Casing connection integrity is a critical element in ensuring structural integrity and pressure containment of casing strings throughout the life of a geothermal well. A primary loading mechanism for cemented intermediate and production casing strings is constrained thermal expansion and contraction. Sufficient temperature variations can lead to thermally-induced axial loads in the casing string that exceed the elastic limit, resulting in plastic deformation.
Accordingly, traditional connection performance envelopes that are based on design margins below the elastic limit are no longer applicable for such scenarios. Therefore, connection evaluation procedures that incorporate post-yield cyclic thermal loading are required to ensure structural integrity and sealability in geothermal applications where such temperature variations can occur.
Existing thermal well casing connection evaluation protocols, such as ISO/PAS 12835, which provides procedures for the assessment of threaded casing connections for thermal well applications with peak operating temperatures between 180°C and 350°C, can form the basis of a connection evaluation program for high-temperature geothermal applications. Although ISO/PAS 12835 was not developed specifically for geothermal applications, the principles and procedures outlined in the protocol can be suitably adapted to evaluate connection performance under geothermal operating conditions.
This paper presents a case study on a customized casing connection evaluation program based on ISO/PAS 12835 procedures for a peak operating temperature of 290°C. The program evaluated the sealability and structural performance of 244.5 mm, 69.9 kg/m and 339.7 mm, 101.2 kg/m grade L80 JFELION™ premium connection designs for a high-temperature geothermal application in Indonesia.
Key components of the evaluation program are described, including the material property characterization, finite element analysis (FEA), and full-scale testing. Important considerations for executing the evaluation program are highlighted, and areas for modification and extrapolation of existing casing connection evaluation procedures to develop a fit-for-purpose geothermal casing qualification protocol are introduced.