Shear Characteristics Of Granite Under Thermal-mechanical Coupling

High-temperature geothermal energy is a new energy that can influence the future,and occupies an extremely important position in the future energy development.Enhanced Geothermal System(EGS)can make good use of geothermal energy to generate electricity.In EGS system,low temperature fluid is injected to extract heat energy from high temperature rock mass.In this process,under the coupling action of heat-fluid-solid,the high temperature fractured rock mass produced seepage shear effect and induced microseismic phenomenon in EGS system.In this paper,indoor experimental research,theoretical analysis and numerical calculation methods were adopted.Firstly,the shear test of granite structural plane after high temperature was carried out.Then,the governing equation of seepage coupling temperature field in high temperature fractured rock mass was established,and the temperature field distribution of seepage coupling in fractured rock mass during water injection and heat raising was studied.Then,the governing equation of seepage shear effect of high-temperature fractured rock mass was established,and the seepage shear process of water-conducting fracture in EGS system was simulated by using COMOSOL Multiphysics numerical software,and the effects of thermophysical and mechanical parameters,fracture inclination angle and pore water pressure on seepage shear effect of high-temperature fractured rock mass were studied.Finally,the reservoir model was established,and the seepage shear effect of high temperature fractured rock mass in the reservoir was studied in the process of water injection to raise heat.The main conclusions are as follows:The shear test of granite structural plane after high temperature was carried out.And the main results are as follows:1)The shear test of granite structural plane after high temperature was carried out.And the main results are as follows:(1)The shear strength of granite structural plane is controlled by temperature,which can be divided into low temperature section,medium temperature section and high temperature section according to the influence degree.(2)The higher the normal stress is,the more obvious the temperature effect of shear is.(3)With the increase of heating temperature,the wear of structural plane is more serious,and the generated debris is more broken.(4)With the increase of heating temperature,the internal friction angle first decreases and then increases,and the cohesion generally increases and then decreases.2)The coupling governing equations of temperature field and flow field in the process of water injection heat extraction were established,which were solved by COMOSOL Multiphysics numerical calculation software,and the influence of different crack widths on the temperature field distribution of rock blocks was compared and analyzed.The conclusions are as follows:(1)With the increase of water injection time,the disturbance range of temperature field gradually increases.(2)The crack width equal to 1.5mm is a threshold value.When the crack width is less than 1.5mm,the greater the crack width,the stronger the cooling capacity will be.When the crack width is greater than 1.5mm,the influence on the temperature field will be extremely small.3)The model of high temperature fractured rock mass was established,which simulated the seepage and shear behavior of fractured rock mass in the process of water injection and heat extraction,and solved it by using COMOSOL Multiphysics numerical software.The conclusions are as follows:(1)The higher the pore pressure is,the more likely the fractured rock mass is to be subjected to shear failure.(2)The shear failure of fractured rock mass is most significant when the inclination angle of fracture is 15° and 60°.(3)Rock mass temperature is the main controlling factor of shear failure of fractured rock mass.4)The EGS reservoir model was established,and the reservoir was injected with water to raise heat.The development process of temperature field,thermal stress,thermal strain,fracture water pressure and shear failure of the model was analyzed.It is considered that thermal stress plays a major role in shear failure of fracture seepage.