Research On The Characteristics And Laws Of Thermal Shock Fracture Of Rock Based On Micro-CT Technology

The thermal shock fracture phenomenon of rock exists in various projects such as the development of geothermal resources,thermal rock breaking,and deep storage of nuclear waste.When the temperature field is disturbed and changes drastically,it will cause thermal shock to the rock,which will cause thermal shock to the rock and change the physical and mechanical properties of the rock.This phenomenon may cause certain safety hazards to related projects,but rational use of this phenomenon can also improve engineering efficiency.Therefore,it is of great significance to study the rock damage under thermal shock for this type of engineering.In this paper,the common sandstone and granite in geothermal reservoirs are selected as the research objects,and the thermal shock fracture characteristics of sandstone and granite after thermal shock are observed and studied by micro-CT.Focusing on the study of the evolution law of pore and fracture morphology and structure,the change law of pore volume fraction,fractal dimension,permeability,etc.,to explore the mesoscopic influence mechanism of thermal shock on rock damage,and to analyze the influence mechanism of thermal shock on rock from the perspective of Explain in detail.The main research contents and conclusions are as follows:(1)Analyze the physical properties of sandstone after thermal shock.With the increase of heating temperature,the color of sandstone changed from blue-gray to reddish-brown after thermal shock,and the mass loss rate and volume expansion rate both increased.The residual elongation upon cooling in 60°C water is slightly greater than that in 20°C and 100°C water cooling.(2)Severe thermal shock fracture occurs in the rock after thermal shock.With the increase of heating temperature,the damage degree of the rock increases,and the pore volume fraction,the maximum connected group fraction,the fractal dimension and the permeability all show an increasing trend.With the increase of thermal shock degree,the difference in the complexity of pore structure at different positions inside the sandstone increases,that is,the heterogeneity increases;The proportion first increased and then decreased,reaching a peak at 600 °C.(3)The fracture forms and damage degrees of high-temperature rocks are different after cooling in water at different temperatures.This kind of sandstone and granite have the strongest damage degree after thermal shock cooling in 100℃ water,followed by 20℃,and the least damage degree at 60℃.In addition to temperature difference,the boiling effect of water will also affect the thermal shock fracture degree of the rock.The temperature of the cooling medium affects the development of the fracture morphology of granite after thermal shock.The hightemperature granite has straight fractures and smooth surface after thermal shock cooling in 20℃water;After cooling in ℃ water,a large number of fissures are overlapped and closed to form a "honeycomb"-like fissure network structure.(4)The thermal shock fracture of sandstone is manifested by the increase and connection of pores,and the thermal shock fracture of granite is manifested by the expansion and overlap of cracks.The thermal shock fracture of sandstone produces a large number of pores and throats,which are interconnected to form large connected clusters.The thermal shock fracture of granite shows obvious brittle fracture,and the fracture is mainly manifested in the increase of the number of cracks and the increase of the size of the crack,and the fracture is more serious where various mineral components gather.(5)Comparing the relationship between rock fractal dimension and void volume fraction after thermal shock,it is found that the change of pore volume fraction and fractal dimension of sandstone after thermal shock with thermal shock conditions is similar,and the linear fitting degree is high.There is no obvious relationship between the void volume fraction and fractal dimension of granite after thermal shock.(7)With the increase of heating temperature,the main expansion directions of the maximum cracks of high-temperature granite at different cooling temperatures are different:when cooled in water at 20 °C and 100 °C,it is mainly manifested as the expansion of length and the increase in thickness,and in water at 60 °C When cooling,it is mainly manifested as an increase in width.After cooling in water at 60°C,the length,width and thickness of the largest crack were significantly smaller than those at 20°C and 100°C.