Study On The Physico-mechanical Properties And Temperature Field Evolution Of Granite Under Different Heating-cooling Treatments

Thermal cracking of rocks caused by temperature gradients is widespread in various processes such as wellbore construction,reservoir reconstruction,and long-term geothermal extraction.During the drilling process of hot dry rock,due to the use of circulating mud,the surrounding rock of the wellbore is rapidly cooled and its mechanical properties are deteriorated,causing the collapse and instability of the well wall;during the construction of the artificial heat storage,the high temperature granite body undergoes thermal shock caused by the rapid cooling of the low temperature medium.Fracture,the physical and mechanical properties of the rock mass change;in the long-term geothermal extraction process,the surface of the reservoir rock mass crack(circulation channel)will be cooled by low-temperature water and new micro-cracks will be generated,which will increase the seepage and heat transfer of the reservoir.Ability to improve heat collection capacity.The essential mechanism of rock cracking under cooling shock is the thermal stress caused by temperature gradient.Therefore,studying the mechanical properties and heat transfer of granite under rapid cooling is of great significance to the development of geothermal resources.In order to study the effect of thermal cracking induced by temperature gradient on the physical and mechanical properties of granite,the physical and mechanical properties of hightemperature granite at 100℃,200℃,and 300℃ after being rapidly cooled in water at 0℃,20℃,and 60℃ were tested.The deterioration of mechanical properties of high-temperature granite under the action of rapid cooling.Secondly,through experimental research on the evolution of the internal temperature field,cooling rate,and temperature gradient of the sample during the rapid cooling process,the fundamental reason for the deterioration of the mechanical properties of the granite under the rapid cooling is explained from the perspective of heat transfer.Based on the experimental results,the software COMSOL Multiphysics 5.4 is used to numerically simulate the granite under different heating-cooling treatments to study the mechanism of the thermal cracking of high-temperature granite induced by the temperature gradient under the action of rapid cooling.Get the following conclusions:(1)Through physical property test,uniaxial compression test,Brazilian split test and permeability test test on granite under different heating-cooling treatments,the physical mechanics and permeability test after different heating-cooling treatments were obtained.The results show that at the same cooling medium temperature,with the increase of heating temperature and the decrease of cooling medium temperature,the longitudinal wave velocity,uniaxial compressive strength,elastic modulus and tensile strength continue to decrease,and the permeability increases.The mechanical strength of high-temperature granite after being rapidly cooled in 0℃,20℃,and 60℃ water is from small to large: 0℃ water abrupt cooling strength,20℃ water abrupt cooling strength,and 60℃ water abrupt cooling strength.(2)There are differences in the full stress-strain curves of granite specimens under different heating-cooling treatments.As the cooling water temperature decreases,the nonlinear characteristics of the axial stress-axial strain curve at the initial deformation stage will gradually increase.Compared with the full stress-strain curve of the original rock at room temperature,the sample after heat treatment usually exhibits more prominent nonlinear behavior and volume compression.This is mainly because the thermal shock caused by the rapid cooling further promotes the development of micro-cracks in the rock.(3)The temperature field distribution test of granite under different heating-cooling effects shows that the heat transfer process of high-temperature granite under the action of rapid cooling belongs to unsteady heat conduction.The internal temperature,cooling rate,and temperature gradient of the specimen vary with time.The change curve presents three obvious stages: the first 200 s is the rapid cooling stage,the 200s～400s is the slow cooling stage,and after 400 s is the temperature constant stage.Among them,the biggest influence on the physical and mechanical properties of granite is the first 25 s of the rapid cooling stage.(4)The evolution of cooling rate,temperature gradient,and thermal stress formed in the high-temperature granite specimens under the action of rapid cooling is consistent,and they all show a trend of a sharp increase first and then a slow decrease.The maximum value always appears on the outside near the solid-liquid exchange interface.(5)The COMSOL Multiphysics 5.4 numerical simulation software was used to analyze the heat transfer in the granite under the action of rapid cooling.The results showed that the physical and mechanical properties of the granite sample were most affected in the first 25 s during the rapid cooling process.The changing trend of cooling rate and temperature gradient is consistent,and the maximum value always appears near the solid-liquid heat exchange interface.At the same time,as the heating temperature increases and the cooling medium temperature decreases,the temperature gradient increases,and the thermal stress increases accordingly,and the thermal cracks induced by the temperature gradient increase,which leads to the deterioration of mechanical properties.Therefore,the mechanical properties are deteriorated under the action of rapid cooling in 0℃ water.The characteristic deterioration is the most serious.