Comparing completion design in hydrocarbon and geothermal wells: The need to evaluate the integrity of casing connections subject to thermal stresses

Tapping for geothermal energy very often requires deep drilling in order to access high-temperature resources. This type of drilling is expensive and is financed by the operator with a long period of debt service before costs can be recovered from the energy sale (heat, electricity or a combination of both). Drilling costs are only a part of the total well expenditure. Tubulars can double the total well cost, especially when complex well completions are required. Together, drilling and well completions can account for more than half of the capital cost for a geothermal power project. A comparison is made of the different completions used for oil, gas and geothermal wells, and geothermal well completion requirements are discussed. Special attention is given to the thermal stresses induced by temperature variations in the casing string of a geothermal well. When the induced thermal stresses exceed the yield strength of the casing material, the fatigue behavior of the latter can be defined as low-cycle fatigue (LCF). The connection threads in the casing body amplify the local stresses and lower the LCF resistance. A theoretical approach is presented to evaluate that parameter, and calculations are compared with preliminary results from experiments on large-diameter Buttress connections, which are commonly used in geothermal well completions. It is shown that under extreme loads the LCF resistance of the Buttress thread connection can be as low as 10 cycles.