Study On Deterioration And Damage Mechanism Of Granite Modified By High-temperature And Chemical Stimulation

As a kind of green and low-carbon renewable energy with huge reserves,geothermal resources are of great strategic significance for China to achieve the carbon peak and carbon neutrality goals and adjust the energy structure in the future.In the development of deep geothermal resources,it is usually necessary to construct artificial reservoirs to extract thermal energy from low-permeability hot dry rocks(HDRs).However,the traditional Enhanced Geothermal System(EGS)faces key problems such as high development costs and difficulty in forming an effective fracture network in artificial thermal storage reconstruction,which has always restricted the high-quality development of deep geothermal resources.Based on this,combining the mining of deep minerals with the exploitation of geothermal resources can provide a new technical solution for reducing mining costs and maximizing the utilization of underground resources.On the one hand,high-temperature and chemical stimulation can improve the connectivity of dense HDR reservoirs and increase the heat exchange area,and on the other hand,it can weaken the mechanical properties of hard rock and improve the excavation efficiency.Therefore,this paper takes the co-mining of deep minerals and geothermal resources as the research background,and takes thermalchemical modified granite as the research object.The physical and mechanical properties,the time-varying characteristics of corrosion and the progressive fracture process of granite specimens were studied by means of laboratory tests,mechanism analysis and theoretical derivation.The meso-mechanism of the deterioration of its macro-mechanical properties was revealed,and a damage constitutive model considering the initial nonlinear compaction was proposed.The main research results are as follows:(1)The physical parameters and thermophysical properties of the modified granite were measured,and the evolution characteristics of mass,volume,density and longitudinal wave velocity with chemical immersion time and heat-treatment temperature were obtained.The quantitative analysis models between longitudinal wave velocity,thermal conductivity and thermal diffusivity and temperature were established respectively,and the correlation between physical parameters and thermophysical properties was discussed.It is found that the change of pore structure is more sensitive to the temperature range of 150℃～450℃,while the solid particle framework is more sensitive to 450℃～600℃.(2)Using scanning electron microscopy(SEM),X-ray diffraction(XRD),nuclear magnetic resonance(NMR)and computed tomography(CT),the evolution of pore morphology with temperature was quantitatively characterized,and the spatial distribution and fractal dimension of the pore structure were reconstructed.The damage evolution process and strength deterioration mechanism of thermal-chemical modified granite were revealed.(3)Based on the uniaxial compression test,combined with energy theory,characteristic stress calculation and acoustic emission(AE)monitoring,the strength and deformation characteristics,energy evolution and progressive fracture process of thermal-chemical modified granite during loading were explored.The"negative Poisson’s ratio" and brittle-ductile transition behavior induced by modification were discussed.It was found that 300℃ and 600℃ are the temperature thresholds for the deterioration of physical and mechanical properties and the brittle-ductile transition of modified granite,respectively.(4)Based on compression tests,the time-varying characteristics of corrosion and confining pressure effect of the mechanical properties of granite at room temperature were analyzed.The sensitivity of the strength criterion parameters to chemical modification was discussed,and the microcrack fracture mode and macro fracture morphology were discriminated in combination with AE characteristics.It was found that the deterioration degree of acid corrosion specimens is higher than that of alkali corrosion,and the lower confining pressure can effectively inhibit the strength deterioration caused by chemical modification.(5)A statistical damage constitutive model considering the combination of thermal-chemical initial damage and micro-element fracture damage during loading was derived.Then the constitutive equation was modified by introducing the compaction coefficient,so that it can better reflect the deterioration effect of the pore structure in the initial nonlinear compaction stage.The validity and applicability of the model were verified by comparison with the experimental results,and the effects of temperature and chemical modification on damage evolution were discussed.It was found that chemical damage plays a dominant role at 150℃ and below,thermal damage plays a dominant role as the temperature increases,and the damage variable evolution changes from nonlinear to linear.