Study On Evolution Law Of Coal Permeability Affected By Dual Pore Pressure

Coalbed methane is an important clean energy source.It is of great significance to study the gas flow law in coal pore for CBM exploitation and coal and gas outburst.Coal pore structure is complex,which is generally simplified as fracture system and matrix block.The fracture system of coal is the main channel of gas flow,and the matrix system is the main gas storage space.Because of the great difference between fracture and matrix permeability,the gas flow rate in fracture is faster than that in matrix,and the change of coal matrix pressure lags behind that of fracture pressure,which shows that the deformation of fracture system and matrix system is not synchronous,and finally leads to the evolution of coal seam permeability in space and time.In order to study the effect of dual pore structure of coal on gas flow,this paper uses physical simulation and numerical simulation to study the unsteady and steady flow processes of gas driven by coal fracture pressure and the interaction between them on permeability,taking into account the interaction mechanism between coal fracture pressure and coal matrix pressure.Influence.The effects of permeability on time,effective stress and gas-solid interface effect are discussed.The main conclusions are as follows:1)To study the effect of effective stress on coal permeability during steady gas flow.The experimental results show that the apparent permeability of coal decreases first and then increases with the decrease of effective stress.Based on the characteristics of gas dynamics,a permeability model considering gas-solid coupling is constructed,and the distribution of coal apparent permeability along gas flow path under gas steady flow is simulated numerically.It is found that when the injection pressure is higher than 1 MPa,the gas flow field of coal body is divided into viscous flow regime and slippage flow regime.Under steady flow conditions,the permeability of these two flow regimes is equal to the overall permeability of coal samples.The contribution of gas slip to gas flow is quantified by numerical analysis,and the error between equivalent permeability and measured permeability is analyzed.2)To study the temporal and spatial evolution of coal permeability during unsteady gas flow.Based on the observed gas pressure gradient attenuation process,the numerical simulation of the flow field permeability with time evolution process.It is found that coal permeability decreases first and then increases with time.In low pore pressure area,the larger the rebound of coal permeability is due to gas slippage.3)The influence of coal fracture-matrix interaction on coal penneability is studied.The test method of pressure attenuation is improved,and the evolution of permeability with time under constant stress and variable effective stress is observed.The evolution of fracture permeability and matrix permeability during gas injection is simulated numerically.The influence of coal matrix expansion on fracture permeability is analyzed by assuming boundary conditions,and the influence of local stress and macro strain on fracture permeability is quantified.