Study On Mechanism Of Coal Structure Evolution And Seepage Response Mechanism Under Alternating Hydraulic Pressure

Traditional coal seam pre-wetting by water injection has good comprehensive control effect in mine dust and rock burst prevention and control.However,with the increase of the mining depth of coal resources year by year,the complex geological environment in deep seam makes the water injection work of coal face new problems such as high in-situ stress,low porosity and difficult permeability,which leads to the increase of water injection pressure,the decrease of water injection flow and the poor wetting effect in deep seam,which seriously restricts the disaster prevention and mitigation effect of water injection in coal seam.At present,the research on permeability enhancement of low permeability coal mainly focuses on high pressure hydraulic fracturing,pressure relief and permeability enhancement.There is no systematic study on the fatigue damage of coal and rock structure under alternating water pressure,especially the damage and seepage evolution of coal and rock under alternating water pressure,waveform and wave amplitude.For this reason,aiming at the outburst problems of high in-situ stress,low porosity and difficult permeability encountered in the application of coal seam water injection in deep and low permeability coal seam,this paper takes experimental research and theoretical analysis as the main means,uses the water pressure below the normal initiation pressure of coal and rock to study the coal and rock structure evolution and seepage influence mechanism under the action of alternating water pressure under the action of in-situ stress,so as to realize the low permeability coal seam.Effective fracturing and permeability enhancement can promote the seepage and water-bearing wetting of pressure water in coal,improve the disaster prevention and control effect of coal seam water injection technology in deep resource mining,and ensure the safety of mine production.Firstly,using the self-developed rock multi-field coupling seepage and permeability enhancement experimental system,pore water pressure under different confining and axial pressures is tested by using two kinds of pore water pressure,constant pressure and rectangular wave,respectively.The results show that the constant pore water pressure will not improve the permeability of coal and rock,and the stress environment of coal and rock will permeate coal and rock.The change of performance plays a leading role;in the high stress state,cyclic alternating water pressure promotes the development of initial damage evolution of coal and rock,expands weak fracture surface and produces local fatigue damage.Then,under certain stress conditions,the macroscopic fluid-solid coupling tests of coal and rock are carried out under the conditions of alternating water pressure with different wave amplitudes,up and down peaks and different cycles.The effects of different alternating water pressure parameters on the stress,strain and permeability of coal and rock are analyzed.The results show that the cyclic alternating water pressure has a certain destructive effect on coal and rock,and the destructive effect is cumulative.When the number of cycles reaches 1000,the strength limit of coal and rock is much lower than 50 cycles of alternating water pressure.In addition,the higher the amplitude of alternating water pressure,the greater the internal force of coal and rock,and more damage to coal and rock.Finally,on the basis of theoretical and experimental research,starting from the principle of effective stress,the macro-deformation characteristics of coal and rock in the process of hydrostatic alternating cycle are analyzed in depth,the structural damage parameters are established,the corresponding relationship between them and the fatigue failure process is analyzed,and the matching relationship among the alternating hydrostatic pressure,macro-structural damage of coal and rock and seepage evolution is explored.The results show that the effective stress of coal and rock is macro-meso-scale.There is a dynamic response mechanism between structural damage and seepage.Cyclic alternating water injection can enhance the permeability stress sensitivity and strain stress sensitivity of coal and rock.