Study On The Mechanisms Of Coal Rock Crack Evolution And Creeping Instability Of Coal Rocks During Permeability-increasing Gas Extraction

Set the coal and rock around the gas extraction holes as the research object, as crack s’creation and development is affected by rheological mechanical properties of the coal and rock,associating breeding, development and occlusion of cracks in low permeability coal seam withcreepage mechanics characteristics of coal and rock, has an important theoretical andengineering significance in revealing mechanism of fracture evolution and creepage instability.Based on the crack and damage mechanics, the rheological mechanics and mutationtheory, establish the mechanics model of coal and rock crack evolution and creepageinstability, their mechanical properties under different stress state are mainly studied withtheoretical analysis, laboratory test and industrial test method, the criterion of fractureinitiation and the stress intensity factor is given. The main research and its results are asfollows:According to the complex mechanical properties of coal and rock material, establish anonlinear time-varying parameters rheological model, which describes completely theviscoelastic plasticity mechanics characteristic of the coal and rock and shows the creepageinstability criterion. Use the Betti theorem and strain energy conservation theory to present theconstitutive relation under both the pressure-shear, tensile-shear stress states, analyze the crackinitiation criterion, and then the crack evolution equation of coal or rock is derived strictly.With its creepage stress threshold value comparison, uniaxial compression strengthmeasured by experiment reflects the creepage properties of coal or rock. Do creepage test byusing acoustic emission technology, acoustic emission characteristics of coal sample properlyreflect the creepage damage evolution process, use acoustic emission information can dopredictive research more factually on the creepage deformation law, which is of greatsignificance for monitoring deformation of coal and rock and coal pillar stability. Real-time monitoring for fracturing process with high-precision microseismic monitoringsystem, according to micro crack distribution regularity, analyze the data collected in theprocess of fracturing operation and explain crack space distribution. Presented from theanalysis of test results, crack’s micro and macro phenomena including initiation, evolution,penetration, divarication, and crack’s cutting-edge form feature are related to external forces,geological conditions and the uniformity of coal and rock, but cracks formation laws also havesimilarity, which generally shows cracks evolution is from a macroeconomic disorder to amesoscopic order, and then to a macroeconomic order under the action of hydraulic pressure.