| Geothermal energy,as a continuous development and renewable resource of cocoa,has been developed and utilized in many countries.During the development of geothermal energy,the deep physical and mechanical properties of the strata are changed due to the deep high temperature and high pressure formation environment.In the process of drilling fluid,it will lead to the change of the wall temperature,which leads to the production of additional thermal stress.Therefore,it is very important to study the stability of the wellbore.Taking GuLong well as a research model,aiming at the wellbore structure of GuLong well,the temperature change,in-situ stress and drilling fluid are considered comprehensively.Specific research content and results:(1)On the basis of the related experiments of granite at home and abroad,the experimental data of granite are summarized,and the elastic modulus of granite,Poisson’s ratio,thermal expansion coefficient and thermal expansion coefficient vary with the temperature,and the physical and mechanical parameters of granite under the influence of different temperatures are very different.In actual calculation,the influence of temperature on the physical and mechanical properties of granite should be considered.(2)In the analysis of the stress distribution in the wall rock,the temperature of the drilling fluid and the formation temperature have the heat transfer that reduces the temperature of the wall rock,and the change of the wall temperature directly leads to the generation of the thermal stress,so that the stress of the wall rock is changed.In this paper,the additional stress produced by drilling fluid seepage is derived by using the related finite element method.On the basis of the theory of thermal stress,the calculation expression of additional thermal stress is derived.Through the comprehensive analysis of ground stress,pore pressure,seepage pressure and temperature change,the calculation equation of the comprehensive stress of the wall rock of the well wall is obtained.By further derivation of the equation,the calculation formula of three main stresses of the well wall rock is obtained,which provides a certain basis for the later paper to use the rock strength criterion to judge the instability of the wellbore.(3)The failure criterion of rock is the basic criterion to judge the instability of the shaft wall.When the stress of the rock is greater than the strength of the rock,the instability of the wall will appear at this time.As for the selection of rock failure criteria,the Drucker Prague strength criterion takes into account the intermediate(4)principal stress and the hydrostatic pressure,but the three main stresses expressed are not in accordance with the actual stress conditions of the rock.Although the Mohr-Coulomb neglects the influence of the intermediate principal stress on the stability of the wall,the Mohr-Coulomb highlights the characteristics of the plastic failure of the rock,and the prediction results are considered to be the most practical.(5)In this paper,ABAQUS software is used to establish the finite element model of geothermal well shaft wall rock.Drilling fluid are analyzed in the process of circulation of borehole wall rock stress change trend,at the same time for different bottom temperature of wall rock stress is studied,it is concluded that casing stress change trend and its surrounding rock under different temperature,according to the coulomb-Moore standards on GuLongJing formation depth H = 6000 m in predicting the stability of wall rock. |
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