Rheology Mechanics Behaviour And Damage Mechanisms Of Sandstone Containing Two Fissures Under High Temperature In Deep Surroundings

The development of Underground Coal Gasification(UCG)is expected to exploit the huge amount of deep coal resources that cannot be mined and utilized by conventional mining,and at the same time,utilizing the UCG cavities for gas storage can maximize the use of resources and space.However,how to effectively control the long-term stability and low permeability of the overburden rock in the UCG chamber under high ground stress and thermally heat-treated during coal gasification is one of the key issues to ensure the safe operation of underground gas storage.Therefore,the research on the time dependent effect and the permeability evolution of high-temperature thermally damaged fractured rocks under high surrounding pressure,has great theoretical value as well as engineering guidance significance in the application of combining UCG and underground gas storage.We completed laboratory triaxial compression tests and incremental multi-step loading and unloading creep tests with permeability measurements at confining stresses of 30 MPa on pre-cracked sandstone specimens thermally heat-treated,and the30 MPa confining pressure is used to simulate the deep surrounding rock environment.A nonlinear rheological model considering the effects of pre-cracked fissure inclination and thermal damage degree was developed based on the creep tests.The thesis also analyzes the cracks initiation and expansion of fractred red sandstone by Two Dimension Particle Flow Code(PFC^2D)numerical simulation method.The main research contents and findings are as follows:(1)Based on instantaneous triaxial compression tests results of pre-cracked dual fissured red sandstone with different crack inclination angles,rock bridge inclination angles and thermal damage degrees,the multi-peak phenomenon exhibited by the dual fissured sandstone under high confining pressure has been found,and the influence of crack angles,rock bridge angles and thermal damage degrees on pre-and post-peak deformation characteristics,strength parameters and damage modes were revealed.Meanwhile,a?value characterizing the degree of thermal damage is proposed based on the image recognition technique,and the larger the value is,the higher the degree of thermal damage is.(2)According to the instantaneous triaxial compression strength,different creep loading stress levels were set,and incremental cyclic and loading and unloading triaxial compression creep-permeability tests were conducted on pre-cracked dual fissure red sandstone with different crack angles,rock bridge angles and thermal damage degrees,and the creep strain was separated,and the effects of stress levels,fissure angles,rock bridge angles and thermal damage degrees on the individual strains,permeability and damage mode were revealed.(3)A plastic body with variable stress yield limit has been introduced in this research,and in parallel connection with a variable cross-section viscous body to compose the nonlinear viscoplastic body,and the nonlinear elastic-viscoplastic rheological model respectively considering the effects of initial damage and thermal damage is established,which can better describe the rheological characteristics of fractured red sandstone under high confining pressure.(4)The PFC^2D was used to reveal the micro-crack initiation,expansion and penetration patterns of dual fissured sandstone in conventional triaxial compression from a microscopic perspective,and at the same time,a numerical model of thermal damage was established based on the degree of thermal damage to simulate the effect law of thermal damage on rock strength and deformation.The Burgers model was used to simulate the three stages of creep phase of the dual fissured specimen,which revealed the evolution of micro-crack,stress field and displacement field in the rheological process.There are 115 figures,20 tables and 192 references in the dissertation.