Theoretical And Experimental Study On Permeability Of Filled Coal Fractures

As an unconventional clean gas resource,CBM(coal seam methane)is a potential alternative energy.At present,CBM mining technology has been deeply explored in the world,but there are still many challenges.Fissures and fissures fill constitute the main passage of coal bed methane transportation.Permeability of filling coal is usually determined based on fracture-filling characteristics and stress-strain field of coal.Under certain effective stress conditions,coal permeability is a function of pore pressure,filling type and injection gas type.In order to accurately describe the mechanism of gas flow in porous materials,many scholars have proposed many theoretical and empirical permeability models.However,up to now,the effect of slippage effect has not been considered in most models,and the slip effect has a significant effect on the gas seepage behavior of coal and rock fractures,especially in low permeability coal rocks.The permeability characteristics of filling coal and rock fissures are analyzed and divided into two parts: coal rock and fracture filling medium.By analyzing the effect of effective stress on porosity and judging the effect of slippage by using Knudsen number,a mathematical model of fracture permeability characteristic of filling coal and rock is established.Then,the fracture permeability characteristics of filling coal and rock were verified by taking the filling coal and rock as the research object.The variation of fracture permeability(different filling material,different confining pressure,different air pressure)in filling coal and rock is studied.By fitting the experimental data,the slip factors of different effective stress and different filling medium are obtained,and the mathematical model is obtained.The result of calculation.The correctness of the mathematical model is verified by comparing the experimental results with the calculated results.The variation law of fracture permeability(different confining pressure and different pressure)for filling coal and rock is analyzed.The main conclusions are as follows::1.The influence of effective stress and slippage effect is discussed through theoretical analysis of permeability characteristics of filling coal and rock fissures.The results show that effective stress can cause volume strain of filling coal and rock,change porosity of filling coal and rock,and then affect fracture permeability of filling coal and rock,and slip off effect is also an important factor in low permeability gas seepage system.The effect of slippage can be determined by the size of Knudsen number of gas and different filling media.A mathematical model of permeability characteristics of filling coal and rock fractures is established.2.The variation of fracture permeability(different filling material,different confining pressure,different air pressure)in filling coal and rock is studied experimentally.The result shows that.(1)Under the condition of constant air pressure,the fracture permeability of filling coal decreases with the increase of confining pressure.(2)Under the condition of constant confining pressure,with the increase of gas pressure,with calcite,kaolin,square high mixture and bentonite as filling medium,filling coal and rock fissures,the permeability decreases gradually and tends to smooth.When coal powder is used as filling medium,permeability increases with the increase of air pressure.(3)Under the condition of constant confining pressure,the effect of slippage decreases with the increase of gas pressure.The effect of slippage is only obvious at low pressure.3.By fitting the experimental data,the kg-1/p relation fitting equation and slip factor of filling coal and rock with different effective stress and different filling medium are obtained.The correlation coefficient R2 of the fitting equation is about 0.85.According to the slip factor and the parameters of filling coal and rock,the calculation results of mathematical model are obtained.The calculation results are in good agreement with the experimental results,which verifies the correctness of the mathematical model.