Study On Evolution Law Of Meso Fracture Of Granite Under Thermal Excitation

Increasingly tense energy and environmental issues have prompted the transformation of the energy structure.In the process of this transformation,geothermal energy stands out due to its abundant reserves,wide distribution,low unit cost and renewable characteristics,and it may become an important component of future energy.The existing methods for constructing artificial heat storage can generally be divided into three categories: hydraulic fracturing,thermal excitation,and chemical excitation.As an important part of geothermal resource mining,As an important part of geothermal resources exploitation,granite will fracture in the process of thermal shock and chemical excitation leading to the initiation,expansion and evolution of its microscopic pores and cracks.This process leads to the reduction of its macroscopic mechanical properties,but greatly improves the compactness and low permeability of granite,in turn,are conducive to the effective exploitation of geothermal resources.Therefore,it is of great significance to study the meso-fracture evolution of granite under thermal-chemical excitation.In the context of hot dry rock geothermal mining,this paper takes the granite taken from GR2#2750m in the Gonghe Basin of Qinghai as the test sample.Not only observe the acidification laws of granite with the help of a microscope,but also use a high-precision CT scanning system to perform mesoscopic analysis of granites under different thermal shock temperatures.Combined with the macro-mechanical experiments of granite,The relationship between meso fracture evolution of granite under thermal shock and chemical excitation and macro mechanical law is obtained.The main conclusions are as follows:(1)Based on the microscopic observation of the acidification law of granite,it is found that chemical excitation will dissolve the mineral composition of granite.In this experiment,both mica and feldspar were dissolved by acid.This process caused the granite to produce pores and cracks,and its mechanical properties were significantly reduced in macroscopic view.However,the compactness and low permeability of granite are greatly improved.(2)Based on the high-precision micro-CT system,the study of the meso-macro-fracture law of granite under the action of thermal shock found that the thermal shock damage of granite first occurred on the surface of the rock sample,and the thermal rupture was uneven thermal rupture.When the heating temperature is 400 ?,the surface and internal pores of the granite are obviously increased,and thermal cracking occurs,but the surface has not yet formed through cracks.Therefore,the temperature threshold for thermal shock damage of granite is400 ?.(3)When the cooling temperature during the thermal shock of granite is 60 ?,as the heating temperature increases,the pores and cracks on the surface and inside of the granite will change accordingly,the length and width of the cracks will gradually increase,and there will be more and more pores,Gather around the cracks,and as the heating temperature further increases,the generated pores will communicate with each other to form cracks,and eventually the cracks will expand to form a "failure surface".After that,as the width of the fracture increases,the "failure surface" will gradually expand.All the granites are thermally damaged.In connection with the macromechanical experiments of granite under different thermal shock temperatures,it is found that when the cooling temperature is the same,the higher the heating temperature,the more obvious the decrease in the mechanical properties of granite.(4)The meso-fracture law of granite after thermal shock is not only related to its initial heating temperature,but also related to the temperature of cooling water,and has nothing to do with the temperature difference between the heating temperature and the cooling medium.By analyzing the macro-mechanical properties of granite after thermal shock at different cooling temperatures,it is found that when the cooling temperature is the same,the higher the heating temperature,the more obvious the decrease in the mechanical properties of granite.(5)With the help of a high-precision microscope and CT system,the evolution law from meso-fracture to macro-destruction of granite under thermal shock and acidification is studied through thermal shock and chemical excitation methods,which provides guidance for the construction of artificial heat storage in hot dry rock geothermal development.According to the test results,the acidification is mainly to dissolve feldspar and other easily soluble minerals,which is helpful for the establishment of large-volume and well-developed three-dimensional water-rock heat exchange channels in the process of hot dry rock geothermal development,which improves the heat exchange efficiency of hot dry rocks and increase production capacity.