Thermal Stress Mitigation Testing of Externally Cemented Flexible Coupling in Geothermal Steam

Abstract

Casing failures in-high temperature geothermal wells occur if the casing is overstressed. The most common failures occur because of axial forces and/or differential pressures. For medium-enthalpy geothermal wells, thermal cycling has the potential to severely deteriorate the integrity of the cemented annulus and for higher enthalpy wells yielding of casings becomes a structural concern. The recently innovated patent solution, flexible couplings, aims to reduce thermal straining by allowing displacement into the connections of each casing segment at approximately 12 m intervals (API R3). In the GEOTHERMICA internationally co-funded GeConnect project a full-scale experiment is performed at a well site in a high-temperature geothermal field. The experimental set-up is composed of two concentric casings cemented together and the inner one equipped with a flexible coupling. The aim is to investigate effects of thermal cycling on well integrity and to test the function of cemented-in flexible couplings using a bypass of a producing well. Cement sheath integrity and cement-casing boundary are evaluated at moderate to high temperatures, e.g. by using fiber optic distributed sensing technology. Structural models are used to analyze casing-cement interactions and to evaluate prospects and potential improvements of well integrity by using the new technology of flexible couplings.