An assembly for circulating tool fluid of a downhole tool includes a tool fluid source line extending into a subterranean well from a wellhead spool assembly to the downhole tool. A tool fluid return line extends out of the subterranean well from the downhole tool to the wellhead spool assembly. A tool fluid reservoir is located at a surface outside of the subterranean well. The tool fluid source line is in one way fluid communication with the tool fluid reservoir and the tool fluid is free of fluid communication with wellbore fluids.