Research On The Deformation And Seepage Characteristics Of Crushed Coal And Rock And Its Application In Abandoned Coal Mine Gob Methane Extraction

In recent years,as an unconventional natural gas resource,abandoned mine gob methane?AMM?has attracted more and more attention.On one hand,the extraction of AMM can relieve the gas pressure of adjacent coal seam and create favorable conditions for the exploitation of residual coal resources;on the other hand,it can be pumped as a clean energy.The extraction of AMM can not only"turn waste into treasure"to improve China's energy structure,but also prevent it from escaping into the atmosphere to increase the greenhouse effect.Therefore,the extraction of AMM is of great significance.According to the research results of the major consulting research project of Chinese Academy of Engineering,the strategy of efficient recovery of coal resources and energy conservation in China,it is predicted that the number of abandoned mines in China will reach 12000 by 2020.The coal recovery rate of these abandoned gobs is generally low before closure,and after the closure of the mine,abundant and considerable methane is enriched with time.The research of AMM extraction technology abroad is earlier than that in China,and the research in this field is still in its infancy in China.On the basis of previous research,this paper takes the abandoned gob of Qinshui coalfield of Shanxi Jincheng Coal Mine Group as the research object,and aims to solve the problems of unclear drilling well layout and unclear deformation and seepage characteristics of crushed coal and rock existing in surface extraction of AMM.By means of theoretical analysis,numerical simulation,laboratory experiment and on-site industrial test,the related technical foundation of AMM extraction in goabs is systematically studied.The main conclusions are as follows:?1?The concept of gas storage space of abandoned mine gob is defined scientifically.The gas storage space of abandoned mine gob is divided into caving type and knife-pillar type.The enrichment range of gas storage space in different types of gobs is delineated by the pressure relief zone and plastic failure zone induced by gob excavation.Three-dimensional numerical simulation experiments show that there exists a dumbbell-shaped three-dimensional pressure relief body in caving gob which is wrapped in a inverted trapezoidal plastic failure platform combination with larger upper and smaller bottom ones.N?n is the number of pillar mining areas?spindle-shaped relief space assemblies in the gob with knife pillars are connected by N inverted trapezoidal plastic failure space assemblies against the bottom.Based on this,the caving gob is divided into epitaxial plastic zone,fracture development zone and compaction affected zone,and the knife pillar gob is divided into epitaxial plastic zone and pressure relief connected zone.?2?The source of methane in gob and the effect of crushed coal and rock on the enrichment zone of methane in gob are analyzed.The seepage model of crushed coal and rock in caving zone of gob and its characteristics are summarized.In order to satisfy the need of studying the law of internal fracture and the law of acoustic emission parameters evolving with stress in the process of compaction of crushed coal and rock,a compaction-acoustic emission?AE?experiment system for crushed coal and rock is developed.Based on the analysis of the differences and difficulties between AE experiment of crushed coal and rock porous media and traditional AE experiment of porous and fractured coal and rock,the specially AE experimental method for crushed coal and rock porous media is put forward.In order to study the gas seepage characteristics during the compaction process of crushed coal and rock,a compaction-gas seepage experimental system and the corresponding experimental methods for crushed coal and rock were developed independently.?3?Compaction deformation characteristics of crushed coal and rock with different particle size and lithology were studied by using compaction-acoustic emission experimental system.The compaction process of crushed coal and rock mass is divided into three stages:initial compaction,linear compaction and plastic compaction.The three deformation stages occur when the axial stress reaches 5-10%,5-30%and 30-40%of the uniaxial compressive strength of the crushed coal and rock respectively.The larger the particle size and the softer the lithology are,the longer the initial compaction stage and the linear compaction stage are.?4?The evolution law of AE counting and AE energy with time and its corresponding relationship with three deformation stages of crushed coal and rock are expounded.In the initial compaction stage,only a small amount of friction acoustic emission?AE?signals are generated,so the cumulative count and energy of AE are very small.In the linear compaction stage,AE signals of particle friction and particle breakage increase sharply,and the cumulative AE count and energy increase from the initial"convex"type to the linear type.In the plastic consolidation stage,the AE signals of crushed gangue and limestone show attenuated growth mode,while the crushed anthracite still shows linear growth mode.?5?There are two kinds of fracture evolution models of AE in the process of axial compaction of crushed coal and rock,and the fracture evolution model is related to the stage of axial compression.At the same time,the crushed coal and rock has the characteristics of delamination fracture in the process of compaction,that is,under the constraints of axial compression circumferential displacement,the internal fracture first occurs in the middle stratum,and with the continuous loading,the fracture will gradually transfer to the upper and lower strata,and ultimately occur in the middle and upper strata to the greatest extent.?6?An experimental study on the relationship between porosity,permeability and gas pressure in the compaction process of crushed anthracite was carried out by using the compaction-seepage experimental system of crushed coal and rock.It reveals the internal mechanism of the transition from Darcy flow to slippage flow with the continuous decrease of gas pressure.It is found that methane migration in crushed coal and rock requires threshold pressure,the pseudo-threshold pressure gradient is between 80 and 103 Pa/m,and the pseudo-threshold pressure gradient increases with the decrease of porosity;the permeability of crushed anthracite is between 10^-12-10^-11 m²,and the permeability increases exponentially with the increase of porosity,and the permeability shows a logarithmic growth with the increase of gas pressure gradient.The crushed anthracite permeability is still more than 10^-12 m² even if the stress is restored to the original rock stress.Therefore,the crushed coal and rock in the caving zone together with the through-bed fracture zone constitute the“U”-type high permeability methane enrichment area.?7?Based on the research results of different types of gob gas storage space and deformation and seepage characteristics of crushed coal and rock,the abandoned gob in Jin Coal Group is classified and classified,and it is proposed that the surface boreholes should be arranged in the"U"type high permeability methane enrichment area.The optimum drilling position in gob with knife pillar mining should be drilled into high permeability methane enrichment area formed by the cavity of gob,connected coal pillars with plastic failure and crushed coal and rock.Drilling should be drilled from the center position along the dip and strike direction of the central knife column gob unit.And the position of the final hole in the surface drilling should be as deep as possible to the bottom plate.The above research conclusion guides the arrangement of drilling wells for surface extraction in abandoned mine gobs of Jincheng Coal Mine Group,and achieves good extraction effect.