| Oil,natural gas,and coal are the main energy resources in the world.The annual growth of the economy and population has not only caused large-scale energy consumption,but also aggravated environmental pollution.Coal usually contain a large amount of methane,which can be recovered in the form of coalbed methane.The exploitation of coalbed methane under unconventional reservoir conditions has become an important research topic.Permeability is an important parameter that characterizes the efficiency of coalbed methane recovery.The change of permeability during the mining process is affected by the pore and fissure structure,pore pressure,temperature,and slippage effect.Therefore,combining fractal theory to study the characteristics of coal and rock pore and fissure structure,and revealing the evolution mechanism of coal deformation and permeability under the combined effects of stress and temperature is of great significance for mine disaster prevention and coalbed methane gas injection.The coal samples in this paper were taken from the 3# coal seam of Zhaozhuang Mine,Jincheng,Shanxi.Using the HCA type high pressure capacity method adsorption device and the gas-containing coal triaxial seepage device,the isothermal adsorption test at different temperatures and the triaxial seepage test at different temperatures and stresses were carried out.With the help of fractal theory,the temperature-induced coal pore crack expansion and the change of slippage factor are quantitatively characterized.And a fractal Langmuir model modified by considering the excess adsorption capacity under the effect of temperature is constructed.Considering the fractal characteristics of capillary tube under stress,a fracture volume stress sensitivity model is established.And a coal permeability model considering the expansion and deformation of the coal matrix,temperature and gas pressure changes is further constructed.The main results are as follows:1)Carried out coal isotherm adsorption test and cryogenic liquid nitrogen adsorption test.A fractal adsorption model considering coal heterogeneity and excess adsorption at different temperatures was established.Discussed the inhomogeneity of coal adsorption.With the increase of the fractal dimension D,the gas adsorption capacity and rate of coal are enhanced.The newly-built fractal adsorption model is used to calculate the coal adsorption deformation,and the coal fractal permeability model considering the effect of slippage effect is further constructed.Provide theoretical basis for vigorously promoting coalbed methane extraction.2)Triaxial seepage tests under different temperature and stress conditions were carried out,based on fractal theory to characterize the temperature-induced crack expansion and slippage factor changes in coal.Further considering the influence of compression deformation and slippage effect on coal permeability,a coal fractal permeability model with the combined effect of stress and temperature is established.It can better characterize the gas seepage laws of complex coal reservoirs.3)Considering the fractal characteristics of capillary under stress,a fracture volume stress sensitivity model is established.On this basis,a coal permeability model considering the expansion and deformation of the coal matrix,temperature and gas pressure changes is established.Based on the established model,the synergistic relationship between adsorption and desorption,compression coefficient,slippage effect and coal permeability is analyzed.It reveals the regulation mechanism of coal pore characteristics and permeability under the combined action of stress and temperature,and lays a theoretical foundation for coal seam gas drainage. |
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