Tunnel though coal seam is one of the most difficult tunnels since its risk of coal and gas outburst and the complex geological conditions.According to the directional cutting of water jet and the characteristic of th...Tunnel though coal seam is one of the most difficult tunnels since its risk of coal and gas outburst and the complex geological conditions.According to the directional cutting of water jet and the characteristic of the coal seam and rock mass,this paper presents a new method of tunnelling though coal seam assisted by water jet slotting,which can be divided into improving permeability of coal seam and directional cracking in the rock mass.The mechanism of improving permeability of coal seam was stated,and the crack criterion of rock during blasting was established based on fracture theory.Then,the evolution law of pressure wave and the crack propagation were simulated by FEM software ANSYS/LS-DYNA,the results show that the shape of the crush zone formed by stress wave is different between the normal borehole blasting and the slotted borehole blasting,and the tension is the main factor which let crack propagation.What is more,for normal borehole blasting,the tension concentration occurred along the direction of 45 degrees and let crack expand,while for slotted borehole blasting the tension concentration occurred along the direction of 0 degrees and 90 degrees,and the maximum tension along the direction of 0 degrees is larger than the maximum tension along the direction of 90 degrees,and the main crack expand along the direction of 0 degrees,which prove that the existence of the slot play a good role of orientation for directional cracking.展开更多
Hydraulic fracturing(HF)technology can safely and efficiently increase the permeability of coal seam,which is conducive to CBM exploration and prevent coal and gas outburst.However,conventional HF fractures tend to ex...Hydraulic fracturing(HF)technology can safely and efficiently increase the permeability of coal seam,which is conducive to CBM exploration and prevent coal and gas outburst.However,conventional HF fractures tend to expand in the direction of maximum principal stress,which may be inconsistent with the direction of fracturing required by the project.Therefore,the increased direction of coal seam permeability is different from that expected.To solve these problems,PFC2D software simulation is used to study directional hydraulic fracturing(DHF),that is the combination of slotting and hydraulic fracturing.The effects of different slotting angles(θ),different horizontal stress difference coefficients(K)and different injection pressures on DHF fracture propagation are analyzed.The results show that the DHF method can overcome the dominant effect of initial in-situ stress on the propagation direction of hydraulic fractures and control the propagation of fractures along and perpendicular to the slotting direction when θ,K and liquid injection pressure are small.When the DHF fracture is connected with manual slotting,the pressure will shake violently,and the fracturing curve presents a multi-peak type.The increase and decrease of particle pressure around the fracturing hole reflect the process of pressure accumulation and fracture propagation at the fracture tip respectively.Compared with conventional HF,DHF can not only shorten the fracturing time but also make the fracture network more complex,which is more conducive to gas flow.Under the action of in-situ stress,the stress between slots will increase to exceed the maximum horizontal principal stress.Moreover,with the change in fracturing time,the local stress of the model will also change.Hydraulic fractures are always expanding to the area with large local stress.The research results could provide certain help for DHF theoretical research and engineering application.展开更多
基金Financial support for this work,provided by the National Basic Research of China(2014CB239203)the National Natural Science Foundation of China(51474158)+1 种基金also supported by Program for New Century Excellent Talents in University(NCET-12-0424)The authors would like to thank the State Key Laboratory of Coal Mine Disaster Dynamics and Control and Hubei Key Laboratory of Waterjet Theory&New Technology.
文摘Tunnel though coal seam is one of the most difficult tunnels since its risk of coal and gas outburst and the complex geological conditions.According to the directional cutting of water jet and the characteristic of the coal seam and rock mass,this paper presents a new method of tunnelling though coal seam assisted by water jet slotting,which can be divided into improving permeability of coal seam and directional cracking in the rock mass.The mechanism of improving permeability of coal seam was stated,and the crack criterion of rock during blasting was established based on fracture theory.Then,the evolution law of pressure wave and the crack propagation were simulated by FEM software ANSYS/LS-DYNA,the results show that the shape of the crush zone formed by stress wave is different between the normal borehole blasting and the slotted borehole blasting,and the tension is the main factor which let crack propagation.What is more,for normal borehole blasting,the tension concentration occurred along the direction of 45 degrees and let crack expand,while for slotted borehole blasting the tension concentration occurred along the direction of 0 degrees and 90 degrees,and the maximum tension along the direction of 0 degrees is larger than the maximum tension along the direction of 90 degrees,and the main crack expand along the direction of 0 degrees,which prove that the existence of the slot play a good role of orientation for directional cracking.
基金supported by National Natural Science Foundation of China(52130409,52004291,51874314)the Fundamental Research Funds for the Central Universities(2022YJSAQ03,2022XJAQ02).
文摘Hydraulic fracturing(HF)technology can safely and efficiently increase the permeability of coal seam,which is conducive to CBM exploration and prevent coal and gas outburst.However,conventional HF fractures tend to expand in the direction of maximum principal stress,which may be inconsistent with the direction of fracturing required by the project.Therefore,the increased direction of coal seam permeability is different from that expected.To solve these problems,PFC2D software simulation is used to study directional hydraulic fracturing(DHF),that is the combination of slotting and hydraulic fracturing.The effects of different slotting angles(θ),different horizontal stress difference coefficients(K)and different injection pressures on DHF fracture propagation are analyzed.The results show that the DHF method can overcome the dominant effect of initial in-situ stress on the propagation direction of hydraulic fractures and control the propagation of fractures along and perpendicular to the slotting direction when θ,K and liquid injection pressure are small.When the DHF fracture is connected with manual slotting,the pressure will shake violently,and the fracturing curve presents a multi-peak type.The increase and decrease of particle pressure around the fracturing hole reflect the process of pressure accumulation and fracture propagation at the fracture tip respectively.Compared with conventional HF,DHF can not only shorten the fracturing time but also make the fracture network more complex,which is more conducive to gas flow.Under the action of in-situ stress,the stress between slots will increase to exceed the maximum horizontal principal stress.Moreover,with the change in fracturing time,the local stress of the model will also change.Hydraulic fractures are always expanding to the area with large local stress.The research results could provide certain help for DHF theoretical research and engineering application.
