Active cooling of a down hole well tractor

Wireline interventions in high temperature wells represent one of today’s biggest challenges for the oil and gas industry. The high wellbore temperatures, which can reach 200 °C, drastically reduce the life of the electronic components contained in the wireline downhole tools, which can cause the intervention to fail. Active cooling systems represent a possible solution to the electronics overheating, as they could maintain the sensitive electronics at a tolerable temperature, while operating in hotter environments. This work presents the design, construction and testing of an actively cooled downhole electronics section, which is able to cool the critical electronics below 175 °C while operating at 200 °C. After the investigation of several cooling techniques and the thermal characterization of the studied downhole electronics, thermoelectric coolers were chosen to implement a novel concept of heat management for downhole tools. The chosen design combined active and passive cooling techniques aiming at efficient thermal management, preserving the tool compactness, and avoiding the use of moving parts. Topology optimization was used, in combination with a finite element model of the system, to develop the final design of an actively cooled prototype, which was able to continuously maintain the temperature-sensitive electronics below 170 °C, while operating at 200 °C for more than 200 hours. Effective electrical integration of the cooling system in a wireline downhole tool was also studied, and a power-width-modulation circuit was developed to adapt the downhole power source to a suitable voltage for the thermoelectric cooler. The implementation of the active cooling system was supported by the study of the thermal interaction between the downhole tool and the well environment, which was relevant to define the heat rejection conditions. Given the lack of information from the scientific literature, a downhole sensor that could experimentally quantify the heat transfer rate occurring between the tool and the wellbore was designed and tested. The concept was proved and the sensor calibrated in a laboratory flow loop. Average and maximum mismatches of 3% and 10%, respectively, were found between the measured and predicted heat transfer coefficients, showing good agreement between experimental results and model forecasts.