Simulation Investigation On Fracturing Law Of Coal With Weak Cleat Planes Caused By Supercritical CO₂ Explosion

Coalbed methane（coal mine gas）is not only a kind of highly efficient and clean unconventional natural gas,but also the root of coal mine gas accidents.China is rich in coalbed methane resources,but the low permeability of coal seam generally leads to the extraction rate less than 40%.The supercritical CO₂gas explosion technology is an effective technology to realize the safe and efficient exploitation of low permeability coalbed methane in China.Natural coal seam contains a large number of weak cleat planes,which makes the coal have strong heterogeneity and anisotropy characteristics,and has a significant influence on the fracturing of the coal caused by supercritical CO₂explosion.In this paper,based on the team’s preliminary experiments,the fracturing law of coal with weak cleat planes caused by supercritical CO₂explosion is systematically studied by combining theory with numerical simulation.The main research work and achievements are as follows:（1）The traditional Johnson-Cook dynamic damage constitutive model,which has similar tensile and compressive properties,is modified to reflect the different tensile and compressive properties of brittle materials,the parameters of the state equation of explosion gas are calculated according to the gas explosion pressure measured in the experiment,and the impact dynamic model of supercritical CO₂gas explosion coal is established.（2）Based on the digital image method,a MATLAB code was developed to identify the actual coal image and obtain the geometric information of its cleats.The geometric information was related to the physical parameters of coal with weak cleat planes and imported into ABAQUS to realize the finite element representation of coal with weak cleat planes.（3）SPH and FEM were used to simulate the fracturing process of coal with weak cleat planes caused by supercritical CO₂explosion.Combined with the theory of stress wave transmission and reflection at the interface of different medium.The influence of the dip angle,the aperture and the distance from the weak plane to the center of the blast hole of the single weak plane on the gas explosion were obtained.The greater the distance from the weak plane to the center of the blast hole and the greater the dip angle or the aperture of the weak plane,the greater the hindrance of the weak plane to cracks propagation,and the smaller the stress wave transmittance.（4）The fracturing process of coal with non-penetrating fractures by supercritical CO₂explosion was simulated.The influence of the length of the crack,the distance from the crack to the center of the blast hole,the angle of the stress wave incident on the crack and the filling medium in the crack on the fracturing effect of gas explosion coal were obtained.A code of randomly distributed fractures was written by Python,the code was imported into ABAQUS,and a geometric representation model of coal containing randomly distributed multiple fractures was established.（5）The method of establishing a coal with weak cleat planes model based on digital image identification processing was applied to the simulation of supercritical CO₂porous gas explosion coal body.First,the gas explosion process of a double-hole coal with weak plane was simulated,the law of a single weak plane between two gas blast holes on the gas explosion effect under different dip angles was obtained.Secondly,the gas explosion process of coal with five gas explosion holes was simulated according to four hole layout schemes.The coal fracturing effect by gas explosion was compared and analyzed,and the reasonable hole arrangement scheme was obtained.There are 63 figures,17 tables and 100 references in the thesis.