Multifield Coupling Processes During Gas Drainage In Deep Fractured Coal Seam And Its Engineering Response

With the depletion of the shallow resource,the coal mining in China is gradually extending to the deep strata.Comparing with the shallow layer,the deep reservoir has its specific characteristics.Intensifying the studies in geomechanics of the deep reservoir is of great significance to improve the ability of resource extraction and disaster prevention.Gas drainage is one of the most important measures to prevent gas dynamic disasters,and it is affected by many coupled fields.Especially,in the deep reservoir,the strong plasticity and timeliness of the rock superposition with the mining disturbance,this makes the gas draiage more complex.In this thesis,the gas diffusion in fractured coals,the permeability evolution in coal containing gas,and the multifield coupling processes in elastically and plastically deformed coals were systematically investigated with a series of comprehensive methods.The following conclusions were drawn:An experiment system was developed for the study of gas diffusion processes in fractured.With this system,the size effect of gas diffusion in granular coal was studied,and mechanisms of the influence of confining stress and gas pressure on gas diffusion processes in fractured coal were revealed.The research results show that the effective diffusivity of coal decreases with the time.Gas diffusion in coal particles is size-dependent,and the critical size for this size effect corresponds to the coal matrix size.The effective diffusivity of coal decreases with an increase of the confining stress and a reduction of the pore pressure.A conceptual model of multilevel pore structure of coal was put forward,and based on this,a time-dependent diffusivity model and a dynamic diffusion model were developed.The results show that for the long-term coalbed methane recovery,both the bulk diffusion and surface diffusion should be considered in predicting the gas production,while for underground gas drainage,the surface diffusion can be neglected.During the diffusion process,gas pressure in matrix of fractured coal is independent of the particle size,and a slow attenuation of the diffusivity leads to a fast decrease of the pore pressure.An internal swelling coefficient of coal matrix was defined,and the interaction between coal matrix and fracture was quantitatively investigated.And then a permeability model based on the matrix-fracture interaction was developed.The results show that because of the "rock bridge" ,only part of the swelling deformation of matrix was used to change the fracture aperture and the rest part was used to change the volume of the bulk coal.The effective stress and adsorption swelling were the essential factors affecting the internal swelling coefficient.The permeability model matches well with the data obtained with different boundariy conditions,indicating its wide applicability.A structural model of "Equivalent Fractured Coal?EFC?" was proposed to characterize the structure of plastically damaged coal.Based on this model,the damage evolution of coal during mining was revealed and a dynamic permeability model applicable to plastically deformed coal was developed.The results show that the damage of the coal can be viewed as a process of segmentation of the raw coal matrix,and the mining disturbed coal can be treated as an elastic medium with smaller matrix size and larger fracture number.In the EFC,the fracture number increases exponentially with the deviatoric stress of coal,and a greater deviatoric stress leads to a larger number of fractures.A "Thermal-Hydraulic-Mechanical" multifield coupling model was developed with the consideration of stress,diffusion,seepage,temperature and heterogeneity of coal.With this model,the gas flow field in heterogeneous coal during permeability test was revealed.The results show that the gas pressure isosurface in heterogeneous coal fluctuates in space,which is different with that of homogeneous coal.The permeability increase with the time,and the permeability close to the outlet is lower than that close to the inlet,therefore,the permeability obtained from the test is the value at the outlet.The pore pressure distributes nonlinearly in the coal,which is different from the assumption of the permeability calculation equation.In order to lower the error,the measure of reducing the pressure difference between the inlet and outlet can be adopted.A multifield coupling model of "Stress-Damage-Diffusion-Seepage" was developed for the "Gas-Air" binary system.With this mode,the flow field evolutions of gas and air around the borehole were revealed during gas drainage,and the mechanisms of influence of different factors on gas drainage effect was discussed.It shows that the results calculated with 2D and 3D models were similar in the trend,and a little bit different in the values,but this error is acceptable for the engineering application.The permeability of the mining disturbed zone increases significantly,and both the gas and air flow rates increase obviously,but the gas concentration decreases gradually.With an increase of the sealing length of the borehole,both the gas and air flow rates reduce obviously,but the gas concentration increases gradually.For Pingmei 8^th mine,the optimized sealing length is identified as 12 m.The evolutions of stress,permeability and flow field in the hard-roof stope and the common stope were studied and gas accumulation and migration in both the stopes were revealed.Based on the results,a gas prevention technique was proposed.The results show that the stress-relieved zone and gas accumulation zone in the hard-roof stope is smaller than that of the common stope.By cutting the hard roof,both the fractured zone and gas accumulation zone can be stimulated,and the gas disaster in the hard-roof stope can be solved.The research findings in this thesis improved the theory of multifield coupling processes during gas drainage in deep fractured reservoirs,and promoted the development of the techniques for gas disaster prevention in deep coal mines.