Corrosion Behavior of L80- Carbon Steel and 13 Cr Casing Materials at 400°C in Simulated Superhot Geothermal Well Environment

Recently, interest has increased substantially globally in utilizing geothermal energy from superhot or supercritical geothermal resources which has led to the initiation of several projects in this field around the world. In majority of these projects this means drilling deeper than conventional high temperature geothermal wells (>3 km) to obtain superhot geothermal fluid (>350°C) with enthalpy above the critical enthalpy (>2.086 MJ kg^-1) and higher flow rates which will allow increased productivity for these wells. But there are several challenges faced in the design and construction of deep superhot (>300°C) geothermal wells due to higher temperatures and pressure, and increased corrosiveness of the geothermal fluid. In this study L80- carbon steel and L80-13Cr stainless steel API casing materials were tested in a simulated superhot geothermal well environment at 400°C in H₂O vapor with H₂S and CO₂ gases for 168 hr in a high temperature and high pressure (HTHP) autoclave. The results showed that both materials had low uniform corrosion rates (CR) (<0.1 mm/year) but were prone to localized corrosion in the form of pitting. The low CR of the materials is likely due to the high superheat and low density of the steam at the HTHP conditions. Interestingly Fe₂Cr₂O₄ formed along with magnetite and silicates on the L80-13Cr stainless steel and iron silicates (Fe₂(SiO₄)) on the L80-CS due to the presence of SiO₂ residue in the HTHP environment.