Experimental Research On Influences Of Coal Micro-Pore Structure On Stress Response Characteristics Of CO₂-ECBM

The efficient recovery of China’s coalbed methane(CBM)still faces problems such as low production and poor benefits.After the CBM in Qinshui Basin and the Ordos Basin formed a commercial production scale,CBM recovery in high-rank coal seams entered the recovery stage of middle-deep depth with complex occurrence conditions.Because of the characteristics of deep burial depth,low permeability,and weak adsorption,the CBM in low-rank coal seams does not have the development technology adapted to different formation conditions.The production increase mechanism of the CBM enhancement by CO2 injection(CO2-ECBM)in different rank coal seams with complex stress conditions needs to be improved.In this paper,multi-scale joint analysis,reconstruction,and characterization of the micro-pore structure in five coal samples with different ranks were performed through mercury intrusion experiments,liquid nitrogen absorption and desorption experiments,and X-ray μCT scanning technology.The numerical simulation was carried out through the micro-pore reconstructed models,and the methane micro-seepage mechanism was discussed.In addition,an experiment of CH4 enhancement by CO2 injection in strssed raw coal was carried out based on a self-developed experimental system,and the results revealed the influences of micro-pore structure on the stress response characteristics of CO2ECBM.The main research results are as follows:Quantitative generation characteristics of coal multi-scale pores was jointly performed based on mercury intrusion and liquid nitrogen adsorption and desorption experiments.Results show that:as the ranks of coal metamorphism increases,the proportion of micropores and transitional pores gradually increases,while the degree of openness and connectivity of the seepage pores decreases,The pore size distribution changed from a multimodal shape to a polarized one.Lowrank coal has better development and higher fractal dimension,so it has good permeability.The increase of aromatic rings in molecular structures of high-rank coals has led to the extensive development of micro-pores,which has a higher proportion of adsorption pores,a more uniform distribution law,and a more complex spatial structure.Therefore,its methane or greenhouse storage capacity are stronger.X-ray μCT scanning technology was used to reconstruct the 3D structure of micro-pores,established threshold model and equivalent pore network model(PNM).Spatial structure,topological structure,connectivity,fractal characteristics and percolation of micro-pores were further analyzed.Results show that:the shape of the isolated pores in high-rank coal is more complex,which is closer to the spherical shape in low-rank coal.The shape factor has a linear relationship with surface area in isolated pores,but it is more sensitive to the equivalent diameter which has an exponential relationship.As the ranks of coal metamorphism increases,the volume,percolation probability and Euler number of connected pore clusters gradually decrease,but its eccentricity and the specificity of spatial morphology gradually increases.In addition,the reduction of the volume and equivalent radius of the pores and throats in PNM are the main factors leading to the decrease in permeability.The inverse optimized structure and numerical model of coal micro-pore clusters were established,and the numerical simulation of methane micro-seepage was carried out.Data shows that:the local fluctuation of pressure at the microscale gradually becomes more obvious with the increase of the pressure gradient.As the ranks of coal metamorphism increases,the area of the high-speed seepage area decreases.The unidirectional and multidirectional micro-seepage of methane in coal pores has an advantageous direction,and the throat size has a greater influence on it.It is more stable in low-rank coal and middle-rank coal,but is more significantly affected by pore structure in high-rank coal.An experiment of CH4 enhancement by CO2 injection in strssed raw coal was performed to analyzed the influences of coal micro-pore structure on stress response sharacteristics of CO2 and CH4 seepage,CO2 and CH4 adsorption and CO2-ECBM.Results show that:coal micro-pore structure will affect the perturbation of stress to gas seepage,and the sensitivity will increase as the size increases.The limited adsorption capacity of CO2 and CH4 decreases exponentially with the increase of pore radius,and its rate slows down.During CO2-ECBM,the stress sensitivity of CH4 concentration decreases with the increase of the pore or throat radius,but the stress sensitivity of breakthrough time and displaced efficiency increases logarithmically.And increasing the gas injection pressure can reduce its stress sensitivity.The stress sensitivity of CH4 production decreases exponentially with the increase of pore and throat radius,and increasing the gas injection pressure can increase its stress sensitivity.