By Sarah Derouin, Ph.D.
The pathway to decarbonizing the energy grid of the United States is moving forward. Recently, WSP USA announced the completion of the drilling operations and mechanical integrity tests for two new hydrogen storage caverns. The caverns are in Delta, Utah, on the Advanced Clean Energy Storage project site.
This location will be a spot where renewable energy will be used to create green hydrogen — a process of splitting water into oxygen and hydrogen fuel by wind and solar power. The green hydrogen will be stored in underground human-made caverns created in salt domes, ready for use in energy production.
This work is the first phase of the ACES Delta hydrogen hub, a facility that is planned to help decarbonize the Western U.S. Phase 1 of the project includes creating two storage caverns in salt formations — which takes a mixture of engineering know-how and geologic expertise.
But what does it take to form such caverns?
The hydrogen storage caverns are created in natural salt domes. Engineers drill through rock layers to get to deep salt domes. Then, they create space — using a process called leaching, where the salt is dissolved by fresh water— within the salt to create the necessary volume to store hydrogen for the long term.
Salt is a good geologic substance for storing gas and liquid and has a long track record of success with fossil fuel storage. The caverns developed in salt provide a resilient, protected, leak-proof reservoir to inject, store, and deliver fuels like hydrogen.
A suitable site for underground gas storage requires a few things. “Right off the bat you need to have salt of a certain depth and sufficient quantity and quality,” explains Scyller Borglum, Ph.D., a petroleum and geologic engineer who is vice president of underground storage and energy at WSP USA. She notes that salt layers occur in many places around the world, but it takes a certain type of salt formation — typically domal salts — to work for long-term, underground storage of hydrogen.
Domal salts form when a layer of salt is put under enough heat and pressure to cause it to ooze upward, intruding into sediment layers above it. The salt moves vertically and creates a water-bottle-shaped salt deposit known as a diapir. Borglum says on the American Gulf Coast, domal salt caverns can be the size of the Empire State Building. Delta is another place where domal quality salt exists.
Drilling into salt is also different than drilling into rock. “You need to be aware of the geomechanical properties of the salt,” Borglum says. Although the salt is treated like a rock, in actuality it is a mineral and tends to be more plastic than solid rock. In addition, over time, heat and pressure cause the salt to creep after the cavern has been created. “Depending on where you're at in the country, it can creep significantly faster than what you would ever see with rock,” Borglum says.
Successfully developing a salt storage cavern also requires access to a lot of fresh water to dissolve the salt. She notes that for each barrel of space created in a salt cavern, 7-10 barrels of fresh water is needed to dissolve the salt. Considering that much of the country is semiarid, access to a lot of fresh water “could be a serious lift,” she says.
And once the cavern salt is dissolved, you have to dispose of the brine. Sometimes this means storing the brine in artificial lakes known as a leach ponds. But companies often pump the brine into saline aquifers located deep underground and not used for consumption.