| Hot dry rock(HDR)stores more than 30% of total proven geothermal resources.Hydraulic fracturing(HF)is an effective technology to create an artificial fracture network in reservoirs to enhance their hydraulic conductivity.When the fracturing fluid is injected into a hightemperature reservoir,the cooling of the rock mass around the wellbore produces a temperature gradient,and the far-field rock mass continues to heat the rock mass around the wellbore,and the whole reservoir is in the state of thermal-hydraulic-mechanical coupling under the dynamic balance of heat transfer.The study of rock mechanical properties and damage mechanism under multi-field coupling is of great significance for constructing complex fracture networks with good connectivity.Peridynamics(PD)is a continuum mechanics based on a non-local theory.The governing equation of PD has been formulated in an integral-differential form instead of the partial differential form used in classical continuum mechanics,which enables it to deal with singular and discontinuous problems without resorting to any exterior criterion.PD introduces the benefit of describing the complex fracturing behavior of rocks.This study was supported by the National Natural Science Foundation of China(42077231).The research object was granite.PD theory and computational methods,laboratory experiment comparison and verification were involved in this study.The following general results can be derived:(1)The derivation process of the equation of motion and thermal diffusion equation in the fully coupled thermal-mechanical ordinary state-based peridynamics(OSB-PD)theory was described,and the PD constitutive model,thermal treatment method and numerical implementation suitable for rock materials were analyzed.A Fortran program was compiled.The simulation results of benchmark examples were compared with the finite element method simulation result,analytical solution and laboratory experiment result,verifying the convergence,capability and accuracy of the program.The superiority of PD in simulating the cracking behaviours of brittle granite materials was demonstrated.(2)Based on Shuffle algorithm,a new method for accurately reflecting the mineral compositions and the corresponding characteristics was proposed to simulate the heterogeneity property of the rock material.The thermal cracking behaviours induced by the incompatible expansion between the different minerals of granite under the thermal cycling treatment was simulated.The thermal-mechanical properties and fracture characteristics of the granite specimen containing a pre-existing fissure under uniaxial compression after high temperature treatment were investigated.(3)A multi-layer computational method was proposed to establish a calculation framework for the mechanical behaviours of granite under real-time temperature(RT)and posttemperature(PT)treatments.The effects of temperature and confining pressure on the strength,deformation parameters and failure modes of RT and PT granite specimens were analyzed from the macroscopic point of view,and the interaction law among cracks with different properties was explored from the microscopic point of view.The influence of heterogeneity on the simulation results was discussed.(4)A coupled thermal-hydraulic-mechanical PD model was developed to simulate the hydraulic fracturing and sleeve fracturing of granite specimens under different pressurization rates.Considering the dynamic change of the temperature field,the simulation of hydraulic fracturing in granite under coupled thermal-hydraulic effects was carried out.The effects of pressurization rate,temperature and wellbore cooling on the hydraulic fracturing behaviours of granite under high-temperature and high-pressure treatment were studied,and the interaction mechanism among natural pre-existing microcracks,thermal-induced microcracks and hydraulic-induced cracks was discussed.The evolution characteristic of granite failure modes from subcritical brittle failure to supercritical ductile failure was summarized. |
