Use of Alloy 718 and 725 in Oil and Gas Industry

Abstract

Schlumberger started using alloy 718 in the early 1980s for downhole equipment. A need for completing high-pressure gas wells required a high strength alloy with good corrosion resistance to gas containing high COz and H2S for completion equipment. Alloys like C-276 were available and offered excellent corrosion resistance to anticipated fluids. However, it was available only as high-strength tubing and did not provide wall thickness and uniformity of strength across the wall thickness. The technology for cold-worked, high-strength bar was not developed at that that time. A study for various heat treatments for under-and over-aging was carried out to achieve desirable strength and toughness. The heat treatment and mechanical properties were finalized based on stress corrosion cracking results. The heat treatment and maximum hardness were governed for sour service application. This limited the maximum strength that can be obtained in 718. A welding procedure was developed for artificial gas lift applications.