Abstract
Ultra-high temperature geothermal wells (>450 °C) have a large potential for increased energy yield as compared to conventional high-temperature geothermal wells (200-300 °C), but several research challenges must be resolved before robust operation in this temperature range can be achieved. In this study, yield- and tensile strength data for several relevant carbon steels and corrosion resistant alloys are generated as a step on the way to enable design of collapse- and tensile capacity for geothermal casings exposed to temperatures up to 500-550 °C. The experiments extend the data set listed in NZS 2403:2015 by providing data for higher temperatures and different material classes. It is found that the carbon steels follow the same near linear decay in strength as the NZS 2403:2015 curves up to 350 °C, and then display a significant drop in tensile strength at higher temperatures, particularly for the lower strength steels. The alloys with high nickel content work harden significantly more than the carbon steels at high temperatures and they tend to retain their strength at temperatures above 350 °C. The tested titanium alloy shows high yield strength and low work-hardening at 500 °C and in contrast to the tested nickel alloys, do not display dynamic strain ageing.
Highlights
Ultra-high-temperature geothermal wells (>450 ◦C) have a large potential for increased energy yield as compared to conventional hightemperature geothermal wells (200-300 ◦C) (Elders et al, 2014, Frið leifsson et al, 2014)
The latter is related to the lack of yield plateau in the K55A material, which result in lower yield strength and relatively high yield strength as function of temperature (YSF) at higher temperature
The serrated stress-strain behaviour at higher temperature in the nickel-based alloys and the austenitic steel is explained by the PortevinLeChateliers effect (PLC) effect or dynamic strain aging
Summary
Ultra-high-temperature geothermal wells (>450 ◦C) have a large potential for increased energy yield as compared to conventional hightemperature geothermal wells (200-300 ◦C) (Elders et al, 2014, Frið leifsson et al, 2014). Today’s high-temperature geothermal wells with depths up to 3000 m are exposed to pressures and temperatures that are dominated by formation conditions and at highest follow the boiling point depth curve down to the critical point (374.15 ◦C and 221.2 bar for pure water) (Thorhallsson et al, 2014). Exceptions of the conditions exceeding the boiling depth curve have been seen in cases where the magma body is found at shallow depth and superheated conditions prevail. It follows that the main design code for geothermal wells, NZS 2403:2015 (New Zealand Standard, 2015), provides mechanical data for the relevant casing materials for temperatures up to 350 ◦C. Since high-temperature geothermal wells still are under development, data on the mechanical properties of traditional casing materials and potential novel materials at elevated temperatures is not readily available
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