Study On Fatigue Life Of Wellbore Of CNG Storage Well With Defects

With the application of CNG increasing,the construction of CNG storage well has been accelerated.However,the wellbore of storage well can cause defeats in the process of production and installation or during use.It is easy to cause fatigue failure under the effect of high pressure alternating load,which can lead to leakage or even explosion.Therefore,it is significant to make a research on the fatigue life of CNG storage well with defeats.In this paper,strength analysis and fatigue life study were carried out on the thread of the connection storage well with wall thickness defeats,surface pit defeats and blowhole defeats respectively.(1)The influence of wall thickness difference δ and wall thickness reduction e on the fatigue life of wellbore of high,medium and low pressure groups were studied.The results show that the lowest fatigue life point of wellbore with non-uniform thickness defects appear at the root of the first thread of the connection between the casing and pipe coupling,and the fatigue life decreased with the increase of δ.The thresholds δ of the wellbore of storage well that meet industry standard for high,medium and low pressure groups were2.0 mm,1.95 mm and 1.9 mm.When e≤2.5mm,the lowest point of the fatigue life of the wellbore with defects appear at the root of the first thread of the connection between the casing and pipe coupling,wall-thinning had no effect on the fatigue life of storage well.When e>2.5mm,the lowest point of the fatigue life of the wellbore with defects appear at the casing inner wall of the thinned area.The thresholds e of the wellbore of storage well that meet industry standard for high,medium and low pressure groups were 4.05 mm,3.95 mm and 3.85 mm.The wellbore in the low pressure group is more dangerous than those in the other two groups,and should be the focus of research and monitoring.(2)The study on fatigue life of wellbore with surface pit defeats was carried out.The results show that when the inner wall at the thread of the connection had single pit defeat,the increase of the circle pit and diameter shortened the fatigue life of wellbore.When the wellbore of storage well with double pit defeats and two circle pits are in axial direction,the closer the distance between them,the lower the fatigue life.When two circle pits were arranged in circumferential direction,the closer the distance between them,the higher the fatigue life.When two pits were in arbitrary direction,the function of axial pits and thefunction of circumferential pits superposed.When the casing inner wall of wellbore have ellipsoidal pits from different angles,fatigue life of the wellbore of storage well increase gradually with long axis angle of ellipsoidal pits and circumferential angle of wellbore αchanged from small to large,but they are not linear increased.(3)The study on fatigue life of wellbore with blowhole defeats was carried out.The results show that the casing with blowhole defeats have greater influence on fatigue life of wellbore than pipe coupling with blowhole defeats.Larger blowhole defeats or blowhole defeats with larger ratio of the transverse axis to the longitudinal axis have greater effects on fatigue life of wellbore.When part of the casing of storage well have two horizontally arranged blowhole defeats,the closer the distance between them,the lower the fatigue life.When the distance was greater than 8.5mm,the interaction between two blowhole defeats descended gradually,and the impact on fatigue life was close to that of the single blowhole.