A wave of developers have announced their intention to extract hydrogen from naturally occurring stores of the gas underground. But while this so-called gold or white H2 is expected to be extremely cheap to produce, it is unknown exactly how it will compare to other methods of generating the molecule in terms of greenhouse gas emissions.

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Natural hydrogen could have an emissions intensity of 0.37kg of CO2-equivalent (CO2e) per kilo of H2 based on modelling in a new study published in peer-reviewed scientific journal Joule.

This would put it within the Tier 1 boundary of up to 0.45kgCO2e/kgH2 for the full $3/kg rate of the US clean hydrogen production tax credit.

The paper, entitled Greenhouse gas intensity of natural hydrogen produced from subsurface geologic accumulations, models the potential carbon emissions from drilling and extracting H2, with a baseline scenario of 50 wells with the extracted gas made up of 85% hydrogen, 12% nitrogen and 1.5% methane, with the remaining 1.5% inert gases such as argon and helium.

In this model, while the wells produce 160.7 tonnes per day of raw gas, this has to be separated, dried and compressed to a net 39.1 tonnes of H2 that can be sold.

The model assumes that some of the hydrogen is burned on-site to power some of these processes.

The most emissions-intensive stage is gas separation, during which fugitive greenhouse gases and those released during venting and flaring can reach more than 2.25 tonnes per day of CO2e in the first year.

The next most emissions-intense stage in the first year is the initial drilling, which has embodied emissions from the cement and steel sourced for the equipment.

And while natural hydrogen has a fairly low emissions intensity, the paper notes that there are a number of scenarios that could significantly increase it.

These include flaring the waste gas or firing it directly to power on-site processes. In the first case, this increases the amount of H2 that has to be used on-site (thereby reducing output), while in the second, the methane content of the waste gas results in higher greenhouse gas emissions.

The model also assumes that waste gas is reinjected into a disposal well at a similar pressure to each producing well, rather than back into the productive deposit.

“Although that kind of reinjection could be done to provide pressure support in the production formation through volumetric replacement of some of the produced gas, it would also result eventually in the breakthrough of large amounts of waste materials like N2 and CH4,” the paper notes.

And if the concentration of methane in the subsurface is high, the carbon intensity of the hydrogen creeps up. If there is 75% H2 and 22.5% CH4 in the deposit, the paper calculates that hydrogen from these wells would emit 1.5kgCO2e/kgH2.

Similarly, while the paper acknowledges that hydrogen in the atmosphere can increase the proportion of greenhouse gases — making leakage extremely problematic — it also notes that drilling for natural H2 may actually offset molecules that already seep to the surface from underground.

“It is possible that long-term interaction between H2 production and natural H2 seeps could exist, such that production and consumption of H2 from the subsurface could result in less natural seepage over time. This would potentially offset leakage impacts from H2 use by avoiding natural seepage.”