| High-temperature rock geothermal resource is a kind of renewable and clean energy.There are abundant geothermal resources in China.The appropriate exploitation of geothermal resources is conducive to the improvement of economic development and the balanced development of energy structure of China.To extract the heat of high-temperature rock,the enhanced geothermal system is usually built to realize the thermal exchange process between refrigerant medium and geothermal reservoir.As a kind of ideal rock for geothermal reservoir,granite has the characteristics of low permeability and high density.During the operation of geothermal system,the temperature of high-temperature granite will change sharply,which will change the temperature field,stress field,and seepage field of the reservoir,and even lead to the damage and fracture of the reservoir rock.Therefore,the study on the physical and mechanical characteristics and the failure mechanism of high-temperature granite after the rapid change of temperature can provide a theoretical basis for the design and mining performance analysis of enhanced geothermal system,which has important engineering significance.This paper relies on the key consulting and research project of the Chinese Academy of Engineering "Research on CO mining strategy of deep mineral and geothermal resources"(2019-XZ-16).Taking granite as the research object,the changes in physical and mechanical properties and thermal damage mechanism of granite at different heating temperatures after cooling with water are studied by means of indoor tests,theoretical derivation,and numerical simulation.The main research work and results are as follows:(1)Through the test of physical properties of different heating temperature granite before and after cooling,the effects of temperature and cooling on physical properties of granite were obtained.It has been found that when the temperature increases from 450℃ to 600℃,the change rate of physical parameters increases significantly,and increases exponentially with the increase of temperature.In addition,the physical parameters of thermally damaged granite were analyzed by using the method of mathematical statistics,and the evaluation method of modification effect of physical parameters of thermally damaged granite was proposed.It has been clear that compared with density and thermal conductivity,the longitudinal wave velocity is the most sensitive to temperature change.The relationship model between multiple physical parameters was established,and the evaluation method of thermal conductivity of different heating temperature granite based on the relationship of parameters such as density,wave velocity,thermal conductivity,and thermal diffusion coefficient was proposed.(2)Considering the change of micro structure of thermally damaged granite,the change process of micro fracture and pore structure with the increase of temperature was observed,and the relationship between new micro fracture,pore structure,and the temperature has been obtained respectively.In order to characterize the complexity of pore structure,a fractal dimension model based on pore radius was constructed.It has been that,microporo us pores do not have fractal characteristics,the fractal dimension of mesoporous pores is the smallest and the reservoir performance is the best at 450℃,and the fractal dimension of macroporous pores is the smallest and the reservoir performance is the best at 600℃.(3)The uniaxial compression test and Brazilian fracture test of different heating temperature granite before and after cooling were carried out to obtain the variation laws of the stress-strain curve,strength deformation characteristics,and failure mode of granite specimens.Using the thermal stress damage reduction factor,a statistical thermal damage constitutive model of rocks at different temperatures has been established.Based on the variation law of tensile strength of thermally damaged granite with the increase of temperature,the phenomenon of "thermal hardening" of rock at 150℃ was revealed,and the segmented strength model of rock has been established.Combined with the digital image correlation method,it has been found that the propagation process of rock splitting fracture is affected by the existence of thermal damaged rock micro fractures.From the perspective of physical and chemical changes of granite mineral particles,the mechanism of rock thermal damage caused by rockmoisture change,mineral phase transformation,and particle expansion was revealed.The influence of temperature on crack initiation strength of granite before and after cooling has been studied and the reservoir rock damage model has been established.The mathematical model of rock Brazilian tensile strength increasing with temperature based on piecewise strength theory has been proposed and the evolution of surface strain field of granite specimens during splitting failure was monitored.(4)Based on the mechanical properties of thermally damaged granite under different confining pressures,the effects of temperature and confining pressure on the stress-strain relationship,peak strength,and elastic modulus of rock were discussed.It has been clear that the increase of confining pressure can limit the propagation of micro fractures and help to improve the peak strength of rock;The temperature rise induces the initiation of micro cracks,which can reduce the elastic modulus of rock.Based on the Mohr-Coulomb strength criterion,the law that the internal friction angle increases with the increase of temperature and the cohesion decreases with the increase of temperature was obtained;a Nonlinear coefficient was introduced λ,the nonlinear failure strength criterion for high temperature and high-stress granite was deduced,and the coefficient was analyzed λ,the accuracy of the failure strength criterion was verified.(5)Taking the Yangbajing Geothermal project as an example,a reservoir model is established based on the geological structure and exploitation scheme of the geothermal field.The geothermal exploitation process was simulated with natural fractures as thermal exchange channels and 20℃ water as injection fluid.The influence of geothermal exploitation on reservoir temperature field and stress field was analyzed.It has been clear that the increase in mass flowrate is conducive to improving the production power of the geothermal system and causes a wider range of stress field disturbances.Combined with the simulation results of the reservoir stress field and applying the failure strength criterion of thermally damaged granite,it has been found that the reservoir failure area first extends from the injection well to the surrounding rock,and then extends along the natural fracture.In addition,the fracture pressure of natural fracture is calculated,and the suggestion of mass flow rate injection from low to high was given. |
