Study On Geomechanical Behavior And Damage Evolution Of Reservoir Rock Under Thermo-mechanical Coupling

The design,excavation and maintenance of some deep geotechnical applications,such as geothermal energy extraction,are much more difficult and complicated than that of conventional rock engineering.One of the critical factors is the elevated temperature in such deep rock formations,which usually significantly alters the structure and properties of the reservoir rock.Therefore,it is of great importance to understand the physical and mechanical response of rock when exposed to elevated temperature and the microscopic mechanism of them.In this study,a series of laboratory tests were carried out to elucidate the damage of microscopic structure and the evolution of physical and mechanical behaviors for granite subjected to high temperature.The effects of elevated temperature on mineral compositions were investigated through X-ray diffraction(XRD),thermogravimetric analysis(TGA)and differential scanning calorimetry(DSC).Three-dimensional optical topography scanner,optical microscope and nuclear magnetic resonance(NMR)were used to study the microstructure alterations of granite after thermal treatments.The mechanical response characteristics were obtained by conducting uniaxial compressive,Brazilian splitting and acoustic emission(AE)tests.The results of tests indicated that there were no violent reactions that significantly altered the mineral compositions of granite during the temperature increase.However,the thermal expansion and the dehydration induced by high temperature could obviously change its microstructure.The granite specimens became more compact after being heated up to 200? because of the dehydration and moderate expansion of mineral grains.The mismatch and anisotropy of thermal expansion occurred when the temperature reached 400?,leading to the propagation of original cracks and the generation of new cracks within the granite specimens.When heated up to 600? and beyond,the thermal expansion of mineral grains presented serious incoordination and distinct anisotropy,and the phase transition of some mineral grains,such as quartz,started to occur.These two factors together caused a sharp increase in the size and number of cracks,inducing serious damage in microstructure of granite specimens.The physical properties and mechanical behaviors of granite were highly related to the variations in its intrinsic structure.Moreover,many test results suggested that the two critical temperatures at which the microstructure and the macroscopic properties underwent two mutations remained highly consistent,respectively.One of them was near 200? and the other was between 400? and 600?.Since the granite became more compact,the longitudinal wave velocity increased slightly while the porosity reduced a little.With temperature increase,the longitudinal wave velocity decreased continuously while the porosity increased markedly.The mechanical properties of granite showed an evolution law similar to the longitudinal wave velocity,which was enhanced at 200? and then gradually weakened,and finally deteriorated at 800?.Both the specific mechanical parameters and the AE characteristics reflected this variation trend.In addition,the failure mode of granite transformed from brittle to ductile after 600?.The cyclic thermal treatments with a desired temperature of 200? could not significantly change the microstructure of granite.Correspondingly,the mechanical properties reinforced to some extent and then decreased slightly and stabilized.It is attributed to a temperature memory that the cyclic heating cannot induce obvious cumulative thermal damage as long as the desired temperature does not exceed a threshold.The thermal expansion can recover partially because the specimens still keep elastic under this condition.By comparison with previous investigations,it is found that the specific value of temperature threshold is related the type of rocks and it is usually above 300? for granite.