Prospectivity analysis for underground hydrogen storage, Taranaki basin, Aotearoa New Zealand: A multi-criteria decision-making approach

The problem

In 2022, 87.1% of New Zealand's electricity came from renewable sources (Ministry of Business, Innovation and Employment, 2022). However, natural gas and coal are still used to meet seasonal shortfalls in supply. One option is to turn surplus renewable electricity into hydrogen by electrolysis, store it underground while demand is low, and withdraw it as demand rises. Salt caverns are currently the only proven stores for high-purity hydrogen, and New Zealand has no salt deposits. Storage in porous rock, such as a depleted gas field, is considered the most viable large-scale option for the country (Nicol et al., 2022).

Using estimates from Yergin (2020) and Firstgas (2021), we assumed a 2050 hydrogen demand of 15 to 42 TWh a year. Ahuroa, the country's one underground gas store, holds about a tenth of annual gas use. If the same share applies to hydrogen in 2050, New Zealand would need 1.5 to 4.2 TWh of usable hydrogen storage. That is 1 to 3% of the country's 2022 energy use of 150 TWh (MBIE data, our arithmetic). These figures exclude cushion gas, the gas left in a reservoir to maintain pressure.

What we did

Taranaki Basin is New Zealand's only basin with commercial petroleum production. We assessed 40 sites there: all significant past and present oil and gas fields, one undeveloped gas discovery and one deep aquifer. We estimated each site's hydrogen capacity from its recoverable gas and oil reserves.

A panel of ten geoscientists chose 18 technical parameters in five groups: site properties (trap structure, capacity, porosity and depth), injectivity (how readily gas flows into and out of the rock), containment, reactions of hydrogen with the rock and fluids, and data availability and quality. We ranked the sites with a decision tree, a sequence of yes-or-no questions on size, depth, permeability, proven oil or gas, reactivity, leakage and data.

We also ranked them with a weighted matrix. To enter it, a site needed 1 to 20 TWh of capacity, a reservoir top deeper than 1 km, and an average permeability above 5 mD. Each qualifying site scored 1 (poorest) to 3 (best) on each parameter, and each parameter had a weight from 1 to 5. The weighted, normalised scores sum to a rank score between 0 and 1. The highest weights were assigned to data quality, top-seal and fault-seal risk, and hydrocarbon phase. We favoured gas reservoirs on the premise that a caprock proven to hold natural gas is more likely to hold hydrogen. These weights are subject to our own biases. We therefore tested the ranking with higher and lower weights on reactivity parameters, and with higher weights on reservoir parameters.

What we found

Both methods placed the same two sites first: Ahuroa, an onshore field now used to store natural gas, and the Kaimiro reservoir of the offshore Maui A field. Their matrix scores were 0.806 and 0.796. However, Ahuroa's position depends partly on confidential data from its gas storage operation. The neighbouring Tariki field has a broadly similar structure, reservoir and seal but scored 0.756, mainly because its public data are poorer.

Miocene reservoirs ranked lowest. Typically, they are small, split into separate pools, and more likely to hold oil than gas. They are also often poorly lithified, and their fine-grained, clay-rich sands may degrade under repeated injection and withdrawal. The deep aquifer scored lowest of all (0.525), mainly because its containment is unproven and few of its data are public.

The top ten scores spanned 0.725 to 0.806. Input from other experts could change their order. When we lowered the weights on reactivity, Maui A moved to first place, and Ahuroa led in the other three. Paleogene reservoirs, mostly gas fields or gas caps above oil, filled the top ten in all four weighting scenarios in Table 5.

Our results suggest that developing one or two depleted fields would provide enough storage for New Zealand by 2050. Together, the two leading sites have an estimated maximum capacity of 23 TWh of hydrogen, including cushion gas. Of this, 21.6 TWh is at Maui A (Table 6), above the 20 TWh entry limit for the matrix.

At the cushion gas share of about 30 to 50% that we cite for depleted fields (Ennis-King et al., 2021), the two sites would provide 11.5 to 16.1 TWh of working gas (our arithmetic). Our estimates of the 2050 need are 1.5 to 4.2 TWh (Section 3.1) and about 5 TWh (Section 6.2). Both are below this range. However, the cushion gas share will be site-specific, may range from about 20 to 80%, and will depend on gas composition. At 80%, working gas would be 4.6 TWh (our arithmetic). Cushion gas needs to be excluded from capacity estimates in more detailed site characterisation.

Limitations

We assessed technical suitability only. Economic, social, cultural and regulatory factors can make a site unsuitable. In New Zealand, rights to pore space, and access to it, also need careful discussion with the indigenous population. We gave lower weights to parameters whose effect on hydrogen storage is still uncertain. Our studies suggest that the main risks needing further research include (1) losses and contamination from hydrogen reacting with CO2, coal and other materials, and (2) leakage through seals, because hydrogen diffuses more readily than methane and has a lower viscosity and density. We aim to select sites for characterisation that can address some of these risks, and then to revise the parameters, weights and criteria.

Glossary

Electrolysis splits water into hydrogen and oxygen using electricity.

A terawatt hour (TWh) is a billion kilowatt hours, a unit of energy.

A depleted field is an oil or gas field from which most of the recoverable petroleum has been produced.

An aquifer is a porous rock layer filled with water. Storage is generally limited to saline aquifers, because of the risk of contaminating drinking water.

Pore space is the small gaps between the grains of a rock, where gas or water is held.

Cushion gas is retained in a storage reservoir to maintain its pressure. Working gas is the part that is injected and withdrawn.

A caprock, or top seal, is a low-permeability rock layer above a reservoir that prevents gas from migrating upward out of it. A fault seal is a fault that prevents gas from flowing across it.

Permeability measures how easily fluid flows through rock, here in millidarcies (mD).

A gas cap is a layer of natural gas that lies above oil in the same reservoir.

Hydrocarbon phase records whether a reservoir held gas, gas above oil, or oil only.

A lithified sediment has been compacted and cemented into rock.

The Paleogene is the geological period of the Paleocene, Eocene and Oligocene epochs. The Miocene is the younger epoch that followed it.