Although 13Cr steels are typically used for base pipe in conventional sand screens, there has been a movement to develop new CRA's for expandable pipe. The 13Cr martensitic steels have poor resistance to H2S induced SCC and do not have the required ductility to survive the high strains generated in an expandable screen base pipe. Austenitic stainless steels such as 316L (UNS $31603) have been shown to be sufficient for many applications with improved ductility and moderate corrosion resistance, but they have restrictive limits in environments where they can be used. Thus, a need for an alloy that can be expanded, but will maintain 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 316L 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 $31277). Standard slow strain rate and U-bend tests will be used to explore environments where alloy UNS $31277 offers the most economic solution for sand control products.
In sand control, 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. While this provides the obvious advantage of larger production ID and ease of installation in horizontal applications, where gravel packing over long distances is problematic, this new method 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 ~ .
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 the more than 20% expansion required for this application. Mack has shown 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 316L (UNS $31603) for this application, since the austenitic stainless steels have ample ductility to survive expansion. UNS $31603 has been shown to work well in many sand control applications where the H2S content and temperature are low 3.
The testing involved developing an envelope of acceptability for temperature, chloride content, and H2S content that pushed the current limits established for UNS $31603 according to ISO15156/NACE MR0175. These limits are essentially a maximum temperature of 60 °C (140 °F) with a maximum H2S pressure of 15 psi (100 kPa) but no limit on chlorides; alternatively the H2S level can be higher with very low chloride limits. Using the SSR method, the practical upper limit for UNS $31603 would appear to be around 3 kPa (0.5 psi) H2S at salinity and temperature no more than about 50,000 ppm chloride and 93 °C (200 °F). This gives expandable UNS $31603 the ability to replace conventional 13Cr in many sand control applications. This replacement is feasible because sand control is often a problem in shallow reservoirs where temperature is naturally low. However, the 13Cr steel that UNS $31603 is to replace has a 10 kPa (1.5 psi)limit, but no temperature cap, so a large region exists that UNS $31603 could not fill. Recently, applications have been identified that demand a m