Evolution Characteristics Of Main Controlling Factors Of Coal Gas Seepage And Its Control Mechanism Of Gas Extraction

At present,in order to improve the efficiency of gas extraction in low permeability coal seams,gas enhanced extraction technologies are widely used,but the same enhanced extraction technology in different coal mine,or even different coal seams in the same coal mine,the application effect is uneven,this means that in the coal mine with poor application effect,the enhanced extraction measures have not specifically solve the main factors controlling coal gas seepage.Therefore,research on the main controlling factors of coal gas seepage and its effect on gas extraction is an important basis for achieving efficient coal gas extraction.Coal gas seepage is controlled by both coal permeability and pore pressure gradient,and the stress field is the most fundamental factor controlling coal permeability and pore pressure gradient.Therefore,based on the multi-stress field of actual coal,this paper systematically develops the main control factors of coal gas seepage and its control effect on gas extraction by physical experiments and theoretical analysis.The main research results obtained are as follows(1)Based on the theoretical analysis of the effect of pore pressure and adsorption on the mechanical state of coal,the in-situ boundary conditions of coal were simulated to carry out experiments on the influence of pore pressure and adsorption on the coal stress field.The results show that: The coal horizontal stress and vertical strain tend to decrease with the decrease of gas pressure,and this trend gradually accelerates with the decrease of gas pressure.The adsorption stress has always played a major role in the evolution of coal stress field.The evolution of the coal horizontal stress and vertical strain(influence of individual pore pressure)has a linear relationship with the pore pressure,and the change of pore pressure alone has no effect on the coal horizontal effective stress.The coal horizontal adsorption stress and vertical strain satisfy the relationship of Langmuir's similar equation with gas pressure and the linear relationship with gas adsorption.The stress state equation of coal considering the triple stress field of ground stress,pore pressure and adsorption stress is constructed,which clearly reveals the control law of each stress on the coal stress field.(2)Experiments on the evolution of coal permeability in single stress field,double stress field,triple stress field-different boundary conditions-different bedding directions and gas permeability flow under different pore pressure gradients are carried out.The results show that: Vertical ground stress has no obvious effect on coal permeability,but under horizontal ground stress,coal permeability shows a trend of rapid decay first and then decay slowly.When the horizontal boundary conditions are the same,the evolution law of coal permeability is similar,and the vertical boundary conditions have no obvious effect on coal permeability.Coal permeability under hydrostatic pressure is always greater than the in-situ condition.The coal permeability in parallel bedding direction is always greater than that of the coal in the vertical bedding direction.The coal permeability under in-situ conditions increases slowly with the increase of pore pressure,while under hydrostatic pressure,it shows a significant increase.Under these two boundary conditions,the coal permeability with the increase of the adsorbed gas pressure showed a rapid decay first and then a slow decay trend.The gas seepage flow rate showed a continuous increase trend with the increase of pore pressure gradient.(3)Based on the experimental results and the coal mechanical state,the control effects of ground stress,pore pressure,and adsorption stress on the coal permeability are analyzed.The results show that: The control of stress field on coal fracture opening is the direct cause of coal permeability evolution.Ground stress can be regarded as external stress to reduce coal fracture opening by compressing coal skeleton.Adsorption stress is internal stress,the interaction between the expansion of the coal matrix induced by gas adsorption and the fractures in coal causes the coal fracture opening to decrease.In-situ condition,the pore pressure increases the fracture opening by compressing the coal matrix,while in hydrostatic pressure,it increases the fracture opening by expanding the coal skeleton and compressing the coal matrix.The final fracture state of coal is the result of stress competition of the above three parts,which conforms to the principle of dominant control.Since the change of coal permeability caused by the compression of coal matrix by pore pressure is neglected,there are some errors in describing the effect of pore pressure on coal permeability simply by using effective stress.The concept of "coal matrix bridge" is put forward to reveal the nature of the dynamic evolution of coal permeability.The opening degree of coal fractures decreases,and the "coal matrix bridge" in coal fractures increases,This will hinder the further reduction of fracture opening and slow down the reduction of fracture opening.However,when the fracture opening degree of coal body increases,the change law will be reversed.(4)The main factors controlling the seepage of coal gas are revealed.The results show that: The permeability of coal is mainly controlled by the horizontal mechanical state(including the stress,strain and boundary condition of coal),but has little correlation with the vertical mechanical state.The sensitivity of coal permeability to hydrostatic pressure condition is stronger than in situ condition,which indicates that the change of in situ coal permeability from displacement controlled boundary condition to stress controlled or free boundary condition can effectively increase coal permeability.Under in-situ conditions,the permeability of coal is highly sensitive to adsorption stress and ground stress,but less sensitive to pore pressure.In addition,although reducing the adsorption stress and in-situ stress can effectively increase the permeability of the coal body,it is not appropriate to reduce the adsorption stress to increase the permeability of the deep coal mine when the ground stress is large.Increasing the pore pressure gradient can effectively increase the seepage flow rate of the gas,and the pore pressure gradient has a stronger control effect on the gas flow in the low pore pressure gradient than in the high pore pressure gradient.(5)The permeability models of coal under the conditions of single stress field,double stress field and triple stress field are established respectively,and the dynamic influence laws of ground stress,pore pressure and adsorption stress on coal permeability are quantitatively described.(6)The control effect of main control factors of coal gas seepage on gas extraction is analyzed,and reveals the main factors controlling gas extraction in coal seam.The coal permeability in the early stage controls the gas extraction,while the gas pressure gradient in the later stage is the main factor to control the gas extraction.Based on the research results of the main control factors of coal gas seepage,combined with its control effect on gas extraction,the efficient gas extraction technology of "drill-cut integration" and "pressure-extraction alternating" was put forward.Engineering practice shows that this technology can effectively improve the gas extraction efficiency of coal seam.During the postgraduate period,I published four SCI papers as the first author or corresponding author(three papers in JCR area 1 and one paper in JCR area 3).Six invention patents were applied for,of which two were authorized.One provincial and ministerial science and technology progress award was obtained.There are 93 figures,31 tables and 215 references in this paper.