The Theory And Its Application On Gas Drainage In Low-Permeability Coal Seams

Gas disaster is always one of the main disasters to threaten the security of coal mine in the coal exploitation. Although the scholar in home and abroad do a lot of trial work in gas drainage, it affect a little at present. It is considered that the gas pre-drainage measure in coal seam is one of the most effective method to prevent the gas disaster. With the depth of coal seam increase, the permeability decrease. Consequently it affect the drainage rate and efficiency. So it become the key link solving the gas drainage in low-permeability coal seams to insure in coal mine security and enhance production efficiency. With the development of society economy and the requirement aggrandizement for energy sources, the gas source exploitation and its using become more and more attention. The most coal field of china is all low-permeability coal seams, so it has important society value and economy efficiency to have the research of gas drainage in low-permeability coal seams.At first ,the thesis analyze the form condition, basic character, source status and the foreground of gas in our country; review the gas drainage status and the matter; summarize the specialty and apply condition of many gas drainage method; explain the gas flowing theory and the research status of every gas storage mathematic model; put forward the basic research idea and theory to reconstruct low-permeability coal seams by hydraulic-cutting.The main research work is developed, that is:1. Based on pore medium and probability theory, it put forward the percolation method of pore and fracture double- medium. It adopt numericalsimulation to research the percolation phenomena of double-porosity medium and the distribution low of percolation threshold to announce its basic character. It indicate that the percolation threshold of double-porosity medium is not a certain value, it is three variable function of porosity rate(n),fracture fractal dimension(D),initial value of fracture quantity distribution(No). Its mathematic formula is:f(n,N0,D) = npc -n-N0Exp(-9.8687)· Exp(4.7917D)here : npc-is the percolation threshold of porous medium ; n -porosity rate, N0-initial value of fracture quantity distribution , D —fracture fractal dimensionwhen f(n,D.N0)>=0, it appear percolation cluster inside the double-porosity medium, and the medium can penetrate. On the contrary, it can't penetrate; when f(n,D.N0)<0, there is not percolation cluster inside the medium.2.The relation between percolation probability(p) and permeability coefficient(K) is announced by the numerical simulation in coal seams of china decade. The formula is :K = 0.1468exp(2.8197p)Based on the permeability coefficient, the permeability classification of coal mass method is put forward, and the new permeability classification of coal mass, method is supplied.3.The mechanism of gas drainage rate and the drainage law in low-permeability coal seam by the hydraulic-cutting are put forward through the experiment of great three-dimentional solid-liquid coupling with the coal sample of Lu'an coal field 3# coal seam. The result indicate, after cutting, the initial drainage speed and quality increase quickly, the initial drainage speed increase for 3 times and quality for 4 times at 400 m depth; the speed increase for 16 times and quality for 9times at 800m depth.The effective stress change law of coal mass is put forward in the course of gas drainage. That is ,with the time extend, the gas pressure(p) follows exponential attenuation law: p = a·exp(-b · t); effective stress (σi) of coalmass follows logarithm incremental law: σi=a ln{t) + b; effective stress coefficient (a) follows logarithm attenuation law: a = a-b ln(t).; effective stress coefficient, effective volume stress ((?)) and gas pressure(p) follow double-crewel relation: a = a0 - a1(?) -a2p + a3(?) p.During the cutting experiment, the coal and gas outburst phenomenon occur. The outburst intensity relate to the depth of coal seam. The deeper the coal seam is, the greater the intensity is. In the terra pressure function of 800m depth, the gas drainage volume is about 10% of coal sample volume, unloading is complete. In the 400m depth, the gas drainage volume is about 2% of coal sample volume.The relation between inhomogeneous coefficient and outburst index E/X is researched by numerical method, the result indicate: the formula of its relation is:(E/λ) = 3.0131 (m-1)-1.16764.Because the coal mass is the pore and fracture double-medium, the homogeneity of the intensity and permeability coefficient of coal mass is very poor. The linear size of Representative Elementary Volume(REV) is difficult to confirm. And its physical character is not easy to describe.Inhomogeneous coefficient is introduced to the solid-liquid coupling mathematic model of coal mass deformation and gas seepage flow. The problem of solid-liquid coupling model of double- medium is settled by continuum medium solid-liquid coupling model.These methods predigest the theory and the numerical calculation model. The thesis discuss not only the solving method and foundation method of coupling mathematic model but also the deformation and failure simulation method. Based on above, work out a set of software named "Rock Testing and Engineering Analysis" to research inhomogeneous.5.Using this software "RTEA2D" to have the numerical experiment of gas drainage by cutting based on 3# coal seam. The result indicate, it appear tensile fracture with different size and shear fracture in the both upper and bottom ends after cutting. The size of fracture group relate to depth and width.When the coal seams is 6m thickness, 400m depth, 4~5m width, the fracture group can connect the coal seam with 6m thickness, and make it appear percolation cluster and improve the drainage rate. Hundred-meter drilling drainage rate by hydraulic-cutting can reach 38.5% for a month at 400m depth , 5m width, the total volume is 22026m3, and 62.2% for two months, the total volume is 35613.8m3.The thesis discuss the influence on gas drainage speed from the depth after cutting with software RTEA2D. The numerical calculation indicate, the drainage speed of gas drainage will increase with the depth by cutting. The initial max speed at 200m depth is 1.7 times to 100m depth. As the same, the initial max speed at the 400m depth is 2.2 times to 200m depth. The numerical result tallies with the experiment result.6.Through the industrial experimentation in 7601 work plane of WuYang coal mine of Lu'an Mining Group, a whole set of technics of gas drainage by hydraulic-cutting is achieved. The result indicate, When the jets of water with 70mPa by hydraulic-cutting is used, the length can reached to about 2m, the height can reach to 3 0-5 0mm. In the experimentation, the coal and gas outburst phenomenon with intermittent and different degree can be discovered. The gas drainage rate for a month after cutting can reach to about 30%.