Study On The Effect Of CO₂-Induced Changes In Coal Structure On Methane Diffusion

Due to the characteristics of coal reservoirs in east of Yunnan and west of Guizhou,the natural production of CBM is low,so it is necessary to take stimulation measures to obtain industrial production capacity,and carbon dioxide fracturing and injecting are the important stimulation technologies of CBM.The phase state of carbon dioxide,fracturing treatment time and so on are the key to affect the fracturing effect.Carbon dioxide of different phase state will have different physical and chemical reactions with coal when it flows in the cleat or matrix of coal,which can change the structure of coal,and thus affect the methane diffusion and seepage.Due to the complex structure of coal in east of Yunnan and west of Guizhou,the simple Euclidean geometry principle can't be used to describe it,and establish diffusion or seepage model of methane.Therefore,in recent years,many scholars have introduced the fractal theory into the structural description of coal,and studied the seepage of methane in coal reservoirs,but it is rare to use fractal theory to study the diffusion of methane in coal.In view of the above problems,knowledge and approaches in other fields,such as oil&gas engineering,mechanical engineering,heat and mass transfer theory,and fractal theory,have been applied to the study on the change of microstructure of coal under the reaction of CO₂ and coal in different conditions,and the change of relative diffusion coefficient of methane caused by structural change.The specific research work has the following three aspects.?1?Developing a new visual experimental device that satisfies the interaction between carbon dioxide and coalAnalyzing the advantages and disadvantages of existing devices.According to the temperature and pressure of coal reservoirs and working conditions of field construction,using mechanical design and other related knowledge to develop a visual reaction device that can simulate the formation environment of coal.?2?Experiment and analysis of property change before and after coal and CO₂ interactionThe orthogonal design method was used to carry out several different types of experiments,at the same time,writing a program to extract the image of SEM for calculating fractal dimension of fracture shape.Comparing and analyzing the changes of specific surface area,average pore diameter,pore volume,fractal dimension of pore,fractal dimension of fracture shape and coal permeability after the reaction,and summarizing the change rule of the above parameters after CO₂ treatment,proving that carbon dioxide can effectively improve the microstructure of coal and increase the permeability of coal.At the same time,I analyzed the influence laws of the three factors of "reaction time","carbon dioxide phase state" and "mineralization degree" on the permeability,microstructure and fractal dimension of coal.?3?Characterization and analysis of relative diffusion coefficient of methane considering fractal characteristics of coal.On the basis of considering the joint action of Knudsen diffusion and bulk diffusion,a relative diffusion coefficient model of the fractal-like tree branching network under methane steady-state condition is established.The tortuosity and microstructure parameters of the fractal network structure are introduced into the model.Then,writhed a model solving program,brought the experimental parameters or other structural parameters into the equation,and analyzed the change rules of the relative diffusivity of methane under different treatment results and different structural parameters.This study indicates that supercritical carbon dioxide has the greatest effect on the improvement of coal structure;within 48 hours,the improvement of coal structure became more obvious with the increase of reaction time,but after 24 hours,the improvement effect gradually decreases;after CO₂ treatment,the methane diffusion coefficient will increase and the improvement effect will increase.Therefore,it is suggested that supercritical carbon dioxide be used to treat coal for 48 hours in site construction.