Creep Failure Analysis And Experimental Study Of Thermal Recovery Wellhead

With the development and utilization of petroleum resources in China increasing,the reserves of light crude oil which is easy to be extracted gradually decreases,and the main trend is to increase the production of heavy oil.At present,the main methods of heavy oil recovery in China are steam stimulation and steam flooding.The main recovery principle is to inject high temperature and high pressure steam into the heavy oil reservoir to make the reservoir temperature rise and the viscosity decrease,then exploit.Thermal recovery wellhead is the main equipment used in the process of heavy oil exploitation.Because it works under high temperature and pressure,strength failure,corrosion failure,fatigue failure and creep failure will occur in the process of field work.The failure analysis of thermal recovery wellhead has been studied by scholars at home and abroad in various aspects,but most of them focus on fatigue,corrosion,strength failure,and creep failure of thermal recovery wellhead is rarely reported.In the field application,due to the lack of theoretical basis of creep failure research,field staff can only establish the service life evaluation standard of thermal recovery wellhead based on the experience.However,the service life of these standards is relatively conservative,resulting in a serious waste of materials.Creep failure is one of the main failure modes for equipment working under high temperature and pressure,so creep failure of thermal recovery wellhead must be considered.Based on this,the creep failure analysis of the thermal recovery wellhead was carried out in this paper by the combination of test and finite element simulation,and finally the creep failure time of each main pressure components was obtained,which provided scientific theoretical guidance for the field use of the thermal recovery wellhead.The main works are as follows:(1)The strength of thermal recovery wellhead was analyzed,The results show that the maximum stress of the wellhead is 175Mpa,indicating that the device meets the strength requirements.The influence of thermal stress is small,and the one-side gas injection leads to the overall inclination to the non-gas injection side,with the maximum deformation of 11mm.(2)The creep properties of 30CrMo for thermal recovery wellhead were studied,Through the tensile test at room temperature and 390℃,the yield strength and tensile strength of 30CrMo decreased by 18.6%and 10%,the elongation increased by 6%,the area shrinkage increased by 11%,and the elastic modulus decreased by 14.4%at 390℃.The creep model parameters of Norton Bailey and combined time hardening model of 30CrMo were obtained by creep test,and verification test result shows that the error is 14.1%.(3)The microscopic creep mechanism of 30CrMo was studied.It is found that during the creep process,the precipitated carbide particles in 30CrMo matrix increase and the number increases,resulting in the decrease of its strength.The creep mechanism of the high stress zone is the dislocation Orowan overturning mechanism,while the low stress zone is the dislocation thermal climbing mechanism.(4)Creep failure analysis of thermal recovery wellhead under field conditions was carried out,Under the condition of constant temperature and load,the main diameter valve fails after 153 days and flank steam injection valve fails after 316 days.Under intermittent loading conditions,the main diameter valve fails after 14 cycles and flank steam injection valve fails after 19 cycles.(5)The influence of typical defects on creep failure law of thermal recovery wellhead was studied,The influence of typical defects in the sealing groove and the inner wall on the creep failure law of the thermal recovery wellhead was studied.It can be obtained that the size of defects will gradually increase in the creep process,and when the size is larger than the critical value,the thermal recovery wellhead fails.The critical value is:the diameter of the corrosion pit at the sealing groove is 2.5mm,the depth is 1mm,and the crack found at the sealing groove is invalid;the corrosion pit on the inner wall has no effect,the length of the crack on the inner wall is 6mm,and the depth is 1mm.(6)Stress relaxation failure analysis of bolts in thermal recovery wellhead was carried out,The tensile test at room temperature and 390℃shows that the yield strength of 42CrMo decreased by 20.3%,the ultimate strength decreased by 14.6%,the elongation increased by 9.5%,the area shrinkage increased by 8%,and the elastic modulus decreased by 15.7%at 390℃.According to the short-time creep test and stress relaxation test,the pre-tightening stress of bolt decreases by 52%after 1200h due to creep in the field.It is concluded that the bolt will fail due to stress relaxation after 26 days of use.