Research On Coking Coal Seepage And Mechanical Properties Under Long-term Effect Of Triaxial Stress And Gas Adsorption

Today's society is developing rapidly,and energy consumption is increasing day by day.Many substances produced after the burning of fossil fuels will have a negative impact on the environment.Therefore,it is very important to increase the proportion of clean energy used in energy.CBM is a kind of clean energy.In order to ensure the safe mining of coalbed methane,it is necessary to understand the characteristics of the coalbed methane.The characteristics of coal seams in China are mostly soft and low permeability,which makes CBM extraction difficult and low in safety.Therefore,this paper simulates the pressure and gas environment in the actual formation,starting from the permeability and mechanical properties of the coal body,the characteristics of soft and low permeability coal seams are studied.Considering the safety of the experiment,this experiment uses nitrogen,which has the same adsorption properties as methane,and uses helium gas without adsorption properties for comparison.Coking coal from Shanxi Liulin No.4 Coal Seam is used.Using experimental and theoretical methods,the research was conducted from three angles of adsorption and desorption,and seepage characteristics undertriaxial stress and long-term gas action,and mechanical properties under triaxial stress and long-term gas action,and the important conclusions are as follows:(1)The pure solid density of the coking coal is 1.33 g/cm~3.When the initial pressure of nitrogen is 4.329 MPa,the saturated adsorption amount of coal is 3.20 ml/g,the maximum desorption amount is 2.59 ml/g,and the maximum desorption rate is 81%;(2)Under the conditions of triaxial stress(both axial confining pressures are 10 MPa)and gas(6 MPa)for long-term effects and pore pressures of 6MPa and 5 MPa,the evolution law of initial permeability is related to the cutoff value.When the initial permeability is higher than this value,the permeability change trend is increasing-decreasing-stable,and the peak permeability increase multiple is negatively correlated with the initial permeability.When the initial infiltration is lower than this value,the change trend of permeability is decreasing-stable.The cutoff value under nitrogen environment is smaller than that under helium environment.The deformation of the sample is larger under nitrogen environment.The sample expands with time,and the expansion rate slows down.The larger the initial infiltration of the sample,the greater the amount of deformation expansion,and the sample with little initial infiltration has basically no expansion.The deformation of the sample in the helium environment does not change significantly;(3)Under the action of triaxial stress(both axial pressure and confining pressure are 10 MPa)and gas(6 MPa),adsorption expansion,triaxial stress and pore pressure destruction will affect the permeability and mechanical properties of the sample.The coal body will appear soft and low permeability.The soft state is mainly caused by triaxial stress and pore pressure destruction.In the nitrogen environment,the increase in permeability with time is small and the decrease is large,so the low permeability state is mainly caused by adsorption expansion.The soft coal sample is not easily destroyed by the whole.The sample expands and compresses the crack due to adsorption in the nitrogen environment.Therefore,the more the action factors,the more lag the volume strain curve.When the axial pressure is between 20–30 MPa,the coal body expands,a large number of pores and cracks inside the coal body develop,and the permeability increases.Keeping the pressure within this pressure has a positive effect on the exploitation of coalbed methane;(4)Under the action of triaxial stress(both axial pressure and confining pressure are 10 MPa)and gas(6 MPa),the long-term effect of the triaxial stress and gas on the sample has little effect on the elastic modulus and Poisson's ratio.