Study On The Influencing Factors Of Gas-containing Coal Permeability And Simulation Of Coal Seam Gas Extraction?

With the demand of national coal rising year by year, coal mining has constantly stretched to the deep. Faced with “three high and one low” coal resource(that is high stress, high ground temperature, high gas pressure and low permeability), gas disaster is more and more serious. Extraction technology of coal and gas together has been the first choice and necessity of deep coal mining. Artificially mining activities have changed the stress and temperature field, and caused gas migration in coal. Thus it is the emphasis and difficult part of deep coal mining that to study deformation and permeability of coal containing gasunder the stress and temperature coupling, to study the influencing factors of gas containing coal’s permeability, to study gas seepage law during the process of coal-seam gas drainage. Taking coal samples from 1# coal seam of Yushe coal mine in Guizhou mining area as the main research object, also including other six coal seam samples, laboratory measurement, theoretical reaserch, engineering application and numerical simulation are all adapted to study the basic physical and mechanical attributes, the deformation and permeability characteristics under heat-fluid-solid coupling and the influencing factors of permeability of gas containing coal in both experimental and theoretical field from the following aspects: the deformation and permeability characteristics, influencing factors of permeability and different gas drainage conditions. Simultaneously, numerical simulation of gas drainage is also analyzed. Conbining the seepage mechanics, rock mechanics and elasticity, a range of useful discussions are given.By these studies, we obtain progresses in the following aspects:1) Inner and outer pore characteristics of coal containing gas are analyzed, and uniaxial compressive strength, triaxial compressive strength and deformation characteristic tests are carried out, the relationship between metamorphic degree and the development degree of inner and outer pore, cracks shows positive correlation. The relationship of coal-seam particle sizes, consistent coefficient and basic mechanical parameters are also obtained. The gas adsorption and desorption properties are experimental studied and the law of isothermal adsorption curve and adsorption constants a, b changing with temperatures is obtained, and also the relationship of this law and inner pore characteristics.2) Existing pseudo triaxial seepage device is improved. Triaxial compression tests of coal containing gas under different temperatures and confining pressures are carried out and the influences of temperatures and confining pressures on mechanical parameters of coal containing gas are analyzed.Moreover, the influencing law of confining pressure and temperature on permeability and axial strain before the failure of coal containing gas is discussed. And also the quantitative relationship between confining pressure, temperature and permeability characteristic value and its change trend in the failure process are explored.The mechanism of permeability lag effect is are overall explained.3) Triaxial seepage tests under different stresses, temperatures, moistures and particle sizes are carried out, by which the changing law of gas containing coal’s permeability under multifactorial heating coupling is analyzed, the expressions of gas containing coal’s permeability under moisture and temperature, moisture and effective stress coupling are derived, and the influencing mechanism of permeability is explored. Triaxial seepage tests under different metamorphic degrees and consistent coefficients are also carried out, by which the relationship between metamorphic degrees and development degree of outer pore and crack, and also the consistent coefficient and initial speed of diffusion, the triaxial compressive strength and permeability are analyzed.4) The pore pressure tests under positive outlet pressure are carried out by adding the positive pressure regulating valve at the outlet of triaxial seepage device, by which the relationship between permeability and pore pressure of coal containing gas is obtained, and the influencing law of gas containing coal’spermeability under pore pressure and temperature coupling is analyzed. The sensitive coefficient of pore pressure is definedand the relationship between the sensitive coefficient of pore pressure and pore pressure is derived. The response degree of coal containing gas on pore pressure explored.5) Triaxial seepage experiments when the outlet pressure is positive, atmospheric and negative pressure are carried out by improving existing triaxial seepage device. And the seepage law of coal containing gas under different effective stresses and temperatures is analyzed, the mechanism of gas flow velocity with heating stress, external load, pore pressure, coal deformation and sorption is explored.6) Triaxial seepage experiments when the outlet has a negative pressure are carried out, by which the influence of three kinds of outlet negative pressures on gas flow velocity is comparatively analyzed, and the mechanism that decreasing negative pressure is good to gas flow during gas drainage is revealed. On the basis of optimal outlet negative pressure, the influence of diameter and tubes length of drainage on gas flow velocity of coal containing gas is studied, the changing law that gas flow velocity of coal containing gas increases with diameter increasing, decreases with temperature adding and tubes length extending is obtained.7) The overall strain equations of gas containing coal system and permeability changing model considering Klinkenberg effect are established, and the fluid-solid coupling equations of gas containing coal system is build when the stress and seepage field contained and the gas Klinkenberg effect considered, combining with the gas-flow continuity equations, equilibrium and geometric equations of gas containing coal’s deformation. By the comparison between numerical simulation analysis and physical simulation experiments, it the accuracy and practicality of model are verified, the changing law of gas pressure along the radial direction and gas phase permeability are comparatively analyzed.8) Taking the penetration borehole in 1# coal seam of Yushe coal mine in Guizhou mining area as engineering calculation example, the changing law of gas pressure with expolitation time in computational domain and sectionis analyzed, and the changing trend of gas pressure in different drainage positions at certain time is compared.The gas flow developing law of the calculation model under different negative drainage pressures is revealed and the optimal negative drainage pressure in different stages is discussed.