The Fracture Distribution Characteristics Of The Overlying Strata In An Abandoned Gob And Its Influence On The Migration Of Gas

There are a large amount of gas resources in the abandoned mine gob(AMG),which has the significance of resource utilization and environmental protection for its development.In the process of gas drainage from abandoned gob by surface well,the fracture of overlying strata,the occurrence characteristics of groundwater and the change of atmospheric pressure have obvious effects on borehole drainage.Among them,the fracture distribution of overlying strata is an important factor affecting the storage and flow of CBM.The fracture distribution characteristics of overlying strata in the AMG and its influence on the migration of gas drainage by surface well will provide guidance and reference for deepening the theory and practice of gas drainage in the AMG.The 3305 panel of No.5 mine in Hebi is an example in this paper.The distribution characteristics of stress,fracture and displacement of overlying strata in the abandoned gob were obtained by similar simulation experiment.The spatial structure of caved strata in the abandoned gob was obtained.According to the distribution type and stress characteristics of the coal(rock)in the abandoned gob,the permeability experiments of coal samples with different sizes and coal(rock)with different types of fractures under cyclic loading and unloading were carried out.The variation characteristics of permeability of coal(rock)were analyzed.ICEM module in ANASYS was used to establish a model of surface well for extracting gas from abandoned gob based on the result of similar simulation experiment and permeability experiment of coal(rock)under cyclic loading and unloading.The parameters of numerical model were set and solved by FLUENT.The optimization of well location parameters has been carried out for surface vertical well in the abandoned gob.The results show that:(1)For the single coal seam,the strata of caved zone in the abandoned gob collapse directly,while the strata of fractured zone collapse in the form of bending subsidence.The caved range decreases gradually with the increase of distance from coal seam,and the caved strata present trapezoidal distribution.The stress state is compressive stress in the middle of the abandoned gob,which is close to the original rock stress;the caved strata are compressed.The stress state is tension stress on both sides of the abandoned gob,and the maximum value of tension stress occurs in the start and end mining position;the transverse abscissions are dominant between caved strata.There are large numbers of vertical fractures in the lower part of the abandoned gob.The transverse abscissions and vertical fractures between caved strata are well-developed in the end mining position.The width of gas-conducting fracture in the end mining position is over three times than that of the start mining position.In the end mining position of the abandoned gob,the development degree,quantity and connectivity of the fracture are obviously better than that of the start mining position.(2)After the abandoned gob affected by mining,the development degree of the transverse abscissions and vertical fractures increase.The caved strata of the middle continue to be compacted,which increases the bending degree of caved strata.The tensile and compressive stress increase in the overlying strata,but the variation is relatively small.The development degree of transverse abscissions in the lower seam abandoned gob is obviously reduced.The development degree of transverse abscissions in the start mining position is greater than that in the end mining position,but the development degree of vertical fractures and the width of gas-conducting fracture in the end mining position are larger than that in the start mining position.(3)During the cyclic loading and unloading axial compression,the relationship between permeability and axial compression is a negative exponential function.The permeability decreases with the increase of axial compression in the loading stage,which indicated that the closure of pore and fracture of coal sample is sensitive to stress.The permeability increases with the decrease of axial compression in the unloading stage,which indicated that the opening degree of pore and fracture of coal sample is sensitive to stress.The permeability difference and loss amount decrease with the increase of cycle numbers.The permeability difference and loss amount mainly occurs in the initial stage of loading and unloading.With the increase of the height-diameter ratio of coal samples,the stress sensitivity and the dimensionless permeability difference decrease,and the dimensionless permeability loss amount increases.After cyclic loading and unloading,the permeability recovery rate decreases with the increase of height-diameter ratio,and the permeability loss rate increases.(4)During the cyclic loading and unloading axial compression,the relationship between permeability and axial compression is a negative exponential function for coal samples and sandstone with prefabricated fracture.The permeability and stress sensitivity coefficient of coal(rock)with vertical fracture are higher than the coal(rock)with horizontal fracture and intact coal(rock)samples.The permeability of coal(rock)with horizontal fracture and intact coal(rock)samples have little difference,but the stress sensitivity coefficient is higher than the intact coal(rock).The permeability difference and loss amount of coal(rock)with vertical fracture are higher than the coal(rock)with horizontal fracture and intact coal(rock)samples.The permeability difference of coal(rock)with horizontal fracture is higher than intact coal(rock),but there is slight difference in the permeability loss amount between them.After the cyclic loading and unloading axial compression,the permeability loss rate is the largest for intact coal(rock),followed by the coal(rock)with horizontal fracture and last is the coal(rock)with vertical fracture.The permeability recovery rate shows the opposite variation characteristics.(5)Based on the results of Fluent simulation,when the drainage negative pressure reaches equilibrium in the period of surface drainage gas from the abandoned gob.The methane concentration is the smallest in the end mining position,the methane concentration is the middle in the start mining position and the methane concentration is the largest in the middle of the abandoned gob.The end mining position is the most favorable area for gas flow.The preferred location of surface well is the end mining position,followed by the start mining position and last is the middle of the abandoned gob.