Fracture Development And Physico-mechanical Characteristics Of Surrounding Rock Drilled By High-temperature Treated Crystalline Rock

Deep drilling is the core technical means to enter the deep part of the earth to develop resources and energy,and to obtain scientific data.It is a national strategic scientific project: both "13th Five-Year Plan" and "14th Five-Year Plan" list "deep earth,deep sea and deep space" as a frontier field of science and technology that needs to be tackled urgently.The implementation of deep drilling projects will be conducive to the research on life evolution,energy cycle and deep space on Earth.In-depth research on major frontier scientific issues such as dynamics ensures the successful exploration and development of deep resources and energy in China,especially the exploitation and utilization of hot dry rock geothermal energy.However,the problems of borehole instability and core fracture in deep crystalline rock formations are prominent,which are the main bottlenecks restricting deep drilling.Such as: what is the development and evolution mechanism of deep high-temperature crystalline rock fractures? What are the physical and mechanical characteristics of high-temperature and high-pressure crystalline rocks?How does deep drilling lead to the development of borehole wall and core fissures and develop into borehole wall deformation and instability and core fracture?In this paper,two crystalline rocks from typical hot dry rock areas are taken as the object.It is planned to pass the self-designed high temperature and high pressure physical and mechanical test,drilling hole forming stability test,element phase state and material analysis,fracture observation and quantitative characterization.The method combines acoustic emission technology to carry out research on the physical and mechanical characteristics of high-temperature crystalline rocks and the development and evolution of fractures.The main work is as follows:(1)Through target selection,sample extraction,and specimen preparation,two typical crystalline rock specimens were selected to carry out mineral thin section,thermogravimetric analysis,elemental analysis,and the mineral composition analysis.Major and trace elemental content and valence distribution were analyzed.Mass loss,mineral and elemental change of crystalline rock under high temperature was studied.(2)The macroscopic physical and mechanical properties and fracture development tests of crystalline rocks treated at high temperature were carried out,and the variation laws of strain,peak pressure,elastic modulus,Poisson’s ratio,shear strength and tensile strength of crystalline rocks treated at high temperature were studied,and the macroscopic failure morphology and burst mode.The change rule of mechanical parameters and fracture distribution under different temperature and pressure was mastered,and the influence of mechanical characteristics and macro-fracture development of high temperature treated crystalline rock on the deformation and fracture development of surrounding rock of borehole was explored.(3)The research on hole formation and hole wall deformation of crystalline rock drilling and coring under the high temperature was carried out.As the test object of hole wall stability,the morphology characteristics of the orifice and the fracture law of the core after drilling were analyzed,and the deformation characteristics of the hole wall and the displacement law of the surrounding rock under the action of biaxial loading were studied.(4)Using high-definition 3D CT microscopic observation and scanning electron microscope microscopic analysis.We focused on tracking the development and evolution characteristics of high temperature and high pressure crystalline rock fractures,and analyzed the structural characteristics and distribution of crystalline rock fractures through fracture structure data extraction,quantitative analysis,and 3D model reconstruction.The expansion and penetration characteristics of fractures in mineral crystals,crystal boundaries and mineral matrices are studied,and the similarities and differences in the development of microscopic and microscopic fractures in two crystalline rocks are compared and analyzed.The influence of high temperature and high pressure on the development and evolution of fractures in crystalline rocks is revealed.(5)Taking advantage of acoustic emission technology in fracture development monitoring,high-temperature heat treatment and rapid cooling of crystalline rock with clear water was selected as the research object.Acoustic emission experiments on the development and evolution of uniaxial compression rock fractures were carried out to analyze the characteristics of acoustic emission and their parameter changes.The development and evolution of fractures in high-temperature and high-pressure crystalline rock was studied.Differences in the development and evolution of fractures in two crystalline rocks were analyzed.The influence of crystalline rock minerals,and physical and mechanical properties on fracture development was analyzed,and the development and evolution of fractures in high-temperature and high-pressure crystalline rock was revealed.