In the search for new oil and gas reservoirs, the industry is likely to drill, characterize, complete, and produce wells with a reservoir temperature higher than 150°C, which are considered to be high-pressure/high-temperature (HP/HT) wells. An increasing number of ultra-HP/HT (high-pressure, high-temperature) wells with reservoir temperature above 205°C are also likely to be drilled. Downhole tools experience high failure rates at these conditions since there is a limited catalog of conventional electronic components that can reliably operate above 150°C Active and passive cooling of electronics are options for extending the operability and reliability of downhole tools in HP/HT and ultra-HP/HT environments.
Passive methods provide cooling for a short duration because they are designed to provide a fixed capacity for heat absorption from the tool. Examples of such methods include the use of a flask (or vacuum-jacketed tool housing) and the use of phase-changing materials. If the tool is likely to be exposed to HP/HT or ultra-HP/HT conditions for a long duration, then the use of active cooling methods may become necessary. Active cooling methods use electric power to reject heat (absorbed from the tool) at relatively lower temperatures to the higher-temperature wellbore fluid (or the formation) by using a suitable heat pump or a thermodynamic cycle. It is important to choose the optimal thermodynamic cycle and working fluid so that the power consumption is minimized. We present a process analysis of various thermodynamic cycles and demonstrate that the vapor compression cycle is the most efficient thermodynamic cycle for cooling downhole tools. In addition, we present an analytical methodology to identify a suitable fluid for this cycle. Water was chosen as the fluid for the vapor compression cycle; it is environmentally friendly, easily available, and has a high latent heat of vaporization.
The vapor compression cycle was demonstrated in a part of a wireline formation tester tool. A special chassis was constructed that permitted flow of refrigerant through it. Heating elements and thermocouples were installed on the chassis to simulate heat dissipation from electronic components during operation. To simulate the high-temperature downhole environment, the tool was heated by two 4.5feet long, 2500 W jacket heaters. Steam was supplied to this chassis at different flow rates to balance the heat load at two reservoir temperatures. Heat generated on the chassis, and heat transferred across the vacuum-jacketed housing was absorbed by the refrigerant. This demonstrated that the vapor compression cycle is efficient and capable of actively cooling downhole tools.
Experimental results indicate that, using the vapor compression cycle with a refrigerant flow rate of 10 mL/min, it is possible to remove 175 W and 100 W of heat from the chassis when the tool is exposed to a reservoir temperature of 200°C and 250°C, respectively.