Fracture Failure Analysis of Wellhead Flange and Casing Pup in Heavy Oil Thermal Wells
Authors: Wang Jiandong , Li Yufei , Zhang Lin , Tian Tao , Wang Chuanlei , Chen Ou , Huang Di 
Published: September 2020

In heavy oil thermal wells, wellhead flanges are typically welded to casing pup-joints, with butt welding on the inner wall, and fillet welding on the outer wall. For the field operation, it was found that after 40 days of steam injection, approximate 50% of the wellhead production casing pup-joints exhibitedleakage, with longitudinal crack in casing joints. Fracture of the wellhead flange and the casing led to leakage of significant amount of toxic and harmful hydrogen sulfide which was not part of the l formation fluid. Ultrasonic flaw detection was performed on the failed casing joints. Andfound initiation of longitudinal cracks in the butt and fillet welds. Further tests were performed to obtain chemical composition, mechanical properties, charpy impact energy, and hardness values for the casing, flanges, and welds. The analysis showed poor weldability for the casing and flange materials, with weldability carbon equivalent greater than the critical value (≥0.6%). Microcracks were presented in the welds. Metallographic structure, material composition nearorigins of the cracks and crack propagation zone, and the welds were examined..Micro cracks along the wall thickness in fusion line and casing heat-affected zone were found. Note that the materialin the casingmiddlewallis tempered sorbite, with the nearby external surface of the weldmetallographic structureismartensite of phase transition. The weld and heat-affected zone had obvious phase transitions of IAF+B+F+P. Blind-hole method was used to quantifyhoop and axial stresses in the casing inner wall and the external fillet welds. It was found that location of the maximum hoop tensile stress coincided with the maximum tensile principal stress. With the maximum tensile residual stress 389MPa at 4 mm from the weld-fusion line in the casing external wall, and with the shielded metal arc welding heat-affected-zone width of 6-8.5 mm, the tensile residual stress in the heat-affected zone was 2.2 times higher than the residual stress in the casing body. The weld material at the maximum tensile residual stress location was tested for SSCC. Stress corrosion was observed for all threes specimens at ambient temperature within 12 hours. Using SEM method, the failure and SSCC analyses showed that the fracture morphology was flat and integunularfracture, and surface was covered with agglomerate corrosion products. Sulfur element was analyzed by means of EDS. It was found that the mass percent of S element in fracture secondary cracks coincided with the measured value of the SSCC fracture test. Therefore, it was concluded that the casing pup-joint longitudinal cracking was SSCC.related. Note that the polysulfone-drilling mud pyrolysis produces hydrogen sulfide was above 150 ° C at which H2S may be produced by the drilling mud. During stages with no steam injectat ambient temperature environment, using high grade SSCC non-resisitant thermal recovery well casing, as well as welding causedhigh residual stress, and high hardness martensite, were the main reasons for the longitudinal cracking in the casing. Therefore, it is recommended that the polysulfone-drilling mudshould not be used in drilling for thermal recovery wells. The use of welded connections should also be avoided or the undesired effects should be mitigated.