Study On Perforated Casing Strength During The Hydraulic Fracturing

Under the condition of volume fracturing,the casing of perforated section is in a complex mechanical environment.The factors such as casing internal pressure,temperature load,perforation operation and non-uniform in-situ stress will affect the integrity of casing and can not be ignored.In the oil and gas development at home and abroad,there are many casing damage problems in perforated well sections under the condition of volumetric fracturing,resulting in the failure of fracturing construction and low single well production.Therefore,taking the perforated casing as the research object,this paper uses the finite element method to study the influence law of casing strength in the process of volume fracturing,and puts forward the perforation parameter optimization scheme according to the finite element model simulation,so as to provide reference for preventing casing damage.The main work is as follows:1.The mechanical model of casing cement sheath formation combination in perforated section is established by finite element method.Firstly,the calculation methods of non-uniform in-situ stress outside the perforated section casing and the internal pressure of the perforated section casing are analyzed,and the casing working condition load of the volume fracturing perforated section is determined by referring to the relevant literature;Secondly,based on the analysis of the solution theory of thermal structural coupling finite element temperature field,the thermal structural coupling finite element model of perforated section combination is established for spiral perforation and fixed surface perforation;Finally,based on the finite element model,the steady-state temperature field distribution of perforated section combination under different displacement is obtained,which lays a foundation for the subsequent study of the influence law of casing strength of spiral perforation and fixed surface perforation.2.Based on the thermal structural coupling finite element model of perforated section assembly,the following research is carried out.Firstly,the influence of two perforation parameters under different casing internal pressure and different displacement on the equivalent stress of TP140V casing is analyzed,and the influence law of two perforation parameters under different casing internal pressure and different displacement on the casing strength is obtained;Secondly,taking spiral perforation as an example,the influence law of spiral perforation parameters on casing strength under different in-situ stress non-uniformity coefficients is studied;Finally,the influence of perforation parameters on casing strength with or without thermal structure coupling is evaluated.The results show that:(1)when the casing internal pressure increases from 90MPa to 120MPa or the displacement increases from 8 to20m~3/min,the increase of casing equivalent stress is small;(2)Without considering the thermal structure coupling,when considering the thermal structure coupling analysis,taking the spiral perforation aperture of 32mm as an example,the equivalent stress of the casing increases by 550MPa,and taking the fixed perforation aperture of 35mm as an example,the equivalent stress of the casing increases by 524MPa;(3)When the local stress non-uniformity coefficient K≥1.2,the casing equivalent stress increases significantly and the casing damage probability increases.3.The influence law of casing strength of perforated section under two perforation modes based on finite element model simulation.Firstly,the perforation parameter optimization schemes of TP140V casing and P110 casing are put forward;Secondly,the influence factors of different mechanical parameters of cement sheath on casing strength are analyzed.The results show that the elastic modulus of cement sheath has a great influence on the equivalent stress of casing,and the Poisson’s ratio and thermal expansion coefficient of cement sheath have little influence on the equivalent stress of casing.