Analysis Of The Temperature Field And Breakdown Pressure Of The Borehole With Cracks

Geological drilling is the most direct technical means of geological survey and mineral exploration.In order to meet our national major demands for energy resources and environmental protection,scientific drilling marches to the deep earth.More complex formations will contribute to the rupture and instability of borehole.In addition,the circulation of the drilling fluid at lower temperature leads to the change of the temperature field around the borehole,and the thermal stress around the borehole also affects the safety of the borehole,which makes 10000-meter scientific drilling and deep dry hot rock drilling face the challenge of safety,quality and economy.We need advanced drilling technology.In teams of the problem of borehole instability,this paper has done a lot of profound and detailed research.The main work and conclusions are as follows:1.The governing equation of the temperature field of infinite earth plane with a complete circular borehole was set up.Using of special functions and differential equations,the approximate steady-state distribution of the temperature field around the complete borehole was deduced.The temperature field of the cracked borehole was obtained using complex variable function mapping method.Compared with the steady temperature field of a thick-walled cylinder with cracks on inner wall,the results showed that an infinite plane can be simulated by a thick-walled cylinder with a sufficiently large outer radius.2.According to the theory of elasticity and simulation,the thermal stress distribution of the complete borehole caused by the temperature difference was analyzed,and the influence of different temperature fields on the stress distribution around the hole was explored.With the increase of crack length,the radial thermal stress and circumferential thermal stress will increase.The study showed that the influence of crack length on thermal stress should not be ignored.3.The continuous dislocation distribution method was used to analyze the quasi-static stress state of the borehole cracks,and a equilibrium equation was established to obtain the rock stress intensity factor,breakdown pressure,and crack opening displacement.The influence of temperature and crack length on the breakdown pressure and crack opening displacement were analyzed.The results showed that the decrease of temperature and the increase of crack length will lead to the decrease of crack breakdown pressure and the increase of opening displacement,which is not conducive to borehole stability.