Study On Damage Characteristics And Permeability Evolution Of Heat Treated Sandstone

In view of the complex geological environment of high temperature and high pressure in geothermal energy development,it is very important to explore the influence of temperature on the evolution of rock permeability.At the same time,the damage characteristics and permeability characteristics of rock under complex stress conditions are the key issues in the engineering design of geothermal resource development and nuclear waste disposal.In this paper,based on the development of dry hot rock resources,taking the sandstone collected in Chongqing as the research object,the physical and mechanical tests of sandstone after different heat treatment,the permeability tests in the process of triaxial compression and the related numerical simulation are carried out,the influence of high temperature on the physical and mechanical properties of rock is analyzed and studied,and the damage and deformation of rock in the process of loading and the evolution rules of rock permeability are revealed Law.The following achievements have been achieved:(1)The heat treatment temperature has a great influence on the physical and mechanical properties of sandstone.After heat treatment of sandstone at different temperatures,the natural physical properties and appearance of sandstone will change obviously.The physical and mechanical properties and appearance properties of sandstone,such as quality,porosity,compressive strength,elastic modulus,etc.,will change obviously: with the increase of temperature,its quality will gradually decrease and porosity will gradually increase.The ultimate strength is also affected by the temperature,resulting in great changes.At the same time,the change of elastic modulus is more regular,and generally decreases with the increase of temperature.The shear strength is also affected by temperature.The cohesion decreases with the increase of temperature,while the internal friction angle increases.(2)In the process of triaxial compression failure,the permeability of sandstone is affected by temperature,but in the process of loading,the whole sandstone can be divided into three stages with the increase of axial strain: the initial decline stage,the smooth transition stage and the rapid rise stage;the permeability in each stage is obviously different.In the initial stage of loading,the effect of compaction and closure of the original micro fracture on permeability is significant.With the initial micro fracture compaction and closure,the permeability of sandstone gradually decreases to a stable value.In the online elastic stage,the primary fracture of sandstone has been fully compacted,and the new fracture has not occurred.The pore compression of sandstone has an important impact on permeability.The initiation and expansion of micro fracture At the same time,with the increase of load,the microcracks of sandstone continue to sprout,expand and run through,and its permeability gradually increases significantly.(3)After heat treatment at different temperatures,the permeability of sandstone varies with the confining pressure and osmotic pressure under different confining pressure and osmotic pressure,which shows that the sensitivity of sandstone after heat treatment at different temperatures to confining pressure and osmotic pressure is quite different.It is found that the permeability of sandstone after heat treatment at 500 ? is higher than that after other temperature treatment The rate is obviously high.The permeability of sandstone increases with the increase of fracture volume strain in the process of loading,which further clarifies the permeability evolution law of sandstone after different heat treatment.(4)On the basis of the test,based on the Weibull statistical law and Drucker Prager criterion,the damage variable is defined,the thermal fluid solid statistical damage model is established,and the correlation between sandstone permeability and damage variable is analyzed in depth.It is found that in the stage of microcrack expansion,the damage variable of sandstone begins to increase gradually,and the permeability also increases gradually.The permeability has an exponential relationship with the damage variable.Through the analysis of the influence of osmotic pressure on the damage evolution of rock,it is found that with the increase of osmotic pressure,the damage speed in sandstone is also gradually accelerated,which indicates that high osmotic pressure can accelerate the induced damage development in rock.(5)Based on COMSOL multiphysics,the damage permeability evolution model of heat-treated sandstone is established.The related theoretical analysis is carried out,and the numerical simulation of fluid seepage in the process of loading is carried out by using the COMSOL multiphysics coupling analysis software.The damage characteristics and seepage characteristics of the heat-treated rock are analyzed.Based on the established damage model and permeability evolution model,the numerical analysis can better simulate the damage evolution and permeability evolution of rock in the process of loading,and the simulation results are consistent with the test results.