Evaluation of Casing Collapse Capacity in Ultra-high Temperature Geothermal Wells

Abstract

A common cause of casing failure in geothermal wells are due to collapse caused by compressive hoop stress that occurs during heating of the well. In traditional design, the empirical formulas from API TR 5C3 are applied for determining the casing collapse capacity. However, the experimental data that constitute the foundation for the API formulas are limited in the range of axial stress and temperature. For high temperature geothermal wells, thermal expansion causes high axial compressive stresses, that might be outside the range of the API formulas. Another factor that is not included in the API formulas is the support from the surrounding cement. In this study, we use a combination of data from ultra-high temperature material testing and Finite Element Analyses to investigate the casing collapse capacity for load cases relevant for ultra-high temperature geothermal wells. The results from the Finite Element Analyses show that the collapse capacity increases for high levels of axial stress. This was observed for a wide range of material input data, diameter-wall thickness ratios and for un-supported and supported conditions.