Study On Rheological Mechanical Properties And Permeability Evolution Of Sandstone After Hightemperature Treatments

Deep underground resources exploitation and space utilization,such as coal and metal mineral resources mining,geothermal energy mining,nuclear waste underground disposal and underground coal gasification,have gradually become research hotspots and difficulties in the field of mining and geotechnical engineering.The major challenges currently faced are the deep highstress,high-temperature conditions and the influence of discontinuous structures on the long-term stability and permeability of rock engineering.Since it is of great theoretical and practical significance to investigate the influence of these factors on the long-term mechanical behavior of rock.Therefore,it is necessary to carry out experimental research on the effects of high-stress,high-temperature and fissure on rock rheological behavior and permeability.In addition,numerical simulations and theoretical models based on experimental results provide great convenience for predicting the long-term stability of rock engineering.Therefore,laboratory experimental research,numerical modeling and theoretical analysis were performed in this dissertation to study the rheological mechanical properties of the sandstone after high-temperature treatments and those with different initial fracture dip angles and the permeability evolution during time-dependent deformation.A nonlinear elastic-visco-plastic rheological damage model considering temperature and crack inclination was established.The relationship between creep and permeability was established based on the theoretical models of porosity-and stress-permeability.Finally,the proposed theoretical model was verified based on the test results.The main research contents and outcomes are listed as follows:(1)The tensile strength,fracture toughness,uniaxial and triaxial compression strength and permeability tests of the sandstones after different thermal-treatments were tested.The influence of high-temperature on the basic deformation,strength parameters and failure modes of the sandstones were analyzed and the quantitative relationships were obtained.From the microscopic view to interpret the mechanism of high-temperature effect.Uniaxial and triaxial compression and permeability tests were also carried out on the intact sandstones and single-fractured sandstones with different initial fracture angles.The effects of confining pressure and crack inclination on the deformation and strength parameters and their quantitative relationships were obtained.(2)The triaxial rheological test apparatus was used to carry out the step-loading and cyclic loading-unloading creep-permeability tests on the thermal-treated sandstones and the pre-existing fissured sandstones with different dip angles.The total rheological test time lasted nearly 220 days.Different deformations were separated and compared at various stress levels.Instantaneous elastic strain,transient plastic strain,viscoelastic strain and viscoplastic strain were obtained.The effects of temperature,stress and crack inclination on the rheological properties of the sandstone and their quantitative relationships were obtained.The permeability evolution during time-dependent deformation was also discussed.(3)Based on the results of the conventional triaxial compression and rheological compression tests on the thermal-treated sandstones and the pre-existing fissured sandstones,the deformation,strength and failure characteristics of the specimen were simulated by the discrete element program-PFC2 D.The quantitative relationships of the parameters in the macroscopic model and the microscopic contact model were obtained.(4)Based on the different types of strains separated by cyclic loading-unloading rheological tests,a nonlinear rheological model which can describe the corresponding deformation was proposed.From the view of strain energy,a rheological damage model was estabilished to describe the accelerated creep.The effective stress principle and the strain equivalent principle were utilized to present a rheological model considering the influence of the initial crack angle.The relationships between temperature and rheological model parameters were researched and then a nonlinear rheological model was presented which can consider the temperature effect.Finally,a creeppermeability model was established and verified based on the theoretical models of porosity-and stress-permeability.There are 132 figures,34 tables and 281 references in the dissertation.