High Temperature and Pressure Testing of Casing Materials For Superhot Geothermal Wells

There are numerous challenges in the design and construction of superhot and deep geothermal wells. These challenges include problems due to cementing and degradation of cement casings due to very high temperature and pressure, and high stresses and strain due to large thermal expansions and contraction of steel casings during thermal cycling. This occurs because the yield strength and elastic strain limits of carbon steel materials commonly used for well casings is exceeded during thermal cycling due to decrease in tensile and yield strength of carbon steel at elevated temperatures. Additionally, due to high temperatures (>300°C) and corrosiveness of the geothermal fluid increased corrosion and erosion-corrosion effects are expected in superhot geothermal wells. In this paper, the main material challenges are described and discussed and recent results from relevant high temperature and corrosion testing are reviewed. The potential application of corrosion resistant alloys instead of standard API carbon steel casing material is discussed based on material properties and corrosion resistance. The state of the art of cement casing materials are also reviewed and discussed. The first steps in high temperature testing of novel CRA and cement casing materials in a custom made high temperature and high pressure autoclave laboratory with incorporation of corrosive gases at temperatures >300°C is described. The material advances needed for successful design and construction of superhot geothermal wells are discussed and the main findings summarized.