Rock Damage Test And Mechanism Research Under Low Temperature Cycle Freezing

China is one of the major oil and gas storage and exploitation countries.In order to increase the amount of oil and gas resources and reduce the fracture pressure of reservoir rocks,scholars at home and abroad have some research on various fracturing techniques.But liquid nitrogen as fracturing fluid or liquid nitrogen as foam for fracturing,domestic and foreign research is relatively few.the liquid nitrogen is injected into the rock reservoir as fracturing fluid,and the rock mass faces a cycle of freezing and melting.During this cycle,the phase transition of water and the anisotropy between diagenetic particles will cause damage to the rock micro structure,and then reduce the bearing capacity of the rock mass until the rock is destroyed,thus reducing the fracture strength of the rock.Therefore,it is of practical significance to study the damage evolution law of rock under freeze-thaw condition to reduce the fracture pressure of reservoir and improve the exploitation of oil and gas resources.In this paper,the mechanical properties of rock freeze-thaw test and damage-damage mechanism are studied closely.The mechanical properties of rock freeze-thaw damage are discussed through freeze-thaw cycle test,uniaxial compression test and tensile test.(1)The freeze-thaw damage deterioration and freeze-thaw failure behavior of three kinds of rocks were analyzed.Three kinds of freeze-thaw damage deterioration modes were found: spalling mode,crack mode and fracture mode.On this basis,the variation law of rock's wave velocity,porosity and moisture content after different freeze-thaw times is discussed.(2)XRD and SEM scanning tests were conducted to obtain mineral composition,surface and internal microstructure images of rock samples before and after freezing and thawing.the test parameters and images were analyzed,and the characteristics of rock characters were obtained,which laid the foundation for the mesoscopic damage detection and mesoscopic parameter analysis of rock.(3)The coal and rock were selected as the test object,the liquid nitrogen freeze-thaw cycle test was carried out,and the rock was XRD?SEM and CT scannedbefore and after freeze-thaw.by analyzing the rock physical and mechanical parameters and CT parameters,the internal structure model of the samples before and after freeze-thaw was analyzed.the change of freeze-thaw meso-characteristics and crack expansion characteristics under liquid nitrogen freeze-thaw conditions were discussed,and the damage mechanism of rock samples under freezing action was revealed.(4)Uniaxial compression and tensile tests were carried out on the limestone after different cycles,and the deformation and strength rock after freeze-thaw cycles were obtained.Based on the theory of damage mechanics,the damage variables are wave velocity,elastic modulus,density and wave velocity.By fitting the test results,the expressions of wave velocity,elastic modulus,density and wave velocity coupled with freeze-thaw times were obtained.The fitting values of density and wave velocity can reflect the uniaxial compressive strength of limestone under freeze-thaw cycle.