May 24, 2018
The typical conventional geothermal well produces energy from water or steam in contact with hot rock at 200-325 °C, with wells typically ranging in depth from 60 to 3,000 m. At this point, the industry has decades of experience and the technologies are relatively well known. Stainless steel and other nickel-containing materials are commonly and successfully employed in these projects on a global basis, with their principal role being to deal with corrosive brines and chemicals.
Different local conditions call for different alloys
Geothermal projects have been developed widely across the planet from Kenya to New Zealand, with well over 13,000 megawatts (MW) of capacity worldwide, and the possibility of reaching as much as 23,000 MW by 2021. Some countries, such as Kenya – which at the end of 2016 had about 650 MW of capacity, representing about half of its electricity production – rely heavily on the resource.
Geothermal projects are developed in widely different environments, with enormous variations in metal ions, corrosive chlorides and gases encountered. As a result, a broad range of stainless steels and nickel alloys are deployed in the piping and process equipment.
For many less corrosive wells, standard nickel stainless steels including Types 304L (UNS S30403), 316L (S31603) and 321 (S32100) are commonly utilised, but in chloride- or sulphide-rich wells, higher corrosion-resistant alloys are often needed. These may include duplex stainless steel Types 2205 (S32205), 2507 (S32750) or austenitic alloys such as 904L (N08904) or 825 (N08825). In extremely corrosive environments, more of the highest corrosion-resistant nickel alloys such as Alloy 625 (N06625) and C-276 (N10276) may be required.
This challenge of finding the right alloy for the job becomes even more critical as developers eye a potential major new geothermal frontier: deep advanced geothermal projects sitting close to the edge of the earth’s magma, at pressures as high as 200 atmospheres.
A potential new approach to geothermal in Iceland: exploiting supercritical fluids
The first of these deep drilling projects is taking place in the Krafla geothermal field in northeast Iceland. The potential of a vastly greater geothermal resource is being explored here, with the goal of tapping supercritical fluids at 400-600 °C. Supercritical is the point at which, under extreme temperature and pressure, distinct phases of gas and liquid do not exist.
After 176 days of drilling, the $15 million Iceland project, using stainless steel liners to address corrosion, reached the targeted 4,659 m in January 2017 and encountered supercritical conditions (452 °C – the highest temperature ever measured in a geothermal well). The next steps have been to conduct further testing and research, particularly flow testing and fluid handling experiments. A final determination on the technology and electricity production economics will not be known until the end of 2018.
With these extreme conditions, corrosion has been a big problem
During the drilling, several key engineering challenges have arisen. One big problem is that the superheated steam contains acidic gases that are extremely corrosive, posing potential challenges to even highly corrosion-resistant nickel alloys that are typically used in conventional geothermal applications. Testing has shown that alloys such as C-276 and 625 fare relatively well at lower temperatures (180 °C), but corrosion rates were significantly higher at 350 °C. It is abundantly clear that the deep drilling environment, with its even higher 400 °C temperatures and highly corrosive environment, has created new challenges across the spectrum of alloys.
If this resource can be harnessed, the potential for additional geothermal resources could be enormous in geological areas of the world where young volcanoes occur and magma is relatively close to the surface. For this resource to be cost-effectively developed, the right nickel-containing alloys necessary for this challenging task need to be identified.