Numerical and Experimental Evaluation of Liner Dual Barrier System in Geothermal Wells

Abstract

Drilling and completion of production interval in geothermal wells is a critical operation. Liner dual barrier system consists of two barrier elements - a seal assembly and cement sheath between formation and liner. Any defects in the barrier system can transform into a potential well integrity risk. Limited research is available on failures of these barriers for geothermal applications. The objective of this study is to investigate performance and failure modes of dual barrier system at high temperature and CO2 conditions. In geothermal CO2 -rich reservoirs, well barriers usually experience sever degradation. For this study, a setup similar to production casing/liner program of geothermal well was fabricated using two concentric pipes with elastomer and cement sheath to seal off the annular space. Sealing performance of ethylene propylene diene monomer (EPDM), a commonly used elastomer material was investigated at 1000 psi and 180°F under CO2 exposure. Autoclave was used for aging elastomer sample. Exposure to aging conditions caused cracks and blisters on elastomers. This resulted in failed sealability test. Finite element model similar to the setup was created. Simulation results also indicated lack of sufficient sealability after CO2 exposure. For cement barrier, the experimental setup was used to investigate cement bonding with pipe. Even at room temperature, cement sheath exhibited weak bonding at inner pipe interface. This caused failure in sealability test. Material properties of cement cured at elevated temperatures up to 350 °F were used in the finite element model to examine effect of temperature and wellbore pressure on cement integrity. Stresses in cement were analyzed to identify modes of failure.