Study On High Temperature Rock Fragmentation Mechanism In Geothermal Drilling

In recent years,geothermal energy resources as a renewable green energy,have attracted more and more attention.Conventional drilling techniques for oil and gas Wells have some limitations when applied to geothermal Wells.Most geothermal reservoirs are granite,and their mechanical properties and fracture characteristics are very complex in the underground high-temperature environment.Therefore,how to scientifically break the granite thermal reservoir in the process of geothermal well drilling is one of the primary problems in geothermal resource exploitation.It is necessary to study the fracture mechanism of granite under high temperature according to the actual temperature of geothermal wells,so as to provide theoretical basis for improving rock breaking technology and efficiency of geothermal drilling.This paper takes granite as the research object,Considering the real temperature of geothermal reservoir drilled by gas drilling method,the basic physical and mechanical properties of granite at high temperature are studied by uniaxial compression test,X-ray diffraction and SEM test.Through the rock breaking experiment of PDC drilling bit under high temperature,the rock breaking rule of PDC drilling bit under high temperature was studied.Using the SHPB rock impact experiment and ls-dyna simulation software to study the stress-strain rule,energy consumption rule and rock fracture characteristics during the granite crushing process at the high temperature.Through the above experimental methods,the macroscopic fracture of granite under high temperature is related to the microstructure changes,and the main conclusions are as follows:(1)Uniaxial compression test,vertical and horizontal wave velocity test,SEM test and X-ray diffraction test under high temperature were used to study the basic physical and mechanical properties of granite and the evolution law of thermal crack with temperature,It is shown that the uniaxial compressive strength,elastic modulus,Poisson's ratio and P-S wave velocity decrease with the increase of temperature,and the basic mechanical parameters of HJC constitutive model parameters under high temperature were obtained.(2)By means of self-developed heating device and uniaxial drillability tester,has carried out the experimental research on rock breaking of granite by PDC bit under high temperature state and the rock breaking rule of granite by PDC drill under high temperature is obtained,and explained it by combining the physical and mechanical properties of granite under high temperature.The results show that the rock breaking efficiency of the bit is higher when the bit is used with low drilling pressure at low rotating speed and higher drilling pressure at high rotating speed.(3)Through self-improved vertical structure SHPB impact tester at high temperature,the impact rock breaking experiment under high temperature was carried out.he results show that the higher the impact pressure is,the greater the peak stress is,and the greater the fractal dimension of rock fragmentation;the peak stress increases first and then decreases,and the peak strain increases.The strain and strain rate of granite at high temperature show a good correlation,and the energy consumption and energy consumption density of rock fragmentation decrease with the increase of temperature.(4)According to the variation law of mechanical properties of granite under high temperature,modified the parameters of HJC model of granite under high temperature.Using LS-DYNA to establish The physical model of SHPB experiment and and simulate the impact fracture process of granite under high temperature.The experimental conditions are basically consistent with the simulated conditions in terms of stress change and energy consumption,and the fragmentation characteristics of granite under the simulated conditions are consistent with the experimental results.Through the above research in this paper,it provides an experimental basis for the study of rock breaking mechanism under high temperature,and provides a theoretical basis for efficient rock breaking in geothermal drilling.