Development of gas-tight threads based on API round threads and its evaluation

Great economic benefit will be made if a kind of low-cost gas-tight thread can be developed on the basis of API round thread with a relatively lowest cost to replace the existing premium thread. In this paper, a kind of gas-tight threads was designed and manufactured based on the round threads according to the API Spec 5B standard. An elastic sealing ring was added in the middle of the collar, and gas tightness was realized by virtue of the interference matching between the elastic sealing ring and the casing thread. Then, its ultimate bearing capacity was analyzed by using the finite element analysis software MSC.Marc/Mentat. Finally, ISO 13679 B series and C series tests, and tension tests under internal gas pressure in the bending setting were carried out on Ø73.02 × 5.51 mm J55 gas-tight threads according to the ISO 13679 standard. It is shown that the gas sealing performance of this gas-tight thread is good, and the results of ultimate internal pressure cycling and the thermal cycling test on sealing pipe ends meet the requirements of ISO 13679. Besides, its internal pressure strength, tensile strength and collapse strength are much higher than those required by GB/T 20657 standard. It is concluded that this type of gas-tight threads can be used in the development of low-pressure gas wells in the Ordos Basin. It has the potential to cut down the casing cost greatly and it can be used as the reference for the design of new gas-tight threads.