The Thermal Stress Of Perforating Casing In Process Of Mass Fracturing

Mass fracturing has the characteristics of large displacement and high pump pressure,a large amount of cryogenic fluid was injected during the fracturing process,and the thermal stress was generated because of the change temperature of casing,which has great influence on the strength safety of casing.Therefore,in this paper,perforating casing is the research object,considering the coupling effect of temperature field and stress field,the mechanical properties of perforation section casing are analyzed.For the study of the thermal stress of the perforation casing,the temperature field of the perforation casing is analyzed firstly.According to the first law of thermodynamics and the basic theory of heat transfer,a reasonable boundary condition was established,and a three-layer thermal model of casing-cement-surrounding rocks was established,and the temperature field analysis was carried out by FEA software.Based on the analysis of the temperature field,the stress field of casing-cement-surrounding rock system is carried out by the ANSYS Workbench,and the thermal stress distribution results are obtained.Finally,the analysis and research of the influencing factors of the thermal stress of the perforation section casing are carried out,the control variable method is applied to establish the finite element model of different properties,and the influence of the geometrical and physical characteristics of the cement on the thermal stress of the perforation section casing is analyzed.The results show that the temperature distribution along the radial and annular direction of the perforation section is uneven during the fracturing process.The casing thermal stress is about 38.3% of the yield strength of the casing material,and the cement thermal stress is about 55% of the yield strength of the cement material.The maximum thermal stress points are distributed on both sides of the horizontal axis of the holes,and if there are micro cracks in the holes,the lateral expansion is made.The displacement direction of the stratum points to the shaft neutral shaft,and the casing has extrusion effect.The increase of cement thickness has reduced effect on the thermal stress of cement itself,but the thermal stress of casing is shown to increase.Therefore,in order to reduce the casing thermal stress and ensure the strength safety of casing,the cement thickness is optimized under the premise of strength safety.In the material properties of cement,the influence of elastic modulus on thermal stress is greatest,and Poisson's ratio is minimal to thermal stress.