Corrosion Resistance and Long-Term Reliability of Super-Austinitic Alloy 27-7Mo for Use as Expandable Pipe

Abstract

Abstract In today's competitive oilfield environment, end-users are faced with choosing the lowest-cost material that meets the technical and environmental needs of the application. With field development expanding into deeper, more corrosive environments, corrosion-resistant alloys (CRAs) must be used. In this paper, the discussion will investigate CRAs and the added burden of maintaining economic efficiency in corrosive environments requiring sand control ? more specifically, materials for expandable sand-control systems.For expandable applications, materials must compete with existing non-expandable lower-cost commodity-type screen materials. 13Cr, for example, is the first choice CRA material for the base pipe in conventional sand screens.Although 13Cr is one of the least costly of the CRAs and provides the necessary corrosion resistance in mild H2S environments under a variety of chloride/temperature conditions, it lacks the ductility needed for 20 to 25% expansion as a solid-expandable.Austenitic stainless steels such as 316L (UNS S31603) have the needed ductility for expansion, but with their low resistance to Stress Corrosion Cracking (SCC) may be insufficient for severe-service applications. Thus, a need for an alloy that can be expanded while maintaining corrosion resistance is required.Furthermore, to prevail in this very competitive arena, the material must offer a cost-effective alternative to standard 13Cr and expanded S31603 pipe.The basis for this study involves measuring the SCC resistance and predicting the mechanical behavior of higher-strength corrosion-resistant super austenitic alloy 27–7Mo (UNS S31277).Slow strain rate (SSR) tests, pitting and corrosion tests, as well as U-bend tests are used to explore and compare environments where S31277 and S31603 can offer the most economical solution for expandable sand control products in corrosive conditions. Introduction For sand control applications, the capability now exists for the sand screen to be expanded against the borehole ID, virtually eliminating the annulus around the screen and the need for gravel packing.A drawing of a typical installation is shown in Figure 1.While this method provides the obvious advantage of larger production ID and ease of installation in horizontal applications, where gravel packing over long distances is problematic, it also has other benefits.The method increases the sand screen surface area, thus reducing pressure drop across the filter and increasing production rates.Finally, if a ‘solid’ expandable base pipe is used, because of its inherent high strength, the expanded screen can provide superior support to stabilize the borehole, which then minimizes the potential for sand production due to collapse and subsequent damage to the screen.[1] The challenge with the expandable screen base pipes has been that many conventional CRA materials, such as 13Cr, do not have sufficient ductility to undergo expansion.The lack of ductility is readily apparent in the photograph shown in Figure 2.Also, Mack has reported that the sulfide stress-corrosion cracking resistance of 13Cr is poor following expansion of only 10%.[2]With this in mind, work has been done recently to characterize the corrosion performance of S31603 for this application, since the austenitic stainless steels have ample ductility to survive expansion.S31603 has been shown to work well in many sand control applications where the H2S content and temperature are low.[3]