Study On High Temperature Burnt Surrounding Rock Movement And Failure Mechanism In UCG

Underground coal gasification technology,as one of the models of green mining and clean utilization of coal resources,has great development space.It can not only reduce the impact of coal resources development on the environment,but also recover the coal resources which are difficult to exploit in traditional well mining,greatly improving the efficiency of coal resources recovery,and proving tremendous technical support for the construction of a resource-saving and environment-friendly society.This paper uses mechanical tests,theoretical analysis,numerical simulation,and other research methods,combining with the high temperature denaturation characteristics of surrounding rocks in the combustion area during underground coal gasification.A systematic study was carried out on the movement and failure law of high temperature burnt surrounding rock and the design method of gasifier-isolated coal pillar in underground coal gasification stope.Finally,the following results were achieved:(1)The variation of thermodynamic characteristics of mudstone-sandstones common in coal-bearing strata with temperature is studied and summarized.The mechanical experiments of coal after high temperature are carried out.The experimental results show that: The coal exhibits brittleness below 100?,and immediately failure when the pressure reaches the compressive strength;When the temperature reaches 200? and above,the coal rock exhibits ductility,and the coal exhibits a yielding flow state when the pressure reaches the limit of the compressive strength.The mechanical parameters of coal such as compressive strength,elastic modulus,tensile strength and cohesion show a three-stage change with increasing temperature:(1)At 20~100?,its mechanical properties are slightly enhanced;(2)At 200~400? its mechanical properties drop sharply;(3)When the temperature is greater than 400?,its strength remains stable and unchanged.(2)An extended model of internal temperature field of underground coal gasification surrounding rock based on dynamic parameters is established.Based on the model,the temperature extreme value and extension range in the surrounding rock are calculated.It is considered that the temperature field propagation range is 11.4m inside the roof and the internal propagation range of the bottom plate is 9.3m,and the coal wall has a propagation range of 9.6m.Based on the research results of the variation of mechanical properties of coal-rock with temperature,a numerical simulation method for the failure and movement of high temperature heterogeneity surrounding rock in the combustion area based on dynamic parameters is established.It lays a solid foundation for studying the movement and failure mechanism of surrounding rock of underground coal gasification.(3)Using FLAC3 D numerical simulation,it is found that the main reason for changing the movement and failure characteristics of UCG surrounding rock is the burnt of coal and rock caused by high temperature,and the thermal stress has little influence on the whole process.A discrete element model is established by using 3DEC to simulate the movement and failure of surrounding rock with different width of mining face under the conditions of conventional well mining and UCG.The results show that the height of caving zone and fissure zone increases with the increase of mining face width.The height of roof caving zone and fissure zone and the subsidence of roof in gasification mining are larger than that in conventional mining.By comparing the movement and failure characteristics of UCG roof under different lithological conditions,it can be seen that the greater the influence of high temperature on the rock mechanical properties,the greater the change of deformation and failure characteristics of roof.Because the mechanical properties of mudstone attenuate more under the action of high temperature than that of sandstone,the movement and failure degree of sandstone roof under the same mining face width is less than that of mudstone roof.(4)With the increase of the number of gasification faces in the mining area,the damage height of overburden and the width of plastic zone of coal pillar increase,but the change are very small.With the increase of the number of gasification surfaces,the maximum roof subsidence value increases,and the maximum value appears in the middle of the gasification area.Through numerical simulation,it is found that when the recovery rate is close to 50%,the stability of gasification stope decreases rapidly,which easily causes roof instability and coal pillar collapse.When the width of the combustion is greater than 24 m,the main roof begins to break and enters the gasification channel,which affects the gasification process.Therefore,on the basis of ensuring the stability of gasification passage,the width of gasifier to 16m~24m,and the recovery ratio to 40%~50%.(5)Based on the theory of large plate fracture and the principle of limit equilibrium,the stress distribution inside the isolated coal pillar is analyzed,and the calculation method of the yield zone of the isolated coal column is established.On this basis,the calculation model of yield zone width of trapezoidal high temperature burnt coal pillar is deduced based on the special isolated coal pillar shape and high temperature burnt characteristics of underground coal gasification.Stability evaluation model of isolated coal pillar based on yield zone width and ultimate load is proposed.(6)The state space equation method based on differential quadrature method is applied to the study of high temperature burnt roof movement and deformation in underground coal gasification,which solves the problem of burnt roof movement and deformation in underground coal gasification.A method for calculating the ultimate span of high temperature burnt roof under fixed and simply supported conditions is established.The results show that with the increase of roof thickness,the ultimate span between burnt roof and non-thermal roof increases in direct proportion.When the roof thickness is the same,the ultimate span of burnt roof is smaller than that of non-thermal roof under simply supported condition,and the growth rate of ultimate span of burnt roof is smaller than that of non-thermal roof with the increase of roof thickness.The ultimate span of burnt roof is larger than that of non-thermal roof under fixed support,and the growth rate of ultimate span of burnt roof is larger than that of non-thermal roof with the increase of roof thickness.(7)Based on the idea of synergistic deformation of surrounding rock in the goaf,the roof deformation of multi-strip gasification working face is regarded as the sum of the compression of coal pillar,and roof,and floor caused by stress concentration of coal pillar in the goaf and roof deformation in comsumption.A roof subsidence model based on synergistic deformation of surrounding rock in the combustion area is established.Based on this model,a prediction method of surface subsidence based on equivalent mining height of roof subsidence space is established.When the width of the combustion area is small and the isolated coal pillar is stable,it can be considered that the roof subsidence space can be transmitted to the surface.At this time,the roof subsidence space can be calculated and transformed into an equal mining height goaf,and the surface subsidence can be predicted by probability integral method.The numerical simulation and practical calculation show that the method is feasible.