Increasing Temperature Range of Geothermal Production Casing by Mechanical Pre-tensioning

Abstract

Thermally-induced forces in a fully cemented geothermal production casing can cause compressive failure during production of high temperature formation fluids. These failures lead to production downtime and costly repairs. The compressive forces can be mitigated by applying a pre-tension load on the casing during the cement curing period. The pre-tension method presented in this thesis is a new technique which consists of applying a drill string weight on the casing through mechanical slips during the cement curing period. A representative case study was constructed to test the feasibility of the method and to reveal the axial performance of both casing and drill string, and corresponding effects on well design, before and after the application of the pre-tension load. Analyses show that neither the casing nor the drill string yields during the pre-tensioning operation, and an increased temperature range of the casing during production is achieved. As pre-tension forces reduce the threshold of tensile failure in the casing, some care must be exercised when cooling down the well.