Formation Mechanism And Multi-scale Seepage Characteristics Of Overburden Fracture In Abandoned Coal Mine

With the elimination of outdated coal mines and the reduction of production capacity,the number of closed mines in our country is increasing.However,there are still a large numerous of resources in the abandoned mines,such as coal,water and gas.The precise development and utilization of this resource will be beneficial to the region enterprise transformation and development.Due to the complex environment in the goaf area of abandoned mines,the coexistence of cracked coal and rock mass,water and gas,the overburden fracture is a key component.On the one hand,it is caused by the fracture of the overburden in the early mining process.On the one hand,as a channel,it plays a leading role in the movement of water,gas and other fluids in the goaf.Therefore,exploring the conduction mechanism and multi-scale seepage characteristics of the overlying rock fractures in the goaf area of abandoned mines provides theoretical support for the secondary development and utilization of abandoned mine resources.This paper takes the development and utilization of resources in abandoned mines as the research background.Aiming at the formation mechanism and multi-scale seepage characteristics of the overburden fissure channel,the research is carried out by a combination of laboratory tests,theoretical analysis and numerical simulation.In this paper,the mechanical properties of coal and rock mass and its failure process were studied through uniaxial compression tests.The crack mechanics mechanism of thick hard roofs was explored from the perspective of elastic-plastic mechanics.The rupture mechanism was studied from two scales of standard specimen failure and rock block rupture.Then,the seepage characteristics of the natural fractured rock and coal were studied respectively.The hydraulic coupling seepage test was carried out on the fractured rock body using a multi-field test press,the pore and fissure characteristics of the fractured coal and its seepage characteristics were studied from the microscopic point view using CT scanning technology.Finally,based on the phenomenon of gas floating in the goaf and underwater seepage from the separated layer,the flow characteristics of the water and gas in the fracture network was simulated through the volume of fluid(VOF)method.The main contents and results obtained are as follows.(1)The mechanical properties of the coal/rock body were tested through uniaxial compression tests,and the rupture process of the coal rock body was monitored in real time by combining acoustic emission(AE)monitoring system and digital image correlation(DIC)fullfield strain measurement system.By comparing the mechanical properties of coal and sandstone,there were obvious differences between coal and sandstone in terms of uniaxial compressive strength,deformation characteristics and damage mode.In comparison,the brittle failure characteristics of coal sample were more obvious.The change in acoustic emission energy reflects the accumulation and release of elastic energy during the rupture process,and the evolution of acoustic emission localization points under different stress levels can effectively reflect the rupture propagation.Using DIC full-field strain measurement method can quantitatively monitor the evolution of the displacement and strain fields at the marking point and surface at the same time,which breaks through the limitations of traditional empirical and qualitative-based rupture process.In the monitoring process,the AE pays attention to the internal rupture of the specimen and the DIC focuses on the surface deformation.They complement each other can more comprehensively to reflect the rupture process.(2)From the perspective of elastoplastic mechanics,the damage mechanism of thick hard rock and the aperture characteristics of fracture were studied,and the cantilever beam model with linear incremental load was established.And then,the analytical formula of the internal stress component was obtained according to the elastic stress inverse solution method.Combined with the Mohr-Coulomb shear yield failure criterion,the implicit function equation of the fracture trace was derived.The shape of the fracture trace was related to the congenital mining conditions and the mechanical properties of the rock.The rupture traces show a "vertical pair" pattern,which indicates that the cracks will be deflected during the failure process,and the location of the inflection point was related to the cohesion and internal friction angle of the rock formation.In addition,as the length of the block increases,the rotation angle becomes smaller,and the corresponding opening degree shows a trend of increasing first,then decreasing and then increasing along the rupture trace,which was beneficial to determine the main seepage channel in the mining area,and was also important for further predict the fluid seepage in the fracture.(3)The seepage test was carried out through multi-field coupling test machine,and the seepage law in fractured rock was given under different confining stress and permeability pressure.During the loading process the seepage flow with permeability pressure showed various trends under different confining stress,and its growth types showed power-law,linear change,exponential and bilinear growth change with the increase of confining stress.However,the seepage flow rate under the unloading path was obviously lower than the loading path due to the influence of load history.The three-dimensional morphological characteristics of the fracture surface were further characterized via three-dimensional laser scanner,and the mean undulation height,standard deviation,root mean square first-order derivative and fractal dimension of each fracture surface were statistically analyzed.Among them,the original joint roughness coefficient was concentrated in the range of 8-10 and the fractal dimension was between 1.07-1.16,and the frequency histogram of the undulation height on the fracture surface was conforms to the Gaussian distribution law.Finally,the scanned data were imported into COMSOL software,and the simulation results of the fluid seepage path in the fissure surface showed that there is an obvious dominant seepage path in the surface,and there was an obvious transition zone in the water pressure distribution,the flow streamline and velocity distribution show obvious non-uniform change characteristics due to the geometrical features of the rough surface.The existence of the dominant seepage path was the main reason for the non-Darcy seepage in the rough fissure surface.(4)Using CT scanning technology to obtain the internal pore and fissure structure characteristics of the original fractured coal,Avizo software was used to extract the REV of the fractured coal,and reconstructed the pore and fissure model.It can be seen that there was an obvious macroscopic fracture inside the primary fractured coal mass,while secondary fractures and pores are distributed around it.The pores show a continuous piecewise distribution in the local area with some isolated pores.The establishment of the pore network model reproduces the distribution of macroscopic fractures,pores,and channels,the internal reservoir structure of the fractured coal was more finely characterized.By analyzing the simulation results of seepage flow in the fractured coal,the simulation results showed that the permeability of the original coal body without fractures was low,and there almost no flow line distribution inside the simulation results,while the the fluid was transported downward along the main fractures in the fractured cell,it also transports to the surrounding connected secondary fractures and pores,and forms a good seepage channel,which leads to the increase of the permeability of the fractured coal mass.(5)Combined with the digital image processing technology and similar simulation experiments,the geometric parameters(width,trace length,area,perimeter and angle,etc.)of the overlying strata fractures were statistically analyzed,and a large-scale fracture network model of the mined-out overlying rock was established.Based on the phenomenon of the water seepage from the separation layer and the gas floating in the collapse zone,the volume of fluid(VOF)model was used to capture the process of water-gas countercurrent flow.Research shows that the overlying rock fractures was trapezoidal on the whole,and the internal interlayer separation fractures and longitudinal fractures crisscross and interconnect each other.The fracture aperture,area and perimeter of the two types of fractures present an exponential distribution,and the crack trace length presents a log-normal distribution,and the inclination angle of the fractures presents a normal distribution.The characteristics of the water-gas two phase seepage in the large-scale fracture network reveals some phenomenon.Due to the imbalance of seepage velocity in the fractures,the fractures on both sides of the goaf have formed obvious dominant seepage channels.There were gas retention zones in lateral interlayer separation fissures,the two ends of the fissures were blocked by water,and there was a maldistribution at the intersections.Which has certain guiding significance for the reuse of abandoned gobs.This paper carried out a comprehensive and multi-scale research on the mining induced fractures,and the research results have certain guiding significance for the reuse of resources in the abandoned mines.