| In the fields of deep scientific drilling,deep geothermal drilling and deep resource exploration,the problem of borehole wall instability induced by fracture development caused by high temperature and high in-situ stress has become the main technical problem facing at present.The regularity and mechanism of borehole fracture development in deep well environment need further study.In this paper,the typical deep well bedrock--granite is taken as the research object,and the indoor test,discrete element numerical simulation and elastic theoretical analysis are combined to carry out relevant research on the development law of shaft wall fissure in the deep well environment,and the main results and conclusions are as follows:(1)Based on the determination of physical and mechanical properties of granite,combined with the contact constitutive characteristics and force-displacement law,the uniaxial compression and Brazilian split discrete element method(DEM)model of granite was established,and the sensitivity analysis of contact mesoscopic and macroscopic parameters was carried out.The microscopic parameters of DEM model are obtained by parameter calibration test.(2)The thermal damage fractures of granite under high temperature and high pressure are analyzed.The results show that the thermal damage mainly produces tensile fractures at the boundary of quartz and plagioclase particles with large thermal expansion coefficient.Horizontal ground stress drives granite to produce compression-shear fractures on the basis of tensile fracture development,and the proportion of shear fractures increases as the horizontal ground stress increases.(3)Based on the elastic theory and Kirsch solution,combined with the tube domain model and the granite thermal damage model,the borehole mechanics model and DEM model under thermo-hydromechanical(THM)coupling were constructed respectively,and the output script of the internal stress of the model was written,which provided the calculation basis for the theoretical research and simulation test study of the borehole wall fracture development.(4)The simulation test results of deep well wall fracture development under THM coupling show that temperature mainly affects the stress concentration of surrounding rock.In-situ stress affects the concentration area of well wall fractures,and the fracture development degree is the highest in the direction of in-situ minimum horizontal principal stress.When the liquid column pressure is less than the in-situ stress,the well wall fractures are mainly circumferential fractures,while when the pressure is greater than the in-situ stress,the well wall fractures are mainly radial fractures. |
