Abstract The harsh environment in high-temperature geothermal wells poses significant challenges to casing integrity. As alternatives to Oil Country Tubular Goods (OCTG) materials, titanium alloys have shown great corrosion resistance in geothermal wells. However, little work has been done to investigate the structural performance and sealability of titanium alloy casing and casing connections. Using advanced Finite Element Analysis (FEA), an engineering study was performed to evaluate the sealability of a premium casing connection with a proprietary titanium alloy material. Coupon tests were conducted to characterize the temperature-dependent mechanical response of the titanium alloy as the key inputs for the study. Thermal cycle loading representative of geothermal well conditions was applied to the FEA model to quantitively evaluate the connection sealability performance. The advantage of the sealability performance of the titanium alloy connection was demonstrated by comparing the model response of the titanium alloy connection to the same connection design with L80 material that was previously qualified for this application by full-scale physical testing. The titanium alloy connection showed a much more stable sealability response when compared to the L80 connection throughout the thermal cycles. This superior performance was primarily attributed to the elastic response of the pipe body, relatively low thermal stress, and minimal plastic deformation within the connection. Preliminary physical make-and-break test results were also shown to demonstrate the galling resistance of several connection types made of the titanium alloy.
The engineering study established a strong technical basis for further development of casing and premium casing connections with titanium alloys for challenging high-temperature geothermal well applications. The technical information in this paper provides a reference for high-temperature geothermal and thermal well operators who are considering alternative materials to improve casing integrity.