Australian engineers are discovering whether depleted gas reservoirs with strong cap rock can safely store hydrogen.
Hydrogen, according to a University of Oklahoma study, is “emerging as a key player in the global effort to reach net-zero emissions by 2050”.
The researchers state that “this clean energy carrier is especially important for reducing carbon emissions in areas that are difficult to address using alternative approaches, such as heavy industries, transportation and heating”.
Hydrogen is the most abundant element in the universe, but natural accumulations in the subsurface are rare. Since the early 70s, hydrogen has been stored underground in salt caverns around the world. Recent initiatives include Advanced Clean Energy Storage in Utah and the UK’s HyDeploy. But what happens in places where there are no salt caverns?
One such place is Victoria’s onshore Otway basin, the site of Lochard Energy’s H2Restore project. Here, engineers are investigating whether depleted underground gas reservoirs might perform a similar function to salt caverns.
Jacqui Sutton, Lochard Energy’s Principal Reservoir Engineer, is involved in the first stage of the project, the feasibility study.
“The challenge is to store hydrogen in large enough quantities to support variable renewable energy generation, energy security and grid stability,” Sutton told create.
“In the onshore Otway basin, there is a good-quality natural gas reservoir and cap rock. Within these formations, the high pressure needed to prevent the hydrogen from escaping the formation occurs naturally.”
Infographic: How do you build a hydrogen refuelling station?
Understanding the risks
According to CSIRO, which is also working on the H2Restore feasibility study, “hydrogen tends to have a high diffusivity and therefore can migrate through a range of materials. In the sub-surface, hydrogen could migrate through the caprock with the potential to escape the storage formation with unforeseen consequences.”
Caprock is the layer which overlies an underground rock formation and can play a crucial role in preventing gas or fluids from escaping a storage reservoir. To perform this function, the caprock must be thick, non-porous and stable enough to withstand pressure changes.
“As hydrogen is a smaller molecule, if there’s a leak, there is higher potential for it to escape from the reservoir and potentially find an ignition source,” Sutton said.
There have been hydrogen explosions, some in storage facilities above ground. But these are rare, and there hasn’t been a single reported case of hydrogen exploding in an underground storage facility.
“Hydrogen does have a larger flammable range than natural gas, but this doesn’t mean it is unsafe,” Sutton said. “Many of the technologies used to monitor potential leaks – well casings, well seals, and abandoned legacy wells – are similar to those used for underground natural gas (UGS) and carbon capture and storage (CCS).
“An important difference, though, is the impact of hydrogen on certain materials in the well design, which could result in the development of leakage pathways. Risks and impacts such as seismicity, seal breaching and leakage, failing cavern integrity and subsidence are site-specific issues that can be monitored and mitigated in a similar manner as in UGS and CCS.”
Investigating microbial interactions
For large-scale hydrogen storage, a depleted gas reservoir must also be durable enough to withstand chemical changes – particularly, Sutton explained, reactions between the hydrogen and microbes in the rock formation.
“Hydrogen is not a native gas that exists in these reservoirs and may be a food source for microbes underground. If hydrogen is consumed by the microbes, the by-product may be hydrogen sulfide, which is toxic at high concentrations, although it can be safely removed before the hydrogen is used.”
To this end, H2Restore engineers have taken water samples from the sub surface, “to see what kind of microbes are present, test if hydrogen is consumed in their presence and, if so, what other gases are generated as a result,” Sutton said.
“Underground hydrogen storage can support the build-out of renewable electricity generation infrastructure in Australia and contribute to the overall goal of carbon dioxide emissions reduction.
“It has the potential to help balance the mismatch between periods of high renewable energy production in the summer months, and higher energy demand and lower renewable generation in the colder months.”
The CISRO website goes further, stating that “depleted gas field storage is likely to be the most promising and widely available [underground hydrogen storage technology] in Australia and will require less initial investment”.
The H2Restore project feasibility study is expected to conclude by the end of 2025. Depending on the outcome, a pilot demonstration facility is planned to test small-scale storage in one depleted gas reservoir, with a commercial facility for large-scale storage mooted for the early 2030s.
At this Engineers Australia event in June, explore how systems engineering practices can help bolster the hydrogen supply chain.
