Dynamic And Static Properties Of Sandstone Under High Temperature-Water Cooling Cycle

With the consumption of non-renewable energy such as coal and oil and the pollution caused by combustion emissions,the development and utilization of renewable new energy such as dry-hot rock geothermal energy has become the trend of future energy development.In the process of geothermal energy production,under the action of water circulation,the physical and mechanical properties of high-temperature rocks will inevitably change,resulting in the reduction of rock strength,which may eventually lead to secondary disasters such as wellbore collapse and earthquakes,thus affecting the exploitation and utilization of geothermal energy resources.Therefore,this paper takes the physical and mechanical changes of high-temperature sandstone after being cooled by water in the development of hot dry rock resources as the research background.and dynamic load compression test.The mass loss rate,volume expansion rate,longitudinal wave velocity,compressive strength,elastic modulus and other physical and mechanical parameters of sandstone under different temperatures and different high temperature-water cooling cycles were analyzed.The following conclusions are drawn from the characteristics of acoustic emission,energy dissipation rate variation and damage morphology:(1)Changes in temperature and high temperature-water cooling cycles have a great influence on the appearance of sandstone samples.The samples are gray-white after being heated at 200 °C,gray-black at 300 °C,and red at 400 °C～600 °C.brown.At500 °C,cracks occurred in the sample after 5 high temperature-water cooling cycles,and after 1 high temperature-water cooling cycle at 600 °C,cracks appeared,and the cracks became wider and deeper with the increase of the number of cycles.(2)The physical properties of sandstone samples are weakened by the increase of temperature and the increase of high temperature-water cooling cycles,which is reflected in the increase of the mass loss rate and volume expansion rate of the sample,and the decrease of the longitudinal wave velocity.(3)Under the action of static load,the sandstone samples after the same number of high temperature-water cooling cycles,the elastic modulus and peak strength of the samples fluctuated with the increase of temperature: 200～300℃ increased,300～400℃decreased slightly,400～500℃ increased slightly,and 500～600℃ decreased greatly,indicating that there is a temperature threshold at 500～600℃ that affects the mechanical properties of sandstone,and 300℃ and 500℃ can strengthen the strength of the sample.Under the same temperature conditions,the strength of the sandstone sample increases with the increase of the number of cycles when the temperature is lower than 300℃,and the strength of the sample decreases with the increase of the number of cycles when the temperature is higher than 300℃.(4)Under the action of dynamic load,with the increase of temperature and the number of high temperature-water cooling cycles,the peak dynamic stress of the sample decreases and the dynamic strain increases gradually.Based on the energy analysis,it is found that the energy dissipation rate of the sample increases gradually with the increase of temperature and the number of high temperature-water cooling cycles.Based on the high-speed camera technology,the whole process of loading sandstone samples was analyzed,and it was found that the failure of the samples under the action of dynamic load was in the form of shear-tension composite failure.The failure mode of the sample after cooling cycle is mainly tensile failure,and the failure mode of the sample after 5 or 7 high temperature-water cooling cycles at 500～600℃ is mainly compression shear failure.