Study On The Effect Of Tectonic Coal Pore Fracture Structure Evolution On Methane Adsorption And Desorptio

Guizhou is rich in coal resources and is the most important coal base in the south,but tectonic coal is commonly found in Guizhou,especially in mines with serious prominence.The research on methane adsorption and desorption characteristics of pore and fissure structure of tectonic coal and solid-liquid-gas conversion law can provide a deeper understanding of the evolution of pore and fissure structure of tectonic coal on methane adsorption and desorption law,which can provide coal miners with corresponding knowledge in improving the efficiency and production of gas extraction.At the same time,this kind of research is of great significance to prevent and control the occurrence of gas disasters.In this paper,we adopt physical experiments,theoretical analysis,adsorption-desorption models and numerical simulations to quantitatively characterize the pore and fissure structure of tectonic coal and explore the methane solid-gas-solid-liquid adsorption-desorption patterns based on the theory of micro-pore filling and liquid-phase adsorption.The main conclusions are as follows:（1）The pore volume and specific surface area of experimental coal samples are22.4-62.5 nm and 11.9-26.7-100.7 nm,respectively.The comparative analysis of the two test methods shows that the pore volume and specific surface area measured by high-pressure mercury injection method are higher in large pores,while the pore size measured by liquid nitrogen adsorption method is 3-160 nm,and the pore volume and specific surface area ratio of micropores and small pores are higher.The two test results are inconsistent.Therefore,according to the principle of joint porosity,the two test results were characterized by joint porosity.The pore volume and joint porosity range of experimental coal samples were 22.4-23.4 nm,57.3-62.5 nm and 57.3-62.5nm,respectively.The pore surface area of experimental coal samples is 26.7-32.4 nm,44.5-50.4 nm and 82.2～100.7 nm,respectively.（2）The pore characteristics of structural coal affect the methane saturation adsorption capacity and methane desorption rate.The methane isothermal adsorption test was carried out on the experimental coal sample to obtain the methane saturation adsorption capacity.Through the linear fitting of the pore volume and specific surface area of the experimental coal sample and the methane saturation capacity,it was found that the correlation coefficient between the pore volume and the saturation adsorption capacity was 0.9581,which was positively correlated.The correlation coefficients between pore volume and specific surface area of small pore and large pore and desorption rate were 0.9919 and 0.9788,respectively.It shows that the pore characteristics of structural coal control the methane adsorption and desorption process.（3）The DA-Langmuir（liquid phase）adsorption and desorption equation based on solid gas-solid liquid adsorption theory can well reflect the isothermal adsorption characteristics of methane in structural coal.The liquid-phase adsorption theory can well explain the process of coal seam gas production.Based on the two adsorption theories,a methane adsorption model controlled by solid gas and solid liquid was constructed,and the DA-Langmuir（liquid-phase）adsorption desorption equation and methane isothermal adsorption data had the lowest fitting degree of 0.9963.At the same time,the volume and specific surface of the critical pore at 11.9 nm are fitted with the parameters D₁and D₂.The fitting degree is 0.9689,indicating that the parameters D₁and D₂can reflect the methane adsorption ratio of different adsorption forms in structural coal.（4）Based on the DA-Langmuir（liquid phase）adsorption and desorption equation and molecular simulation results,methane occurrence was divided into solid,gaseous and liquid pores.The occurrence state of methane in the small aperture range of coal combined holes is defined as solid hole,the occurrence state of methane in the aperture range of combined holes is defined as gaseous hole,and the occurrence state of methane in the large aperture range after combined holes is defined as liquid hole.