Casing and tubing in either oil and gas or geothermal wells are designed to withstand pressure loads (burst and collapse) and axial tension - the typical parameters that should be considered in any casing design. However, in geothermal applications, temperature is a major consideration in casing design because of thermal stresses generated by the high temperature geothermal fluids. Elevated temperature reduces the yield strength of the casing which essentially reduces the burst rating, collapse resistance and tensile strength of the material.
High temperature will be experienced by the well once it is on production. The cemented casing strings will be subjected to significant cyclical axial stress caused by changes in temperature due to production of high temperature geothermal fluids that can then be followed by the injection of cold water for well kill operations to perform wellbore maintenance. During production, the cemented casing will enter a compressive state where plastic deformation occurs since the casing is not allowed to expand. When cold water is injected to kill the well, axial tensile stresses are generated. This paper will explore this thermally induced cyclical loading in geothermal wells, and discuss how to account for these loads in the design process.
Another consideration in casing design is the selection of threaded connections that can maintain strength and seals at elevated temperatures. It is also important to evaluate the potential of trapping water in the annulus between two casing strings during cementing operations. When the well is put on production the water expands due to the large increase in temperature, creating a significant amount of pressure between the two casing strings, which can lead to a collapse failure of the inner string or a burst failure of the outer string. Examples of these failure mechanisms will be discussed in detail.