Study On Mechanical Properties And Damage Mechanism Of High Temperature Marble Under Liquid Nitrogen Cooling

In order to sustain the development of energy resources and solve the increasingly prominent environmental pollution problem,it is urgent to develop and utilize clean and renewable geothermal resources.In the process of geothermal resource exploitation,traditional hydraulic fracturing has unfavorable factors such as single fracture formation,uncertain initiation position and environmental pollution.Liquid nitrogen,as an emerging reservoir anti-permeability method,has a prominent ability to create fractures in rocks.Therefore,studying the mechanical properties and damage mechanism of high-temperature rocks under the action of liquid nitrogen cooling has important theoretical guiding significance for the exploitation of hot dry rock geothermal resources and the safety and stability of underground rock engineering.Based on the above-mentioned engineering background,this paper has carried out relevant research,and the research results obtained are as follows:(1)Under different cooling methods,uniaxial compression tests of marbles with different initial temperatures are carried out and the changes of basic mechanical parameters are analyzed.Through the acoustic emission monitoring method,the variation law of the characteristic parameters of the acoustic emission of the sample under different cooling methods is studied.In addition,based on the energy evolution theory,the law of energy evolution during the failure process of the specimen under different cooling methods is analyzed.Experiments show that the degradation effect of liquid nitrogen cooling treatment on the mechanical properties of rocks is far better than natural cooling.High temperature will aggravate this degradation effect.(2)Based on the wave speed test results,the law of marble wave speed change under different cooling methods is analyzed.At the same time,experiments on mercury intrusion in marble under different cooling methods are carried out to explore the microscopic pore structure changes.By analyzing the cumulative mercury ingress curve,pore size distribution,pore size ratio,porosity and pore fractal dimension,the evolution characteristics of marble pore structure under different cooling methods are obtained.The results show that: compared with natural cooling,after liquid nitrogen cooling treatment of high temperature marble samples,the internal porosity of the rock mass is significantly increased.The complexity of the pore structure is improved.(3)With the help of compact scanning electron microscope,the change of marble microstructure is observed.The apparent characteristics and failure mode of marble before and after cooling treatment are analyzed.The thermal damage mechanism of marble is revealed.The fracture surface morphology of marble is scanned by 3D profilometer.The changes of fracture morphology and surface roughness parameters of marble before and after cooling treatment are studied.The results show that: compared with natural cooling,the macro micro structure failure characteristics of marble after liquid nitrogen cooling are more complex,which can meet the assumption of establishing fluid flow channel in high temperature rock.(4)The two-dimensional axisymmetric thermal solid coupling model is established by COMSOL 5.3a finite element simulation software.The sensitivity analysis of the basic mechanical parameters of marble samples is carried out.The influence of different initial temperatures on the distribution of temperature field and thermal stress field of marble is studied.The heat transfer range and damage area of marble treated by liquid nitrogen cooling are evaluated.The simulation results show that the liquid nitrogen cooling can cause considerable damage area in the high temperature marble with the cooling time.This paper has 47 pictures,13 tables and 93 references.