Research On Laws Of Fracturing And Increasing Permeability By Supercritical CO₂ Burst In Low Permeability Coalbed

Clean energy of gas is the strategic choice of China' energy development,and coalbed methane is the most realistic and abundant clean gas source in China.Low permeability of coal is the main problem which restrict the rapid growth of coalbed methane production.In order to solve this problem,supercritical CO2 is put forward to be served as physical blasting source in the gas explosion to enhance the permeability.Combined experiments,theory analysis and numerical simulation,the law of cracking and antireflection for the low permeability of coal seam was carried out by supercritical CO2.The main research works and results are as follow:(1)The Multi-pulse gas explosion device of supercritical CO2 was developed.In order to obtain the law of gas explosion cracking with the inherent physical and mechanical properties of coal and rock,experiments of explosion and fracturing were performed on specimens of coal and concrete with electro-hydraulic servo physical simulation test-bed under no loading.The deformation and failure were monitored during gas explosion by dynamic strain acquisition.The specimens' failure characteristics were divided into crushing zone,fracture zone and seismic zone,and the mechanism was revealed.The medium near the explosion hole was impacted beyond their compressive strength,forming crushing zone.Circumferential tensile stress generated by attenuated stress wave caused tension damage of brittle medium and the radial crack was formed.Cracks were extended and the fracture zone was formed under the quasi-static loading of CO2 gas wedge.The medium outside the fracture zone mostly vibrated under the low-energy stress wave,forming vibration zone.(2)The supercritical CO2 explosion and fracturing were performed under different initial stress,using the developed tri-axial anti-explosion loading device.Based on the strength theory,impacts of initial stress on fractures were revealed.Initial stress in specimens produced static stress field,and explosion load added made the explosive body load in different positions different from that without the initial stress field,which influenced the damage of medium.Smash district decreased with the increase of initial stress.When the initial stress is unequal,it's like a spindle and the long axial direction was the same as that of highest initial stress.Due to initial stress hindering the expansion of perpendicular fracture,the fracture extended in the direction of maximum initial compressive stress.(3)The relationship between the expansion velocity of the crack and distance showed Logistic function,and the velocity was corresponding to the crack mechanism and failure region(rapid extension-compression failure-smash district,slow crack propagation-tension failure-crack area).The larger the initial compressive stress was,the more quickly velocity of crack propagation decreased perpendicular to the initial pressure stress,then the cracks' number and expansion length decreased under the same blasting conditions.(4)The impact dynamic model of supercritical CO2 explosion was established and the Johnson-Cook model was modified with the material hardening properties.Based on the coupling calculation of SPH and FEM,the simulation of supercritical CO2 explosion in coal was performed by ABAQUS.The simulation and experimental results were consistent.Based on the image recognition program to simulate fracture geometrical characters during gas explosion,The finite element model of methane production in low permeability coal with explosion cracks was established.The simulation of enhancing permeability was conducted.The results show that impact of supercritical CO2 explosion was remarkable and that became worse with the increase of initial stress under the same gas blasting conditions.(5)Compared with single-pulse explosion technology,the explosion fracturing range with supercritical CO2 can be effectively improved by multi-pulse explosion technology.In the explosion field under the initial stress,the initial compressive stress hinders crack propagation in the direction perpendicular to its own direction,thereby making the blasting-induced crack extending along the direction of the maximum initial compressive stress.Blasting efficiency can improved by enlarging distance between gas explosion holes in the direction of the maximum principal stress,reducing the distance between holes in the direction perpendicular to the minimum principal pressure stress,or reducing the number of drilling holes based on ensuring crack range and antireflection combined with supercritical CO2 multipulse gas explosion technology.There are 144 figures,15 tables and 111 references in the thesis.