Non-condensable gases (NCGs) occur naturally in geothermal fluids. They result from degassing of magma chambers underground and fluid-rock interactions within a geothermal reservoir. Non-condensable means that, unlike water, these gases can’t condense (i.e. be turned into liquid) by the cooling which occurs in geothermal power stations at the surface.
Geothermal reservoirs store NCGs dissolved in geothermal fluids. In the absence of any development, natural geothermal features, such as fumaroles and bubbling hot pools, can slowly vent the gases from deep underground into the atmosphere.
Geothermal fluids have a wide range of NCG concentrations — their composition depends on the geological and geochemical condition of the reservoir and the temperature of the reservoir fluid. Common geothermal NCGs include carbon dioxide (CO2), hydrogen sulfide (H2S), ammonia (N), methane (CH4), nitrogen (N2), hydrogen (H2), and Argon (Ar). The most abundant geothermal gases are CO2 (the most dominant NCG), CH4 and H2S, with the other gases considered trace gases.
No source of energy is zero emissions, but all renewables have life cycle emissions, which are an order of magnitude less than fossil fuel energy sources. Geothermal is a critical source of renewable energy for Aotearoa New Zealand as the nation transitions to a low-carbon energy sector and economy. This video explains why.
For geothermal projects, NCGs are brought to the surface with the geothermal fluids where, currently, they are mostly released during the power generation process. But NCGs must be managed to minimise their environmental impact: CO2 greenhouse gases, while has other environmental considerations.
Geothermal electricity generation does not create CO2, as there isn’t any combustion like in fossil fuel power plants. Harnessing the geothermal energy just alters the rate of emissions, which would eventually end up in the atmosphere anyway via natural surface features. Some New Zealand geothermal reservoirs have naturally high levels of CO2 (such as Ohaaki, which is an outlier) and others have naturally low levels (such as Mokai).
During the geothermal electricity generation process, NCGs are removed from condensers (or the binary plant equivalent: vaporisers) and released into the atmosphere (‘vented’), so they do not build up and prevent the efficient operation of the plant.
But not all the NCGs are vented. Reinjection is used to return NCGs to the reservoir where they originated. In some binary plants a significant amount (up to ~20%) of NCGs remains in the geothermal fluid and is reinjected with no modification to the plant (called “passive reinjection”).
The NCGs can also be reinjected actively, by modifying the plant to dissolve the NCGs into the geothermal fluid in the reinjection line (or at depth in a reinjection well via tubing). This type of “active reinjection” is being trialled in New Zealand at several geothermal power stations, and has been used in Iceland, Turkey and the USA.
You can download this short explanation on CO2 management made by the NZ Geothermal Association.
To better understand the effects of NCG reinjection on the geothermal reservoir, factors to consider include rock and fluid interaction (permeability/porosity alteration caused by mineral precipitation and dissolution), and potential pressure changes.
It is also important to understand the type of geothermal system before starting reinjection, which is why our Geothermal: The Next Generation research programme includes an investigation of NCG reinjection strategies. Our team is testing injection of NCG into various reservoir rocks to determine the NCG sequestration potential (permanent storage of NCG in mineral form) of common rocks at supercritical conditions. There is a significant amount of permanent mineral storage of NCG at the Carbfix site at Hellisheidi in Iceland, where the rocks are basaltic. This is not the dominant volcanic type in the vicinity of New Zealand’s geothermal power stations and this option for NCG sequestration requires investigation, as the mineralisation potential may be different.
However, even without permanent mineral storage, NCG reinjection is still worthwhile as the gases will be cycled back underground, where they will be dissolved in geothermal fluid rather than being released into the atmosphere.
More details on emissions values for New Zealand geothermal power plants can be found in these recent publications: