| Methane is not only the major cause of gas disasters, such as coal and gasoutburst, gas explosion, but also a clean, efficient and non-renewable energy. Thesimultaneous extraction of coal and gas can not only heighten the level of safeproduction of coal mine, and also improve the energy structure in China, which has aimportant significance for the energy security. However, the characteristics of coalseams in china generally are high gas content, low permeability and strongadsorptions, in which the permeability of the coal seams is2-3orders of magnitudelower than that in American. So the gas extraction is extremely difficult and themining-enhanced permeability of coal seams is an effective way to realizesimultaneous extraction of coal and gas. Therefore, it is so important for thesimultaneous extraction of coal and gas to fully understand the deformation, failureand permeability evolution law of gas-saturated coal and to describe the effect ofmining-enhanced permeability quantitatively. In order to study characteristics ofdeformation, failure and permeability evolution of gas-saturated coal, the laboratorytest, theoretical analysis and numerical simulation research were conducted. The maincontents are as follows.(1) By using MTS815and high pressure gas permeability test systems, theuniaxial, triaxial compression tests and gas permeability tests for raw coal andbriquette coal were carried out. The tests results show that the uniaxial compressionstrength and triaxial compression strength of the raw coal is respectively10.3timesand3.7times of the briquette. The elastic modulus of raw coal is respectively75.1times and12.7times of briquette coal in uniaxial and triaxial compression. The plasticfailure model of briquette coal is distinct from the brittle failure model of raw coal.The initial permeability of briquette is20times of raw coal, but after failure, thepermeability of briquette is about90%of the raw coal. Experimental research resultsshow that using the test results of briquette coal to guide the design and constructionof coal mine gas drainage is easy to cause significant safety risks.（2）With the use of triaxial compression test, mechanical properties of the rawcoal under different gas pressures were studied. Results show that as the gas pressureincreases, the strength and stiffness of the raw coal decrease gradually and the failureintensity at the peak stress weaken gradually, in addition, the failure modes of thesamples transform from brittle splitting failure into ductile shear failure. So, according to the damage and failure features of the coal and rock under different gas pressures, adouble-scale damage model was presented. Considering the pore gas pressure andadsorption expansion stresss, a new elastic damage constitutive equation forgas-saturated coal was established.（3）The permeation properties of raw coal during damage and failure wereresearched by gas permeability test under triaxial compression. The test results showthat the permeability of raw coal decreases with increasing gas pressure. Thepermeability and volumetric strain present well linear relationship. Based on flatfracture flow equations, relationship between fracture volumetric strain andpermeability was established. Considering the effects of gas pressure and adsorbedgas on fracture volumetric strain, and for the first time, by introducing a ratiocoefficient to characterize the effect of fracture surface roughness and fracturetortuosity on permeability, a new permeability evolution model was established.（4）Based on the established damage constitutive model and permeabilityevolution model, a3D finite element analysis program coupling the elastic damageand permeability was developed. Using this program, the quantitative description ofthe damage and permeability evolution law of the mining coal and rock was achieved.Take a single coal seam mining for example, a numerical simulation was performed,in which the damage characteristics and permeability evolution law of the mining coaland rock were studied under different gas pressures. The results show that as theworking face moves forward from30meters to150meters, the damage regionextends from5meters to30meters accordingly, and the permeability in the damageregion may increase likely by2-3orders of magnitude. Besides, compared with thecase under a1MPa gas pressure, a6MPa gas pressure may lead to a damage increaseby19%and a permeability ratio increase by85%in front of the working face.Calculation results coincide basically with the engineering practice and can be used asa scientific guidance for coal mine gas extraction design. |
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