While there are rechargeable batteries rated to 125ºC, many service companies prefer not to use them because a rating of 150ºC is preferable. The industry typically uses one-time-use lithium primary batteries.
According to Robert Estes, manager of emerging technology at Baker Hughes, lithium batteries have been used in the oil field for 30 years or more and they are now fairly reliable up to somewhat above 150ºC.
However, he said, “lithium metal melts at around 180ºC. So if you go much above 150ºC with a standard lithium thionyl chloride one-time-use primary cell, then you risk getting the temperature externally as well as the internal temperature of the battery to the point where it will melt the lithium metal. And that can be a risk factor.”
The lithium metal can melt and combine in a very fast reaction called an explosion. It can blow a hole through the pressure housing and damage the tool. If the explosion occurs at the surface, it can be dangerous.
According to Estes, “the industry would like to go away from these solid lithium metal batteries—the thionyl chlorides or sulfuryl chloride types.”
But he explains that these batteries are very energy dense. “They are the most dense source of energy we can supply for downhole tools in a small space that will work up to 150ºC.” He says they can be alloyed to work up to 200ºC, but the performance of the battery becomes compromised.
Still, downhole energy storage is so important that research continues to produce better high-temperature chemistry and construction for all types of primary and rechargeable battery cells and supercapacitors.