The Research On Crack Propagation Of Oil Tubing In Acidic Environment

In the oil exploitation process of well,The acidification process will be used to acidize the wells in order to increase the productivity,which makes the acidity of the acidic well will been enhanced.These acidic medium cause serious corrosion damage to the oil tubing,at the same time,the oil tubing will undertake the annulus external pressure,internal pressure,bending,lode and torque.The cooperation of acdic medium and loads can produce corrosion fatigue and stress corrosion damage,which lead to crack initiation,expansion or even fracture.Therefore,studying the crack propagation rate of oil tubing in acidic environment can be contributed to predict the residual life of oil tubing.In this paper researched the crack growth rate of a oil well pipe when corrosion fatigue or stress corrosion happened in different acid environment.First,the paper researched the influence of the H+ concentration(PH=3,4,5),the loading frequency(5Hz and 1Hz)and the material(P110,13Cr)on the crack growth rate of a oil well pipe,which is named da/dN,when corrosion fatigue took place,and established a Corrosion fatigue residual life rate model based on the experimental result.Secondly,the paper employed the Slow Strain Rate Test to explore the effect of H+ concentrations(PH=3,4,5)on the characteristics of stress corrosion crack propagation when stress corrosion happened,and measured the stress intensity factor for slow crack growth KIscc in different PH,analyzed the function relationship between Stress corrosion crack growth rate da/dt and Stress intensity factor range ?K.Main conclusions are reached as follows:(1)In the research of corrosion fatigue,when the frequency is5 Hz and the material is P110,the crack growth rate of a oil well pipe in 3.5wt % NaCl corrosion environment more obvious than which in the atmospheric environment.At the meantime,when the PH value reduced from 5 to 3,the crack growth rate is to be higer;when the frequency reduced to 1Hz,the extension of the corrosion fatigue crack of an oil well pipe was accelerated because of the corrosion damage time increased as the frequency decreased.When he material 13 Cr and the frequency 1Hz,the crack growth rate of a oil well pipe in 3.5wt % NaCl corrosion environment is more obvious than which in the atmospheric environment,and the value is decreased a little as the H+concentrations increased,which is opposite to the P110 material.therefore,the corrosion resistance of 13 Cr is higher than P110's in acid solution.(2)In the research of stress corrosion,set the alternating load as static load in experiments and set a constant displacement rate for 5×10-5mm/s,the stress intensity factor for slow crack growth KIscc of P110 was increased as the H+ concentration decreased in3.5wt% NaCl corrosion environment,so as the total energy absorbed by the process did,which declared that the crack growth rate increased with the increase of H+concentration,and meantime the stress corrosion was promoted.