Study On Evolution Law Of Stress And Damage Of Pulsating Fracturing In Coal Seam

The core of coal seam pulsating fracturing technology is to apply cyclically varying pulsating pressure.The disturbance stress is generated inside the coal seam by the reflection,superposition and energy accumulation effects of pulsating stress wave.Under the repeated loading-unloading of the disturbance stress,fatigue damage of coal rock is caused.On the one hand,the strength of coal rock is reduced.On the other hand,when the disturbance stress reaches the corresponding strength limit of the coal rock,the coal rock is destroyed to for m a crack.However,most of the researches on coal seam pulsating fracturing at home and abroad focus on determining the critical pressure of crack extension under pulsating conditions,and analyzing the effects of different pulsating stress wave parameters and the mechanical properties of coal rock on crack initiation and extension mechanical conditions.In this paper,the mechanical parameters of coal are tested,and the key mechanical parameters such as tensile strength,shear strength and tri-axial compressive strength are obtained,which provide basic data for theoretical research and numerical simulation.According to elastic superposition theory,the in-situ stress field and the pulsating stress field are superimposed,and the numerical model of the pulsating fracturing disturbance stress field is established.The effects of pulsating parameters,geostress conditions and coal parameters on the cracking disturbance stress field of the wellbore are discussed by numerical model,and the influence of pulsation parameters,the length of crack and the angle between crack and horizontal maximum principal stress on the crack propagation stress field is analyzed.Based on the energy evolution characteristics of the hysteresis loop of the stress-strain curve under pulsating fracturing,a new calculation model describing the rock damage evolution is established,and the model is verified by experimental data.The study shows that for a specific coal fracturing,when the pulsating stress level is constant,increasing the pulsating frequency is beneficial to accelerate the damage and reduce the strength of coal.When the pulsating frequency is constant,the cumulative damage variable increases with the increase of pulsating stress level.When the pulsating frequency is constant,the pulsating stress level has a significant effect on the cumulative damage,and when the ratio of the dissipative energy to the total energy increases as the stress level increases,the cumulative damage variable increases with the increase of the pulsating stress level.In this paper,the numerical calculatio n model of pulasting fracturing stress field of coal seam is established.The variation of the maximum circumferential stress and the maximum tensile stress at the crack tip under pulsating load is calculated and analyzed.The damage evolution model based on the energy evolution characteristics of stress-strain curve hysteresis loop under pulsating cyclic loading is established.The damage evolution law of coal under pulsating fracturing is revealed,which provides a theoretical basis for the design of coal seam pulsating fracturing scheme.