Research On Statistical Damage Constitutive Model Of High Temperature Hard Rock And Rock Breaking Efficiency Of PDC Cutter

The sound and sustainable development of China's economy is facing severe energy shortages and environmental problems.The development and use of clean energy is of great strategic significance for improving the energy structure and easing environmental pressures in the development process.Geothermal energy is an important development direction of clean energy.High-temperature geothermal resources have become an important part of geothermal resource development because of its high commercial development value.China's high temperature geothermal resources have large reserves,wide distribution and low development level.Drilling technology is one of the key factors restricting the development of high temperature geothermal resources in China.The first problem faced in the high-temperature geothermal resource exploitation process is the drilling problem.In the high temperature environment,the rock exhibits mechanical properties that are quite different from the normal temperature,which seriously affects the drilling efficiency.Therefore,an in-depth understanding of the mechanical properties of high temperature rock has great engineering significance for high temperature geothermal exploitation.The purpose of this study is to establish a statistical damage constitutive model which can describe the stress-strain relationship of high temperature hard rock.Based on this,the rock-breaking efficiency of PDC cutter breaking high-temperature hard rock is studied.It is expected to provide reference for the study of mechanical properties of high-temperature hard rock and the design of tools and processes in high-temperature geothermal drilling.The main research work is as follows:(1)Considering the damage threshold,residual stress,thermal damage and effects of confining pressure,the Drucker-Prager(D-P)criterion is used as the failure criterion of rock microelement.The Weibull distribution is introduced to describe the strength distribution of rock microelements.And the damage evolution equation and statistical damage constitutive model of hard rock at high temperature are established.The solution formula of the statistical damage constitutive model parameters is derived by the peak point method.By introducing shear stress and dynamic pointer considering dynamic damage into the proposed model,the subroutine of ABAQUS numerical analysis is established.The results of theoretical calculation and numerical simulation are compared with the experimental results of triaxial compression of high temperature hard granite.And the rationality and applicability of the model are proved.The physical significance of some model parameters is discussed.The results show that the proposed model can describe the stress-strain relationship of high-temperature hard rock well,and the definition of model parameters is reasonable.(2)Based on the proposed model,the influence of rock material parameter ?0?ET??and statistical parameter m?n on stress-strain curve is studied.Based on the proposed damage evolution equation,and a rock extension-brittleness evaluation method based on damage evolution is proposed.The effects of temperature,rock material parameters?0?ET?? and statistical parameters m?n on damage evolution and damage evolution rate were studied.The results show that the brittleness of rock decreases with increasing temperature,increases with the increase of parameter ?0?ET?m,decreases with the increase of parameter n,and is little affected by the parameter ?.(3)Based on elastoplastic mechanics and rock mechanics,the Drucker-Prager criterion is used as the strength criterion of rock,and the proposed rock damage evolution equation established is used as the basis for the value of rock damage parameters.A three-dimensional numerical simulation model for dynamic rock breaking of PDC cutter is established.The influences of cutting depth,temperature,back rake angle and cutting speed on the rock breaking efficiency of PDC cutter under 60 MPa confining pressure and the influence mechanism are studied.The results show that:?Under the confining pressure of 60 MPa,the PDC cutter cut the rock at a speed of 0.5 m/s,and the mechanical specific energy(MSE)of the PDC cutter(back rake angle 5°?25°)decreases with the increase of the cutting depth.With the increase of temperature,MSE shows a trend of increasing first and then decreasing,and the critical temperature of conversion is 200?;?Under 60 MPa confining pressure,PDC cutter cut rock at 0.5m/s cutting speed.The MSE of PDC teeth(cutting depth 1/3 mm)shows a tendency of decreasing first and then increasing,and the optimal back rake angle is 20°.?Under 60 MPa confining pressure,the rock temperature is in the range of 20??300?,and rock breaking with 5° back rake angle of PDC cutter,the MSE increases with the increase of cutting speed.Under any cutting speed,the MSE decreases as the cutting depth increases.