Research On Fracture Characteristics Of No.3 Coal Seam In Northern Shizhuang,Qinshui Basin

In this dissertation,through the fracture observation of related structural layer in outcrops,combined with laboratory physical experiments of core samples and logging data,by taking insitu stress of single well and fracture parameters as constraints,quantitative numerical simulation on spatial distribution of fracture parameters of No.3 coal seam of northern Shizhuang in the southern part of Qinshui basin is taken on the basis of paleo-structural stress field and current in-situ stress field numerical simulation,fracture development regularity and static and dynamic influence factors are analyzed,thus the quantitative analysis of coal seam fracture characteristics is realized.Recognition and description of fracture in Triassic sandstone formation in outcrops of the southern part of Qinshui basin demonstrates the similarity between fracture of No.3 coal seam in northern Shizhuang and fracture in surface exposed sandstone.Fractures in outcrops are mainly NW and nearly EW strike,with the characteristics of conjugate shear fractures.Most of the fractures are shearing fractures with high angle.Geological model consistent with true burial depth is established on the basis of structural configuration of No.3 coal seam and the surface topography of northern Shizhuang in the southern part of Qinshui basin.Static mechanical parameters of No.3 coal seam and samples of roof and floor are achieved by rock physical measurement.Combining with rock dynamic mechanical parameters from logging data and static parameters obtained form the mechanical test,the proper parameters of No.3 coal seam and roof and floor in the geological model are selected.Statistical analysis of in-situ stress in single well,which can be calculated through hydraulic fracturing and other method,confirms that in-situ stress switches twice in the burial depth of 600 m and 1000m;desired result of stress distribution of No.3 coal seam of northern Shizhuang is achieved by repeated loading and fitting of current mechanical model,and the calculation result of in-situ stress in single well can be used as inspection standard.Divided into many typical structural zones,fitting analysis of relationship between principal stress and coal seam structure and burial depth demonstrates that,structural configuration is the main influence factor of stress distribution of coal seam.Combing with regional structure evolution and structural stress field evolution,the main structural configuration of No.3 coal seam of northern Shizhuang is formed in early Yanshanian with strong NE compressive stress,and multiple structural movements in later stages were only inherit transformation on the basis of former structural configuration.So,the main generation period of structural fractures of No.3 coal seam is Yanshanian.On the basis of current finite element model of No.3 coal seam of northern Shizhuang,decreasing structural topography,and using processed node coordinates to establish paleo-model.Through repeated loading experiments of nodes,it can be considered as the best loading way of equivalent paleo-stress,when the vertical strain overlaid paleo-structural configuration corresponds with current state.Then the structural stress field numerical simulation result of Yanshanian is achieved.Quantitative numerical simulation on spatial distribution of structural fracture of No.3 coal seam of northern Shizhuang is taken combing with calculation results of paleo and current stress field,establishing mathematical relationship between fracture parameters and stress-strain.Fracture density distributes mainly between 20?40 per meter.Fracture aperture changes under the transformation of current stress field,and its distribution is opposed to the density,fracture density is low but aperture is high in anticlinal area,fracture density is high in the western district and synclinal area,but most of fractures are closed,the interior of fault belts is broken,it has the highest fracture density.Coal seam fractures are mainly NEE and SEE conjugate strike,while fractures are SEE strike near fault belts.Distribution regularity of fracture porosity is similar to aperture.Different districts and belts have different values,high values distribute in the structural high position such as the axial part of anticline,low values distribute in the structural low position such as the axial part of syncline.Structural fracture permeability of coal seam has obvious anisotropy,the E-W permeability of fracture is close to N-S permeability,while the vertical permeability is lower.Fracture permeability in different direction has similar distribution regularity,low values distribute in the axial part of syncline,high values distribute in the axial part of anticline.Fracture simulation result shows that porosity and permeability of coal seam are mainly influenced by geological structure,but the variety characteristics of porosity and permeability are different in different structural districts,the original distribution regularity of porosity and permeability is influenced by inregular burial depth which caused by complex surface topography,and the relationship between porosity and permeability and burial depth is weak.As the effective exploitation of CBM,the reduction of formation pressure leads to the increase of coal seam effective stress,and fracture porosity and permeability decrease with it;but porosity and permeability of coal seam increase by shrinkage strain of coal matrix resulted from coal gas analysis.Based on the analysis of coal seam reservoir pressure and matrix shrinkage effect in the development,adopted tested simulation to proceed dynamic analysis of fracture porosity and permeability,comprehensively considering two dynamic factors,demonstrate that coal seam fracture porosity and permeability decrease slightly with the reduction of formation pressure,but the whole distribution regularity has no obvious change,it has little influence on the exploitation of CBM.