Evaluation of Sorbents at Elevated Temperatures for Downhole Application

The drilling of geothermal wellbores provides a basis for an efficient exploitation of geothermal energy. It is usually more demanding than conventional high-temperature applications for the oil and gas industry. The temperatures are generally higher, and all components of the downhole system will be exposed to temperatures between 200 °C and 250 °C and to harsh environmental conditions like strong vibrations, shocks, and high pressures. The high wellbore temperatures may prevent the utilization of electronic components and can negatively influence accuracy and precision of sensors. Due to the drilling depth of deep geothermal wellbores a failure of a downhole system seriously impacts the cost and economical efficiency of the geothermal project. Therefore, reliable technical solutions are paramount for the success of geothermal projects. To date, there is no existing commercial technology capable of providing instrumentation that meets the requirements for the operation of downhole instrumentation in deep and hot geothermal wellbores. One solution under investigation is to use an active cooling system to cool existing sensors and electronics. Techniques that lower the temperature inside the cooling systems below the downhole temperature are considered as active cooling methods, in contrast to passive heat dissipation. Active cooling systems based on phase change from solid to liquid as well as from liquid to gas provide a good compromise between cooling power, temperature drop, and operation time. While phase change from solid to liquid was used in the past, it provides significantly lower enthalpy compared to a phase change from liquid to gas. Therefore, the liquid-to-gas concept is appealing. Unfortunately, the large gas volume is not easy to manage downhole. The sorption of gas in a desiccant is one potential solution and will be further investigated in this paper. Important for the considered cooling system are selection of an optimal phase change material and the design and application of a suitable sorption process. We designed two special experimental setups to investigate the sorption process, and to characterize the sorption rate and capacity of different desiccants at elevated temperatures and pressures. We show results of a characterization of two potential desiccants for downhole application.