Damage Induced Permeability Evolution In Stressed And Gas-bearing Primary Coal-rock Combination And Its Application

The pressure relieved gas drainage technology of the protected layer is an effective regional measure to prevent coal and gas outburst.Its effectiveness largely depends on the pressure relieved and anti-reflection effect of the protected layer.While mining the protected layer,the results of pressure relieved and permeability enhancement are significantly different between coal seam containing gangue and coal seam without gangue affected by coal seam structure.Therefore,the research on the deformation damage of coal-rock combination body and the evolution mechanism of gas seepage is an important content to realize efficient gas drainage under pressure relieved in coal seam with complicated gangue structure.Based on the engineering background of pressure relieved gas drainage of 11# coal seam containing gangue in Baode coal mine,Shanxi Province,the primary and artificial coal-rock combination body were taken as the research object,and the theoretical analysis,experimental test and numerical simulation were employed in this work.The meso structural characteristics and mechanical properties of the primary coal-rock combination body were analyzed,and the deformation damage and the evolution law of seepage of the gas bearing primary coal-rock combination body were investigated.Also,an optimization technical scheme of gas drainage with pressure relieved in the protected layer with gangue was proposed and applied in practical engineering.The main research conclusions of this work are as follows:The three-dimensional microstructure characteristics of the primary and artificial coal-rock combination body were obtained.By using CT scanning and Avizo threedimensional reconstruction technology,the structural characteristics of the primary and artificial coal-rock combination body were also obtained.It is found that the interface structure of the primary coal-rock combination body is complex and zigzag,and the and the coal rock matrix is infiltrated and inlaid with certain fractal characteristics,with the fractal dimension between 2-3.The internal mineral structure and the distribution characteristics of the pore fissure structure for the primary coal-rock combination body were depicted.Also,the mineral structure and pore fissure structure in different scales are quantitatively analyzed based on the statistical method of fractures and mineral equivalent diameter,which reveals the heterogeneity of the primary coal-rock combination body.The mechanical and strength characteristics of the primary and artificial coal-rock combination body under load conditions were obtained.Based on the results of the CT scanning reconstruction,a grid model of primary coal-rock combination body with primary interface structure was established,and uniaxial loading tests and Abaqus simulation loading tests are carried out.It is found that the primary interface structure has a certain weakening effect on the compressive strength of coal-rock combination body,and the uniaxial compressive strength of rock samples,artificial coal-rock combination body samples,primary coal-rock combination body samples,and coal samples decreases in turn.With the influence of the primary interface structure,the radial deformation of the primary coal-rock combination body is smaller than that of the artificial coal-rock combination body.Also,the inward force exerted by the rock mass on the coal body reduces the equivalent axial differential stress of the coal body and enhances the compressive strength of the coal body.The primary interface structure has significant influence on the evolution characteristics of the plastic zone of the coal-rock combination body,and the plastic deformation firstly occurs near the coal-rock interface and gradually develops towards the interior of the coal body.The strength deformation and damage failure laws of gas bearing primary coal-rock combination body under different mechanical paths were obtained.Two mechanical paths of constant confining pressure plus axial pressure and constant axial pressure minus confining pressure were designed,and the simultaneous experiments of mechanicaldamage-seepage were performed considering the effects of different confining pressures,unloading rates of confining pressure,and loading and unloading modes.The results show that confining pressure,unloading rates of confining pressure,and loading and unloading modes have significant influence on the damage degree and seepage characteristics of samples.It is found that the strength of samples increases with increasing of confining pressure.The strength relationship of these three samples under the same confining pressure is as follows: gas bearing artificial coal-rock combination sample>gas bearing primary coal-rock combination sample>gas bearing raw coal sample.High confining pressure conditions restrict the deformation of the coal and rock parts of the primary coalrock combination body,and the primary coal-rock combination body is easier to arise interface slip failure with low confining pressure conditions.The interface structure has a great influence on the development of coal-rock interface fractures.Under the same mechanical conditions,the destruction coal body of the primary coal-rock combination body have more obvious cracks,and it is also much easier to expand to the rock mass.Also,the interface structure significantly affects the failure mode of the coal-rock combination body.Besides,the failure of the primary coal-rock combination body mainly occurs at the interface or coal body,which have three failure mode including the coal shear failure,the interface slip failure and the simultaneous of coal shear and interface slip failure.The evolution law of gas seepage of gas bearing primary coal-rock combination body under different mechanical paths was obtained.The results show that the permeability of these three samples decreased first and then increased in the whole stressstrain process.The permeability of raw coal samples after destruction is significantly increased compared with the initial value,but due to the influence of low permeability of rock mass,the permeability of the raw coal-rock combination samples after destruction is slightly increased compared with the initial value.The failure mechanism and permeability evolution mechanism of gas bearing primary coal-rock combination body under the influence of coal-rock interface interaction force were revealed.Considering the difference of mechanical properties,effective stress coefficient,and gas adsorption effect of coal and rock,an analytical formula of the interaction force between coal-rock interfaces is proposed,and then the failure mechanism of the loaded gas bearing primary coal-rock combination body is revealed.It is found that the larger the difference of mechanical parameters effective stress coefficient,and other parameters between coal and rock,the relative deformation of coal and rock is more obvious,and the radial interaction force at the interface is also larger.Based on Griffith’s theory of solid material fracture,the influence of the relationship between the stress intensity factor of the interface crack and the fracture toughness of the coal body on the interface crack growth was analyzed,and the mechanism of interface crack growth of primary coal-rock mass gas bearing under axial loading path was obtained.The permeability evolution models of gas bearing coal and single rock under the influence of coal-rock interface interaction are respectively constructed,and then the permeability evolution model of gas bearing primary coal-rock combination body was established.The effect of its main control parameters was analyzed,and then the evolution mechanism of permeability of gas bearing primary coal-rock combination under the influence of coalrock interface interaction was revealed.The evolution law of deformation damage of coal seam containing gangue during mining unloading is expounded.According to the geological conditions and mining conditions of Baode coal mine in Shanxi Province,the evolution characteristics of unloading damage deformation of the protected layer(11 # coal seam containing gangue)under the influence of the mining of the protective layer(8 # coal seam)were simulated.It is found that the stress of the protected layer has the rule of increasing-decreasingincreasing,and the change rule of deformation show the rule of compressing-recoveringinflating-shrinking.However,there is no obvious plastic failure in the surrounding rock,which hinders the gas flow through the layer.The optimized technical scheme of pressure relieved gas drainage in the coal seam containing gangue is put forward.Under the influence of the mining of the protective layer,the permeability enhancement effect of the gangue layer in the protected layer is not obvious,which has an obstacle effect on the gas flow through the layer.The best unloading point of the protected layer appears about 50 m behind the working face of the protective layer,where the permeability is 10.8 times of the initial permeability,and the permeability after re-compaction is 6.2 times of the initial permeability.Based on the above,Based on this,an optimization scheme of gas extraction from protected layer containing gangue with pressure relieved by directional drilling rig combined with horizontal branch drilling and cross-layer branch drilling was proposed.The engineering practice shows that this scheme can significantly improve the efficiency of gas drainage in the coal seam containing gangue and effectively reduce the risk of gas outburst in the coal seam containing gangue.