Effects Of High-temperature Cooling Conditions On Mechanical-acoustic-thermal Performance And Roughness Of Granite After Thermal Cracking

The development and utilization of Hot Dry Rock(HDR)geothermal resources in China is still in the very initial stage,and there is no large-scale exploitation of such resources so far.Due to the limitation of the specific rock structure,geological characteristics,thermal mining conditions and other factors,there are still several issues to be addressed in the HDR mining technology.For instance,the exploration of HDR thermal physical characteristics under different high-temperature cooling conditions,the prediction of thermal fracture,and the study of fracture surface roughness are among the urgent engineering issues to be solved.Therefore,the present thesis takes the HDR(the target area is granite)in the central paleouplift zone of Daqing area in the northern Songliao Basin as the research object,establishes the thermoelastic mechanical model of composite mineral rock under high-temperature cooling condition,and studies the influence mechanism of internal and external factors on the thermal fracture characteristics of the rock.Uniaxial compression test,Brazil splitting test,ultrasonic wave velocity measurement test,thermal conductivity measurement test,and splitting surface roughness measurement test were carried out on the specimens after heat treatment.Different test conditions and parameters are considered in these tests,such as heat-treated schemes of natural cooling,water cooling,combined cooling,different water temperatures,different cycle times of high-temperature cooling and combined cooling with different cooling ratios.The rock's changing laws of physics,mechanics,acoustics,thermal and splitting surface roughness characteristics under different high-temperature cooling conditions were obtained,and the relationship between different characteristics was studied.Then the parameters obtained from the test were adopted in the Comsol software,in order to numerically simulate the HDR mining process under different thermal mining conditions.The relationship between production capacity and each field was discussed.The research results provide a theoretical basis for the selection of HDR thermal mining conditions and the prediction of thermal cracking degree,and have important engineering values for the development of HDR mining technology in our country.The specific research content is as follows:(1)Because of previous researches,a new thermoelastic model is founded for the rock of single minerals and binary composite minerals under high temperature and cooling conditions.Compared with the traditional thermoelastic models,the new model is more consistent with the practical situation of the HDR thermoelastic stage and provides a theoretical basis for the study of the follow-up thermal fracture stage of the rock body.The influence mechanism of external factors and internal factors on the thermal fracture of rock was discussed,the thermal fracture phenomenon of rock was explained,and the theory of HDR thermal fracture penetration was further developed.(2)Exploring from the perspective of the thermophysical properties of the rock during cooling and standing.First of all,the variation trends of immersion cooling height,apparent temperature,and thermal shock velocity of the rock during the cooling process were investigated.Thermal shock velocity was found to be one of the main factors causing the thermal cracking of rocks,and the specific time for thermal cracking of the rock after water cooling was obtained.Additionally,abnormal apparent temperature and increasing height of wetted surface were observed under combined cooling,indicating a dense crack zone between the natural cooling zone and the water-cooling zone.Then,the rock's physical,mechanical and acoustic performances after water cooling were investigated.The results showed that timeliness would affect the accuracy of test results measurement.(3)From the perspective of the rock after cooling,the variation trends of the rock's physical,mechanical,acoustic and thermal performances under different high-temperature cooling conditions were studied.The sensitivities of these features on thermal cracking of rocks were analyzed,and the correlation of acoustic-mechanical performances and acoustic-thermal performances was get started.Meanwhile,a new acoustic method for the prediction of mechanical and thermal performances of rocks was proposed.(4)In terms of internal fractures in rock after cooling,a new three-dimensional roughness profile tester was developed to verify its accuracy.Founded on the self-made roughness profile tester,a scheme was proposed for measurement of roughness of rock splitting surface after high-temperature cooling.The influence of different high-temperature cooling conditions on the roughness of the splitting surface of the rock was analyzed,the relationship between the tensile strength and the roughness characteristics was established,and a new method for inferring the roughness of the crack from the mechanical point of view was proposed.(5)Based on the experimental data,using Excel,graphics,and the Comsol software,an Enhanced Geothermal System(EGS)thermal mining model with double permeability of random fractures and upper and lower rock formations was established.The trends of EGS productivity,reservoir temperature field,strain field and displacement field were simulated and analyzed in terms of thermal mining conditions,matrix and crack parameters,demonstrating interaction and restriction among the productivity,temperature field,displacement field and strain field.This study also points out that while considering the productivity,it is necessary to consider the impact of reservoir subsidence and shrinkage on geological disasters such as thermal mining life and microseism.This paper has 92 figures,14 tables and 258 references.