Study On Coal Matrix Shape Factor Based On Gas Desorption

Accurate prediction of gas migration characteristics in coal seam is the premise of efficient gas extraction and utilization.In the dual pore model of coal,the matrix and fracture are divided into two independent systems,and the matrix shape factor is an important parameter reflecting the geometric shape of the coal matrix,which together with the diffusion coefficient controls the mass exchange rate between the matrix and fracture.For the convenience of application,most scholars use the concept of adsorption time instead of diffusion coefficient and shape factor.However,adsorption time measured by the current methods cannot accurately reflect the gas diffusion characteristics of coal seam.In order to obtain the accurately gas diffusion characteristics of coal seam,study the gas transport rule of coal seam and guide the gas extraction of coal seam,it is necessary to study the diffusion coefficient and shape factor respectively.In this paper,the primary coal and tectonic coals from the 15#coal seam of SiJiaZhuang coal mine are taken as the research object,and the coal particle gas desorption experiment is carried out.By means of theoretical analysis and experimental measurements,the adsorption time,coal particle gas diffusion characteristics and shape factor under different assumptions were studied.The main conclusions of this paper are as follows:1)The adsorption time of primary coal and tectonic coal measured in SiJiaZhuang mine was between 1.5min and 120min,almost across the whole determination time.The adsorption time is greatly affected by particle size.When the particle size of coal is reduced from 0.074-0.2mm to less than 0.074mm,the adsorption time of primary coal is shortened by nearly 5 times,while that of tectonic coal is only shortened by 3times.2)The gas desorption rate of primary coal is lower than that of tectonic coal,and the difference of gas diffusion coefficient between primary coal and tectonic coal with different particle sizes is between 2-10 times.With the development of the desorption,gas diffusion coefficient with time,negative exponent relationship can be a very good fitting attenuation process of diffusion coefficient with time.3)In the process of coal particle gas desorption,the matrix shape factor decreases with the increase of time and finally tends to be stable.The shape factor at the initial stage of desorption differs by 2-3 times of magnitude from the stable value,and the use of fixed shape factor will produce a large error in the calculation of mass exchange capacity between the matrix at the initial stage of desorption.4)Particle size is the main factor affecting matrix shape factor.With the decrease of particle size,the matrix shape factor increases continuously,the tectonic coal increases by nearly 130 times,and the primary coal increases by nearly 118 times.The shape factor of tectonic coal matrix with the same particle size can be up to 5 times of the primary coal.The tectonic coal undergoes long-term geological tectonics,and the matrix is constantly destroyed and reconstructed.The tectonic coal body is similar to the particle aggregate under the weak binding effect,while the grinding effect on the transformation of the primary coal matrix is relatively small.5)The main reason for the variation of shape factor with time is that,at the initial stage of desorption diffusion,the diffusion characteristic distance of matrix gas migration to the fracture increases continuously.When the pressure drop wave is transferred to the center,the flow tends to be stable and the time-dependent shape factor also becomes constant.Based on Fick diffusion law,shape factor under the parabola,linear and uniform concentration distribution of coal particles are 60?~2?48?~2and~2?~2,respectively.Under the parabolic concentration distribution,the gas concentration gradient near the surface of the spherical matrix is large,the gas desorption diffusion speed is fast,and the mass exchange rate between the matrix and the crack is large,so the shape factor is large.6)The adsorption time is not exactly equal to the inverse of the product of diffusion coefficient and shape factor,and the difference between them is between 1.5 times and5 times.The measured adsorption time and shape factor of SiJiaZhuang coal mine were used to simulate gas extraction in coal seam,with time going,the difference value of matrix gas pressure distribution between them is going large.