Sebastian Jung

Active cooling of downhole instrumentation for drilling in deep geothermal reservoirs

To ensure efficient geothermal energy exploitation, hot and deep reservoirs have to be drilled. During the drilling process all components of a downhole drilling and evaluation system, which can be a measurement while drilling (MWD) or a wireline tool, are exposed to harsh environmental conditions like strong vibrations and temperatures up to 250°C. These high temperatures can impact the electronics or sensors of the bottom hole assembly. Therefore, for deep and hot geothermal wellbores, active cooling provides an option to extend the life time of the components and improve the reliability of the entire system. Techniques that lower the temperature inside the cooling system below the downhole temperature that exists in the wellbore are considered as active cooling methods. Thermal insulation is critical for the efficiency of an active cooling system. Phase change from solid to liquid as well as from liquid to gas provide a good compromise between cooling power, cooling time and independence from electrical power. Several active cooling methods were investigated. An example of a phase change from liquid to gas in combination with a sorption process is explained in more detail and in combination with experimental results.

Sinter-attach of Peltier dice for cooling of deep-drilling electronics

We evaluate Ag particle sintering as a highly reliable die attach technology for the assembly of thermoelectric modules. Bismuth telluride (Bi2Te3)-based Peltier coolers are realized using Ag sintering and tested for high-temperature applications (e.g. Measurement-While-Drilling, MWD). Bonding takes place at a pressure of 6 N/mm2 and a temperature of 250°C for 2 min. Shear tests are performed for evaluating the adhesion according the American military standard for chip-substrate contacts (MIL-STD- 883H, method 2019.8). Sintered layers are analyzed for porosity, Youngs modulus, electrical conductivity, and thermal conductivity. It is shown that due to specific additives (i.e. micro-diamond particles) the coefficient of thermal expansion (CTE) of sintered Ag layers can be reduced to meet the requirements of Bi2Te3 as well as next-generation thermoelectric materials (e.g. skutterudites) for minimized thermally induced stress during drilling.