| As it is clean,green and renewable,geothermal energy has been the focus of increasing attention by researchers all over the world.In the process of geothermal energy exploitation,both the drilling of the wellbore and the heat exchange of water in the artificial reservoir during the mining process involves the alternating cooling and heating of a high-temperature rock mass.Under the alternating action of cold and heat,damage and degradation of rock mass inevitably occurs,which leads to the change of its physical and mechanical properties.On the one hand,this adversely affects the wellbore stability in the drilling process of a high-temperature rock mass;on the other hand,it promotes the further expansion of the fracture channel network and the generation of new fractures in the thermal reservoir.Aiming at the actual process of heat exchange of an artificial reservoir involved in geothermal mining,this paper studied the changes in the physical and mechanical properties of granite under different temperatures during cyclic temperature changes.It is of great significance to further understand the physical and mechanical properties of high-temperature rocks and to reveal their changing rules and fracture mechanism in these conditions.Meanwhile,its study can provide the theoretical basis for wellbore stability and artificial thermal reservoir construction in high-temperature geothermal development.In this paper,a self-developed multifunctional servo-controlled three-axis testing machine is used to test high-temperature rocks at different temperature levels after heating and also granite real-time temperature recording during uniaxial compression test.The study then compares the experimental results,especially analyzing the temperature effects of the thermal stress of granite on the uniaxial compressive strength.Finally,seepage tests were carried out on the granite after alternating cooling and heating at different temperatures,and the permeability evolution law after the process was analyzed;the internal damage and degradation mechanism of the granite was also analyzed in terms of micro view by means of scanning electron microscopy,microsections and other means.The main conclusions are as follows:(1)Testing the physical and mechanical properties of granites at different temperatures under the action of water cooling and natural cooling respectively,it was seen that the physical and mechanical properties of granites change to some extent.Water cooling and natural cooling alter the apparent color of granite,compressional wave velocity,mass loss rate and compression strength,elastic modulus and failure characteristics.For example,water cooling of the granite sample from room temperature(20?)film gradually changes to the 600?khaki,and natural cooling condition of present the colored sample color change is not obvious,and the rise of temperature becomes shallow gradually.For example,a water-cooled granite sample heated from room temperature(20?)to 600?gradually changes color to become khaki;this color change is not obvious in a naturally-cooled sample,and the temperature rise rate is also more shallow.Using both cooling methods,the average wave velocity and mass loss rate of the granite decreased with the increase of temperature,and the observed decline rate was faster in the range of 400??600?.The longitudinal wave velocity of the samples under natural cooling was higher than that under water cooling,indicating that water cooling induced greater damage to the interior of the specimens.Other granite samples subjected to two different cooling mechanisms show different stress-strain curve characteristics:The natural-cooling specimens show brittle failure characteristics,with hardening phenomena appearing between 200?and300?,and the stress-strain curve shows lower variability before the peak.The water-cooled specimens show a brittle to ductile transition,and a bimodal gradual damage phenomenon for temperature greater than 300?;Under both cooling conditions,the uniaxial compressive strength and elastic modulus of the specimens decreased with the increase of temperature.At the same temperature,the decrease of uniaxial compressive strength and elastic modulus of the specimens under water cooling were larger than that under natural cooling.(2)On the basis of research on alternating heating and cooling granite samples conducted using uniaxial compression test and Brazilian split test,analysis of the changes in the law of compressive strength and tensile strength of the samples,found that the number of alternations of both ways of cooling,in different temperature regions has severe influence on the mechanical properties of granite.After alternating cooling treatment at different temperatures,the internal structure of the sample changes significantly,from no crack present at low temperature to small or large cracks appearing at high temperatures.The damage of the sample becomes more serious with the increase of the number of alternations:particles were easy to fall off after a big number of alternations.In the same conditions of alternating frequency,the tensile strength,compressive strength and elastic modulus decrease gradually with the increase of temperature,while,at the same temperature the compressive strength and tensile strength of the specimens all show a trend of decline with the increase of number of alternations;the decline is greater in natural cooling than in water cooling,the range of change is more affected by the change of temperature gradient(100?)than the amplitude as a function of number of cycles(5 times).Brazilian splitting100??400?most of the specimen along the diameter of splitting and failure surface can overlap,however,increasing the number of alternations,more samples show certain changes in the direction of the split,broken lines gradually become bent and cracks split.Finally,when the temperature reaches 700?,particles broken off the slag lead to sample destruction.The uniaxial compressive failure mode presents shear failure,tensile failure and mixed failure mode,with the shear failure mode accounting for about 40%of the specimens,while tensile failure accounted for 35%.In the water-cooled samples,the shear failure mode accounted for 43.3%of the samples,the largest proportion.In the natural cooling group,shear and conical failure samples accounted for the most samples,at 36.7%,forming fracture zones.This indicates that the number of heating and cooling alternations has a certain influence on the failure mode.(3)Using a self-developed multifunctional servo control,high temperature,rock triaxial testing machine at different temperatures after heating of granite to measure the real-time temperature during a uniaxial compression test,the specimen is burned at different temperature regions under the condition of red,and it is compared with the experimental results under the condition of analysis.It is seen that the thermal stress generated under the action of temperature has a great impact on the physical and mechanical properties of granite.Firstly,the compressive strength and elastic modulus of the sample under the action of temperature are obviously lower than that at normal temperature.Under heating,the strength of the sample is mainly concentrated in the range of 20?60MPa,while at normal temperature the strength is mainly concentrated in the range of70?180MPa.Also under heating,the elastic modulus ranges between 15?99GPa while at normal temperature,it is between 38?234GPa.With the increase of the number of temperature alternations,the strength and elastic modulus both showed a trend of decrease at room temperature and high temperature.Moreover,the decline range of water-cooled specimens was greater than that of natural-cooled ones,indicating that the internal damage of water-cooled specimens was relatively bigger.At the same time,at normal temperature,water cooling and natural cooling both show a compressive strength that is greater at 200?than at 100?,while at higher temperatures,this only appears in the water-cooled samples.On the one hand,this shows that the hardening phenomenon at 200?is not accidental,and this effect increases the strength of the specimen.Moreover,this shows that water cooling promotes hardening,but the hardening effect decreases under the effect of temperature.(4)By means of microsections and scanning electron microscopy,the micro morphology characteristics of the granite after alternating cooling and heating at different temperatures were observed.It can be seen that the sample underwent not only chemical changes but also physical changes after alternating cooling and heating.Due to the difference in particle size and thermal expansion coefficient of the various mineral particles inside the granite,thermal expansion of particle boundaries is inconsistent under heating,and the tensile and compressive stress between mineral particles results in microcracks inside the sample,with a gradual increase of the number and width of the cracks,as well as an extension of the crack's length,It can also be seen,by observing the pore and fracture network model of granite after the alternating action of cold and hot at 600?,that the connected fractures and seepage paths increase significantly with the increase of heating and cooling cycles.A good seepage channel is formed,which directly leads to a significant increase in permeability.The permeability under normal temperature is 0.001×10-18m2,while after alternating heat and cold for 25 times,it increases to 1.934×10-18m2,an increase of nearly 2,000 times. |
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