Effect Of Gas Adsorption On Seepage And Mechanical Properties Of Coal

Coal seam is of dual porosity media, the fracture is the main contributor to permeability of coal seam and the pore structure of coal influence the gas adsorption on coal, the vast majority of coalbed methane is in absorption state in the pore. The adsorption of gas will swell the coal matrix and narrow the fractures, resulting in the decrease of permeability. Also, the adsorption of gas will weaken the mechanical strength of coal, resulting in the instability of coal seam during the process of drilling and gas injection. Therefore, based on the analysis of pore structure in coal, studies the effect of gas adsorption on the permeability and mechanical properties of coal deeply, which has positive significance in recognizing the permeability of coal and optimizing the engineering work.This thesis focuses on the No.9coal seam in Ningwu Basin, based on the analysis of coal mineral composition, physical properties and pore structure properties, studies the coal adsorption characteristics to different gas. By the high temperature/pressure triaxial gas adsorption-seepage experiment testing system studies the effect of gas adsorption on the permeability of coal, experiments on the effect of gas adsorption on the mechanical strength of coal, the influence of gas adsorption on the stability of coal seam is discussed.Deepen understanding the pore structure characteristics of No.9coal. The results from integrated thin-section analysis, scanning electron microscopy (SEM), CT scan point out micro-fractures in coal are developed, yet poor in connectivity. It can be concluded from high pressure mercury, nitrogen adsorption and nuclear magnetic resonance (NMR) that the pores in coal matrix are mainly nanoscale pores, the structure is quite complex, the connectivity between pores and fractures is poor.Make clear the gas adsorption capacity of coal to different types of gas on No.9coal. The characteristic of single component gas adsorption on coal is tested by the HKY-II automatic test system, which quantitatively calculate the relationship between the change of surface energy value and the amount of expansion with the adsorption capacity. The experimental results demonstrate that the CO2has the strongest adsorption capacity on coal, followed by CH4, and N2is the least; with the increasing of adsorption capacity, the reduced value of the surface energy increases and the amount of expansion increases.The high temperature/pressure triaxial gas adsorption-seepage experiment testing system is self-developed. Test the permeability of coal under different conditions such as pore pressure, confining pressure, axial pressures and tress-strain process, and discuss the effect of gas adsorption on the permeability of coal. The results show that:gas adsorption on coal may cause permeability reduced in some degree; the stronger of gas adsorption is, the lower of the permeability; hysteresis phenomenon occurs in the relation curves of permeability and pore pressure during the loading-unloading process of pore pressure; under the constant axial stress and confining stress, there is a critical phenomenon when the permeability changes with the variation of pore pressure, presenting the control of gas adsorption and effective stress; under the constant pore pressure, with the increase of axial pressure or confining pressure the permeability decrease; under the constant confining pressure and pore pressure, with the increase of axial stress the coal go through the process of compaction-damage and re-compaction, the permeability experience as decrease, increases rapidly and drop slightly; considering the effect of backpressure in outlet, the model of which considers the non-zero lateral strain condition during the permeability test under triaxial is perfected.Reveal the effect of gas adsorption on the mechanical strength of coal. Experiments on the tensile strength, uniaxial and triaxial compressive strength are carried out before and after gas adsorption on coal. The results show that the adsorption gas enhance the plasticity of coal, decrease the tensile strength, uniaxial and triaxial compressive strength, the more of gas adsorption on coal, the bigger the content of decrease; the improvement in calculated mode of swelling capacity after the gas adsorption on coal, the constitutive model, which can predict the relationship of strain and stress with considering the gas adsorption is perfected.Discuss the effect of non-hydrocarbon gas adsorption on the stability of coal seam in situ conditions. Analysis of the borehole stability in coal seam using the discontinuous medium mechanical model. Based on properties of coal consider gas adsorption, optimize the density of drilling fluid in site and point out that the improvement of the sealing characteristics of drilling is the key to guarantee the stability of coal seam. The equation of the crack propagation velocity is deduced, the simulation calculation point out that with the higher gas injection pressure and the stronger of the gas adsorption, the bigger of the rate of crack propagation.