A New Approach for Material Characterization to Predict Casing Behavior for High Temperature Geothermal Applications

Abstract Manufacturers follow the Qualification of casings connections for thermal wells Protocol (ISO/TS 12835:2022(E)) to design and qualify casing threaded connections for geothermal applications. This standard considers Finite Element Analyses (FEA) to highlight the critical configurations. To better simulate connection behavior according to the ISO/TS 12835 protocol, an enhanced material law is required. This paper proposes an approach to characterize and model the material behavior of an API carbon steel (L80) for geothermal applications considering cyclic behavior, temperature dependance and stress relaxation. Simple strain driven cyclic tests at various temperatures including holding times were used to separate the cyclic hardening and the viscoplastic responses. This allowed to characterize the behavior of the material over the target range of temperatures, from 20°C to 450°C. A specific data treatment was used to properly estimate the creep strain rate as a function of the stress level allowing to formulate a creep law able to predict the stress relaxation which occurs during the holding times. Tensile tests were also performed to assess the static plasticity of the material. Yield strength (YS) derating for L80 described a reduction of YS up to 18.1% at 350°C compared to YS at room temperature. The specific strain-controlled cyclic tests have displayed stress relaxation phases during each constant strain step which emphasize a creep behavior even at low temperature but with a very significant increase between 350°C and 450°C. It also shows a progressive softening of the material (reduction of the maximum stress with the number of cycles achieved) especially at room temperature. The material test results were used to create a material law in the commercial software ABAQUS®. The impact of this enhanced material law on the response of a threaded connector to the ISO/TS 12835 protocol highlights new potential structural and functional failure modes consistent with those reported on real cases in literature.