Study On The Influence Of Large Flow Lift On The Fatigue Characteristics Of Sucker Rod

With the continuous exploitation and development of oil fields in China,some of them have entered an extremely-high water cut period.The water content of oil well production fluids exceeds 95%,and increasing oil well production is an important way to maintain stable production or increase oil production.In the process of increasing the discharged fluid,the working condition of the oil well is often degraded,the stress distribution on the pumping rod is more complicated,and then the break-off accident often occurs,resulting in a shortened period of the check pump.This paper analyzes the sucker rod in the oil well with large displacements.The geometry of the sucker rod in the three-dimensional space is described by using differential geometric space curves.Aimed at the main problems in the lifting process of the inclined shaft,the mechanical behavior of the sucker rod string inside the casing was studied in details The stress change of the sucker rod string was comprehensively analyzed,and a large row of the sucker rod string in the well bore was established.The theoretical model of the load change at the time of lift and the calculation model of the axial force and the lateral force generated during the actual production of the sucker rod and column downhole.A method for calculating axial and lateral loads at arbitrary positions was established.For the lift rods with different displacements during the lifting of large displacements in inclined wells,including new rods,1-year rods,2-year rods,and 3-year rods,the alternating time is estimated based on the use of these rods.The universal material tester was used to test the sucker rod specimens,and the tensile strengths of the sucker rods of different years were tested.The lift limit method was used to count the fatigue limit distribution of the sucker rods of different years.According to the experimental results,the stress-fatigue curve of steel grade D rods was plotted by using the fatigue limit determined by the group method combined with the lifting method.The mathematical method was used to verify that the fatigue curve of sucker rod obey normal distribution.Using Origin,an academic plotting software,to fit the sucker rod fatigue life data,the S-N curve of the sucker rod with different reliability was obtained.Combined with the test data,the parameters of the S-N curve were obtained by using the power function to obtain the S-N curve formula of the sample under the stress ratio R=0.1.The formula can be used to predict the life of downhole sucker rod string.The structure of connecting thread and coupling part of the sucker rod joint is complex,and it is difficult to calculate in the force analysis.The mechanical model of nonlinear finite element analysis of joint thread is established by using ABAQUS.Through simulation analysis,the maximum stress occurs at the root of the first thread that the sucker rod and the collar are engaged with,and stress concentration occurs in the load-shear zone between the shoulder and the stress relief groove.Using ABAQUS to perform a finite element analysis mechanics model,the cyclic load of the thread section is converted into the axial equivalent stress.Based on the GOODMAN equivalent stress formula the P-S-N formula with different reliabilities,the equivalent life of the crew connection is obtained.Finally,the theoretical and fitting P-S-N life curve is calculated for the sucker rod string.Combining the statistical analysis,the theoretical analysis and the field data,a set of design softwares for the trajectory simulation and load distribution optimization of the downhole sucker rod string is established.