Analysis Of Wellbore Stability And Critical Conditions Under The Action Of Thermal-Mechanical Coupling

During the development of deep energy,whether the traditional oil and gas resources or the development of the hot dry rock geothermal resources,the key issue that needs to be solved first is the problem of deep drilling construction and well-bore stability control under the action of high temperature and high stress coupling.During the construction of deep drilling,due to the high crustal stress and high ground temperature,the drilling stability is greatly reduced and the maintenance cost is increased,which seriously restricts the development of deep energy resources.In the process of drilling,when the stratum is granite and based on the analysis and summarize of the “The mechanics experiment of Granite well-bore under the action of high stress and high temperature”,through numerical simulation method,using MATLAB and COMSOL Multiphysics5.2 software studied “The thermal elastic deformation of granite under the action of thermal-mechanical coupling”,and “The rheological behavior of granitewell-bore under the action of thermal-mechanical coupling”,and “The relationship between the failure zone radius and the load stress and temperature is analyzed” and finally obtained “The critical conditions of failure zone of granite well-bore under the action of thermal mechanical coupling”.(1)Analyzed the experiment data of the mechanics of granite well-bore under high stress and high temperature,and obtained the following conclusions:1)Under the action of thermal-mechanical coupling,the growth trend of thermal strain of borehole is different in different temperature range.At different temperatures,the thermal strain of granite borehole with the increase of buried stress,and its value decreases.2)There is a significant threshold for the creep failure of granite well-bore under the action of thermal mechanical coupling,that is,the stress threshold is4000 m buried stress and the temperature threshold is 480?.3)The well-bore failure characteristics under the action of thermal mechanical coupling have significant end effect,necking,damage along the inner wall of the well-hole;there is a significant displacement contour of the dislocation slip,there are traces of significant shear damage,eventually forming the cone shaped hole around the failure surface.4)The influence of high temperature on the basic mechanical parameters of granite is significant.Its basic mechanical parameters,such as elastic modulus E,Poisson's ratio,and thermal expansion coefficient can be considered as a function of temperature.(2)Studied the thermal elastic deformation law of granite well-bore under thermal-mechanical coupling,and obtained the following conclusions:1)In the process of heating,the specimens has a significant temperature gradient,and thermal stress generated by the temperature gradient has influence on the thermal strain of granite specimens only during heating;Under the conditions of thermal-mechanical coupling,the stress of granite specimen is mainly compressive stress,and the maximum value is located on the outer wall of the well-bore and the stress value of the inner wall of well-bore is relative small.The maximum displacement is located on the outer wall of the drilling well,and the minimum value is located on the inner wall of the drilling well.2)The temperature threshold for plastic failure of granite specimens under thermal-mechanical coupling:The temperature threshold under 3000 m buried depth is 210 degrees?,and the temperature threshold under 3000 m or 4000 m is195?.3)In the thermal elastic deformation of granite well-bore under thermalmechanical coupling,the end effect is remarkable and there has a significant slip,dislocation along the contour of the displacement which result in the failure zone near the inner wall of well-bore.(3)Studied the rheological law of granite well-bore under thermalmechanical coupling.The following conclusions are obtained:1)The rheological displacement under different thermal-mechanical coupling had a similar law,that is,the inner wall of well-bore has the largestrheological displacement,besides the rheological displacement decreases gradually from the inner wall to the outer wall along the radial direction.2)Under the same condition of buried depth stress and loading temperature,after considering the creep effect,the rheological displacement of the well-bore increases significantly,which makes the threshold of failure of well-bore decreased,that is:under 2000 m buried depth and loading the temperature of200?,the well-bore will produce failure zone.What is more,under the same conditions,the radius of failure zone is much larger than that of elastic deformation.(4)The critical condition of granite well-bore failure under the action of thermal-mechanical coupling is ?=241.9-0.3998·T.