Geothermal Well Design, Construction and Failures

Abstract

When a high temperature geothermal well is completed, there is a risk that the well fails on the first heat up or that there is a delayed failure due to the formation environment like the presence of corrosive fluids. When a failure occurs, the loss of production and cost of repair can be quite significant compared with the cost of completing the well. The components in these failures are the casing and cement used in the well. Design and construction techniques have been developed to minimise the risk of catastrophic failure of casing. Also, studies have shown that implementing code design requirements will not necessarily contribute to reducing or eliminating the risk of casing failures due to high temperatures. The use of investigative techniques like down hole video cameras has shown how failures can be categorised. Cement and casing design alone cannot minimise the risk of failure. Construction methods are examined. It is apparent that any oversight or slight deviation in implementing good construction techniques increases the risk of casing failure. Case studies from various geothermal developments in the Pacific - South East Asia region are used to illustrate casing failure mechanisms and causes of such failure. Large and standard diameter wells are compared. In addition, this paper examines two common design and construction options used in geothermal wells, namely, single string design and the use of the tieback liner arrangement. Statistics of successful wells versus failures are assembled to provide risk profiles for each design option and casing failure mechanism.