of organic matter,inorganic matter and intergranular space were quantitatively characterized by the separation of organic matter and inorganic matter from shale.The difference of CH₄ adsorption performance between organic matter pore and intergranular pores was obtained by molecular simulation,and the results revealed that organic matter pore could be the key influencing factor to the shale gas storage.?2?Competitive adsorption characteristics of CH4/CO2 in kerogen of shale were evaluated by means of high pressure isothermal adsorption experiments and molecular simulations.The applicabilities of three-parameter Langmuir model and linear fitting method in correcting excess CO₂/CH₄ adsorption were examined.A kerogen molecular model with a disordered porous structure was constructed,and the competitive adsorption characteristics were analyzed at the molecular scale.The selective coefficient of CO₂ to CH₄ was investigated at different gas injection ratios,and the dominant adsorption ability of CO₂ to CH₄ was evaluated quantitatively.The distributions of adsorbed CO₂/CH₄ in kerogen at different injection ratios were observed through visualization software.?3?A tortuosity model describing the fluid diffusion property in molecular/nano--scale pore structures was established.The diffusion mechanism of fluid in kerogen were analyzed by molecular dynamics method.The effects of different influencing factors,such as adsorption capacity,gas molecular size,porosity,temperature and pressure,on fluid diffusion property were researched.It is proposed that the diffusive tortuosity in nano-scale pores was an extrinsic value closely related to the properties of the fluid.The differences between the conductive tortuosity and diffusive tortuosity of the kerogen model were compared,and the results verified the the importance of the interaction between gas molecules and pore wall to diffusion property.?4?Models with simplified and fractal rough surface were constructed respectively to simulate and reveal the adsorption,diffusion and flow characteristics of CH4/CO2 in rough nanopores.The reliability of Hagen-Poiseuille equation with slip boundary condition in describing CH₄ flow process was verified,and the main controlling factors affecting CH₄ nanoscale flow were analysized.Based on the analysis of the molecular motion trajectories,the effect of rapid CH₄ movement in the adsorption layer and the local smoothness of the pore wall on the slippage phenomenon were clarified.The influence of fractal roughness on the diffusion characteristics and flow velocity of CO₂/CH₄ in micro-scale pore structure was evaluated,and the internal mechanism of gas separation happened in the process of mixed gas flow was revealed.The selectivity coefficients of CO₂ to CH₄ were calculated,and the influences of gas injection ratios,system pressure and temperature,and pore size on the competitive adsorption performance of CO₂/CH₄ were investigated.The paper has 80 figures,15 tables and 245 references.