The current technology for geothermal well drill ing is characterized by the use, often unsatisfactorily y, of drill pipes made of carbon steels, according to the standard practice in oil and gas fields. An experimental work has been carried out on laboratory heats and on industrial drill pipe in order to define criteria for the selection of innovative steels for geothermal applications. The Electro-Slag Remelting (ESR) technique applied to drill pipe materials was found to be effective in decreasing impurity levels (mainly sulfides), with positive effects on the mechanical characteristics (especially toughness) as well as on the resistance to
environmental y assisted fracture. Nevertheless, the positive effect of steel cleanliness and of the improved mechanical properties on stress corrosion and fatigue corrosion sensitivity was found unable to completely solve the practical problems of failure in service. As a matter of fact, all the steel examined proved to be subject to stress corrosion cracking in the simulating geothermal environment. Environmental control by lime addition to the drilling mud, together with the use of ESR
treated steels, proved to be very effective in overcoming the strong sensitivity to environmental assisted cracking in sour environments.
Keywords: carbon dioxide, hydrogen sulfide, corrosive gas, geothermal wells, sulfide stress cracking, drill pipe, clean steel