Study On Mechanics And Permeability Of Raw Coal Under Loading-Unloading Stress

This paper takes the raw coal sample taken from the C1 coal seam of a coal mine of Furong Company of Sichuan Coal Group as the research object,through the micro-pore crack test and gas permeability test of raw coal,the mechanical and permeability experiments of raw coal under different axial loading rates,the mechanical and permeability experiments of raw coal under different loading-unloading stress conditions,and the fluid-solid coupling numerical simulation of coal under loading-unloading stress conditions.In order to obtain the mechanical and permeability characteristics of the total stress-strain process.The details are as follows:?1?Pore diameter of raw coal distribute below 160 nm.The proportion of small pores and micropores below 100 nm is 81.5%,the proportion of mesopores above 100nm is 18.5%,and the porosity of raw coal is 5.65%.The raw coal particles conform to the variation characteristics of the type II isotherm and there is no hysteresis loop phenomenon.The coal rock has a BET specific surface area of 0.7192 m²/g,and average pore diameter is 317??31.7 nm?,the pore volume is 0.00098 cm³/g.Three pore structures of plant pores,metamorphic pores and intergranular pore.can be found in scanning electron microscopy.When loading in elastic stage,The permeability of raw coal satisfies the functional relationship(K=?A?₁+B?e^C?₃with confining pressure and axial pressure.?2?In the experiment of mechanical and permeability characteristics under different axial loading rates:The stress drops rapidly when the raw coal failure,and the brittleness characteristic is obvious.Volumetric expansion stress,peak stress,elastic modulus and deformation modulus are four mechanical indexes,which increase with the increase of axial loading rates.During the pre-stress peak stage,the energy dissipation of coal sample is mostly converted into elastic strain energy,and the rate of dissipation strain energy increases slowly.During the post-stress peak stage,The elastic strain energy drops sharply and is transformed into a large amount of dissipative strain energy.The total energy at the peak of the stress increases with the increase of the axial loading rate.During the loading stage after hydrostatic pressure of raw coal,the permeability-strain curve generally presents a"V"shape.During the whole stress-strain process,the relationship between permeability and axial strain of coal and rock satisfies quadratic polynomial function?K=A?₁²+B?₁+C?,which is more suitable when the axial loading rate is small.?3?In the experiment of mechanical and permeability characteristics under different loading-unloading stress:Under the same loading-unloading control point,with the increasement of loading-unloading rate ratio,the volumetric strain decreased,but the axial strain and the radial strain increased,the failure pattern of coal gradually developed from several macroscopic fracture to single macroscopic fracture.Under the same loading-unloading rate ratio,with the increasement of loading-unloading control point,the intensity of coal increased,but the failure angle of coal decreased.During the yield stage,with the increase of loading-unloading rate ratio,the response variation of permeability decreased with a linear relationship.However,the response variation of strain from loading-unloading control point to permeability transformation point increased with a linear relationship.?4?In the numerical simulation of loading-unloading RFPA^2D-Flow under fluid-solid coupling:the failure mode of coal rock is shear failure,and the failure patterns is looser with the increase of confining pressure unloading rate.A negative relationship between the rate of confining pressure unloading and the final compressive strength has been found.Acoustic emission energy are single peak type,and there will be obvious dissipation of acoustic emission energy before the peak point,which can predict the failure precursors of coal and rock under various confining pressure unloading rates.The acoustic emission energy at the peak stress point increase with the increase of confining pressure unloading rate.The permeability decreases gradually with the increase of loading step until it approaches 0 in compaction and elasticity stages.With the continuous unloading of confining pressure in the yield stage,cumulative damage occurs in the coal sample,resulting in a small increase in permeability.Permeability increases sharply in the stage of failure and residual deformation...