Development and Application of 140 MPa Mandrel Casing Head in Ultra-High Pressure Gas Well
Authors: Xiong Maoxian , Xie Junfeng , Liu Hongtao , Zhang Jingcheng , Liu Weilong , Hu Kaiyin , Zhou Jianping , Xu Xingliang , Ma Lei , Zhao Mifeng , Fan Wei , Hu Fangting 
Published: March 2021

Abstract
In view of the high shut in pressure of gas wells in Kuqa mountain front ultra-high pressure block where the highest shut in pressure of KeS X is 115MPa, the 105MPa casing head currently used can not meet the shut in demanding, so the risk of well control is high. A new 140MPa mandrel casing head was developed. Its sealing structure adopts the form of X Metal sealing at the upper end and rubber seal at the lower end, which has the characteristics of high pressure bearing and reliable sealing performance.
The structural design verification of the 140 MPa mandrel casing head was conducted by finite element analysis(FEA) of the structural strength and sealing performance of the key components of the casing head, including casing head body and hanger. Then indoor evaluation tests were carried out on the material, strength and sealing performance of the casing head and hanger, as well as the overall structure, and the 140MPa mandrel casing is completed Finally, the quality control level of 140MPa mandrel casing head product has reached the requirements of ultra-high pressure field working condition through field trial in ultra-high pressure gas well, and it has the conditions for promotion and application in other ultra-high pressure gas wells.
The results of and FEA show that the maximum bearing capacity of the mandrel type casing head is 793t, and no yielding occurs under the conditions of bearing capacity of 473t, external pressure of 140MPa and safety factor of 1.35; the maximum internal pressure resistance of the hanger is 212MPa, and no yielding occurs under the conditions of bearing capacity of 200t, internal pressure of 140MPa and safety factor of 1.35. The indoor evaluation test shows that: ① there is no sulfide stress cracking (SSC) and hydrogen induced cracking (HIC) in the casing head body (0Cr18Ni9) and hanger (718); ② there is no leakage in the casing head body under 210MPa clean water and hanger under 140MPa nitrogen; ③ there is no yield in the casing head step and hanger under 673t pressure in the mandrel type casing head. The field test shows that the test pressure of the mandrel type casing head is 117MPa and it is qualified under 280t setting and hanging tonnage. At present, the 140 MPa mandrel casing head has been successfully used in Kuqa mountain for 15 wells, which provides a reliable guarantee for the safety production of ultra-high pressure gas wells.
The 140MPa mandrel casing head developed in this paper has the following three innovations: ① adopt the structure without top wire, fix the wear-resistant sleeve by installing the top wire flange during drilling, and avoid the leakage caused by the top wire hole in the later production; ② adopt the form of upper metal seal + lower X-type rubber seal in the sealing structure of hanger, which can not only avoid the metal seal of hanger during the lowering process The seal assembly is damaged and fails, and in case of unqualified pressure test, the metal seal assembly at the upper end of the hanger can be replaced; ③ a limited step is designed at the contact part between the metal seal assembly at the upper end of the mandrel hanger and the casing head body, which can transfer the excess pressure to the casing head body, so as to avoid the failure of the rubber seal and bearing step at the lower part of the hanger.