Fracture Characteristics And Damage Mechanism Of Deep Coal Impacted By Water Jet Under True Triaxial Stress

Deep ground is the main battlefield of resource exploitation in the future.Deep coalbed methane resources are rich in reserves,Efficient development of deep coalbed methane is important to optimizing my country’s energy structure,reducing foreign dependence on natural gas,and ensuring energy security.In recent years,hydraulic measures(hydraulic punching,hydraulic slotting,tree type drilling,etc.)based on water jet rock breaking have been widely used as an efficient method for increasing permeability of reservoirs.However,as mining gradually enters deep mines of more than 1000 meters,the rock mass is in a complex environment of high in-situ stress and high geothermal temperature,and the fracture characteristics and damage fracture mechanism of coal have changed significantly.The existing theory of shallow coal breaking and related hydraulic technology parameters and technical equipment cannot be directly applied to the deep,and hydraulic measures face new challenges in the deep application.It is urgent to carry out research on the damage and fracture mechanism of water jet impacting deep coal,so as to lay a theoretical foundation for the efficient development of deep coalbed methane by water jet technology.Therefore,this paper adopts the methods of experimental research,theoretical analysis and numerical simulation to carry out research on "the fracture characteristics and damage mechanism of water jet impacting coal under different in-situ stress and geothermal conditions".Based on the self-developed experimental system of jet rock breaking under true triaxial stress,researches were carried out on the shape,threshold pressure,damage distribution,and microscopic fracture mode of water jet impacting coal breaking zone under different in-situ stress(true triaxial)and geothermal conditions.The damage and fracture mechanism of water jet impacted deep(geostress,geothermal)coal is revealed.The following main innovative results were achieved:(1)The damage and fracture characteristics of coal under the synergistic effect of water jet impact and in-situ stress are revealed.Based on the self-developed "jet rock breaking experimental system under true triaxial stress",the water jet impact coal breaking test under different stress conditions was carried out,and the damage distribution law of coal and rock was obtained.The damage distribution range of coal shrinks rapidly with the increase of in-situ stress.The microscopic fracture mode of coal has been proved,and the water jet coal breaking is dominated by tension failure and transition to shear failure dominated by deep high in-situ stress.The shape of the crushing pit changes from "V" shape to "U" shape with the increase of ground stress,the volume of broken rock decreases,and the specific energy consumption increases.When the in-situ stress is 10 MPa,the rock-breaking volume of the jet decreases by 79.7%compared with the condition without in-situ stress,and the specific energy consumption increases by nearly 5 times.(2)The calculation model of coal rock fracture threshold pressure under different in-situ stresses subject to water jet was established,and the intrinsic relationship between coal rock fracture threshold pressure and in-situ stress state and strength parameters was clarified,and the experimental verification was carried out.The water jet impacting coal fracture threshold pressure increases exponentially with the increase of in-situ stress,and there is a threshold pressure stagnation point for jet rock fracture.When the stagnation point of the coal threshold pressure is at 30 MPa,the coal exhibits strong ductile deformation characteristics under high in-situ stress,so that the fracture threshold pressure is stable at 19 MPa.(3)The damage and fracture mechanism of coal under water jet impact with different geothermal conditions is elucidated.Under the condition of deep high geothermal temperature,the strength of coal is deteriorated,and the combined thermal stress of jet impact can greatly enhance the ability of jet damage to break coal.When the ground temperature is 75°C,the coal-breaking depth of the water jet increases by81%,and the specific energy consumption decreases by 52%.Combined with CT and SEM methods,the law of the influence of ground temperature on the failure mode and damage distribution of coal and rock was obtained.With the increase of ground temperature,the jet coal breaking mode changed from "water wedge" dominated to "water wedge-thermal fracture" mode,and the ground temperature increased.The thermal stress increases,and the impact force of the water jet and the non-uniform thermal stress together strengthen the damage process of coal and expand the damage range.The law of the influence of geothermal temperature on the threshold pressure of coal rock fracture is proved.The threshold pressure of coal first decreases rapidly and then tends to be stable with the increase of geothermal temperature,and there is a threshold pressure stagnation point when the geothermal temperature is 75℃.(4)The failure response mechanism of water jet impacting deep coal was revealed.Based on the change characteristics of deep coal physical properties,combined with impact dynamics,fracture mechanics and other theories,the temporal and spatial evolution equations of the stress field and displacement field of jet impact on deep coal are deduced.The failure criterion of coal and rock under the action of jet impact load is constructed,and the scope of shear and tensile fracture area of coal is clarified.It shows that in-situ stress and geo-temperature are the key factors affecting the stress and deformation response characteristics of jet impacting coal,resulting in the change of coal tensile and shear failure.The fracture mode of coal and rock under the action of in-hole static pressure combined with in-situ stress and geothermal temperature and its judging criteria are put forward,and the experimental verification have been carried out,the results indicates that the rock in the crushing pit is easy to fracture on the side of the maximum principal stress,the temperature of the coal rock increases,and the fracture angle decreases.