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ABSTRACT
The Salton Sea KGRA has long been known for its power generating potential, however, the corrosivity of the hypersaline fluid has limited power generation as well as the production casing material choices. In the 1990’s UNS R56404 (titanium Grade 29) was utilized very successfully to line nearly all of the energy producing wells in this field. The material proved to be an excellent choice and has given 20-25 years’ service life to date. Despite its excellent performance, Grade 29 is a high strength titanium alloy that is expensive to manufacture and finish. Recent wells (2012 and beyond) have utilized other (non-titanium) materials that were less expensive upfront, but appear to be degrading at a much faster rate than the Grade 29 titanium alloy. To date, these newer wells have required repairs in as little as 2-3 years after start-up. Several new alloys that will address both the cost and performance issues seen in this geothermal field have recently been developed. This paper will highlight the corrosion performance of these new alloys, along with their more cost effective manufacturing processes.
INTRODUCTION
The development of supercritical geothermal wells will require materials of construction that can withstand the harsh corrosive environment at these elevated temperatures. Titanium is not usually considered under these conditions due to the lack of experience and the perceived high cost of the alloys. However, operators in the Salton Sea region have come to depend on titanium for production well casing over the last 25 years. Titanium is one of the only alloy systems that shows minimal corrosion and a higher resistance to scaling. In addition, the low thermal expansion and modulus of titanium significantly reduces the compressive stress on connections, thus typically limiting the strain developed during thermal cycling to the elastic region.
The goal of harnessing high power geothermal wells has been underway at several sites around the world since 1980. Two deep wells were drilled in Iceland under the guidance of the Iceland Deep Drilling Project (IDDP). The IDDP-1 was drilled by Landsvirkjun in 2009 and became the hottest geothermal well in the world at that time. However, the aggressive conditions caused extensive corrosion of the well casing materials. HS Orka drilled the second deep well, IDDP-2 in 2017. This became the deepest and hottest well in the world, but yet again; the materials of construction were not able to withstand the environment. A third well, IDDP-3, is under the direction of Reykjavik Energy and is now under discussion as to the materials of construction. The goal of IDDP is to achieve 450°C supercritical steam which will produce 50MW power with 7in. OD well casing.
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