Why and When Does Casing Fail in Geothermal Wells: a Surprise Question?

Abstract

Casing failures in some geothermal wells cannot be easily explained using simple casing string mechanics. Some of these failures appear to be time dependent while others happen a few hours after the well is brought onto production. The detrimental effect of thermal stresses on casing are well known from the years of experience of steam injection in heavy oilfields and a geothermal well poses even more complex loading issues. After more than 100 years of geothermal experience, geothermal wells are still designed driven by oil and gas rules. However, oil and gas rules have never been developed for geothermal application, even though high pressure, high temperature wells pose some similarities. This paper describes why and when casing fails in geothermal wells by means of analytical and numerical models. Casing failure by buckling is generally accepted to occur at high temperatures, but it is problematical to understand such events when cement does not allow buckling to occur. One may ask the question why does casing fail if no buckling occurs? This paper describes the basic casing mechanics of such a failure event using analytical methods and integrates the connection behavior into this modeling. By explaining how casing can collapse and what the possible available solutions are, the results can be used to improve the design of future geothermal casing. The case studies are supported by FEM and analytical calculations, so that the paper can become a good compendium of how to improve your casing design.