Study on law of longitudinal wave velocity evolution of dry hot rock filled after thermal shock

Due to the influence of tectonic movement,the dry hot rock mass granite produces large-scale joint fissures,forming a fractured dry hot rock mass,these fissures are later filled with hydrothermal fluid to form a late hydrothermal filling fractured dry hot rock mass. The cementation surface between the filling body and the parent rock is a weak surface,which is very easy to build geothermal artificial reservoir of dry hot rock and greatly saves the construction cost. In order to study the evolution law of parent rock,filling body and cementation surface after injecting low temperature hydrothermal shock in the geothermal development of actual dry hot rock,four kinds of standard samples (granite parent rock,filling body,filling granite with longitudinal cementation surface and filling granite with transverse cementation surface) were studied by ultrasonic testing technology under the condition of repeated thermal shock from room temperature to 600 ℃ and different shock atmosphere (water cooling shock at 0 ℃ and cooling shock at room temperature).The results show that:at room temperature,the porosity of granite with cemented surface is lower than that of parent rock and filling body,while the wave velocity is higher than that of parent rock and filling body.During the repeated thermal shock of room temperature-300～0 ℃ water,the wave velocity of the four types of samples can be reduced by 40%-50% after the first thermal shock,and then the wave velocity decreases by about 3% with the increase of the number of thermal shocks. At the same high temperature,the wave velocity drop of four kinds of samples after water cooling shock at 0 ℃ is larger than that at room temperature,which indicates that water cooling shock at 0 ℃ has stronger damage to rock.Under the same thermal shock mode,the wave velocity drop of the filling granite with cemented surface is larger than that of the parent rock and the filling body,because the size difference of the mineral particles near the cemented surface is the largest,and the degree of thermal cracking is the strongest. The research results are of great significance to the understanding of the formation and structural evolution of the reservoirs in the fault model dry hot rock geothermal field.