Executive summary: Green hydrogen, generated from excess renewable electricity, will be an important component of the future zero-emissions energy system. Hydrogen has a wide range of applications including transport fuel, industrial feedstock, and electricity generation to meet excess demand. Hydrogen is projected to account for at least 10% of the global energy system in 20 years. At current energy demand, this corresponds to 72 PJ (~600,000 tonnes) of hydrogen annually in New Zealand, of which 7 to 18 PJ may need to be held in storage. Storage allows production to take advantage of intermittent renewable surplus at low cost, to accommodate peaks in energy demand, and to provide a strategic reserve. Meeting this storage demand would require up to 200 cryogenic tanks of the most advanced design, or about 500 vertical shafts, which are currently an unproven technology. It could also be met through subsurface storage in a small number (e.g., <10) of porous reservoirs. The large storage volumes of porous reservoirs, which are here referred to as Underground Hydrogen Storage (UHS), will permit larger-scale hydrogen production and provide sufficient capacity for managing out-of-phase supply and demand cycles. Globally, underground storage of pure (>98%) hydrogen has only been achieved in caverns excavated from salt deposits, which are not present in New Zealand. Instead, this report focuses on the potential for UHS in subsurface porous rock formations of Taranaki. Two types of storage systems are considered here, depleted oil and gas reservoirs and saline aquifers. Depleted oil and gas reservoirs are the most attractive of the two options because containment of buoyant fluids over geological time is proven, they are usually associated with extensive subsurface characterization, and have existing infrastructure that could be repurposed (e.g., wells and pipelines). Optimism in the published literature is high for storage in both aquifers and depleted natural gas reservoirs with foundational research and technical trials showing promising results. Effective UHS in porous-media requires the geological system to have sufficient storage capacity, deliverability and security of containment to meet operational specifications. Taranaki UHS systems require three key elements; i) a porous reservoir sandstone to store the hydrogen, ii) an effective geological cap rock (top seal) to prevent hydrogen migrating upwards out of the reservoir, and iii) a suitable geological trap to prevent the hydrogen migrating around the cap rock. The preponderance of hydrocarbon accumulations in the Taranaki region demonstrate that sedimentary strata commonly form effective reservoir-cap rock trap systems. The largest and geologically simplest traps are at the crests of anticlines at depths of <2.5 km. The suitability of five reservoir sandstone formations and three cap rock mudstone formations are considered here. These rocks range in age, mineral composition, physical properties, and burial depths. Reservoir sandstones range in thickness from 10s to 100s of metres, with permeabilities of mainly 10-100 mD. The Tariki Sandstone in the Ahuroa field and McKee Sandstone in the McKee field have the greatest potential for UHS. Cap rocks typically comprise fine silt and clay sized particles with porosities of a few percent, permeabilities of <5 mD and thicknesses of >10 m. Reservoir and sealrocks in Taranaki are dominated by silicate minerals (quartz, feldspars, clay) that are unlikely to undergo significant alteration over a typical hydrogen storage cycle. Geochemical modelling of hydrogen-brine-rock systems suggests that reservoir and cap rocks containing sulphates, carbonates (e.g., calcite or dolomite) or pyrite could react with injected hydrogen resulting in mineral dissolution and/or precipitation. Sulphate minerals are generally absent in New Zealand reservoir and seal rocks; many also do not have significant carbonate or pyrite and are considered unlikely to produce adverse reactions. However, carbonate and pyrite content is variable and will need to be assessed for each UHS site. Furthermore, hydrogen-rock reactions are dependent on sub-surface conditions (e.g., temperature, pH, pressure, chemistry) and studies are recommended to predict the degree of rock reactivity and potential resulting changes in rock properties. Preliminary reservoir models were constructed for UHS at three depleted gas fields (Ahuroa, McKee and Rimu) and one saline aquifer (Ahuroa shallow sand). These were characterized using data from well and field reports and published literature. Dynamic (annual) modelling suggests that the depleted reservoirs could have storage capacities up to 850 TJ per well. The McKee scenario has the largest storage due to its high permeability and porosity, thick storage interval and relatively large pressure depletion. Dynamic storage between 55 and 290 TJ at Rimu per well is probably sufficient to accommodate (10 to 15% of) the ~600-770 TJ of hydrogen production estimated for a nearby Waipipi windspill scenario. Static modelling suggests total storage capacity could be 5 PJ at Ahuroa (if converted from natural gas), and 1 PJ at Rimu, which collectively are approaching the estimated requirements of a future hydrogen economy (7 to 18 PJ). These volumes exceed the capacity of other large storage options (cryogenic storage, artificial caverns, linepack). Modelled hydrogen transfer rates are lower at Rimu and McKee (0.45 and 7.0 TJ/d) than Ahuroa (18 to 33 TJ/d). These hydrogen rates are less than the current estimated energy transfer performance for natural gas at Ahuroa (65 TJ/d).Monitoring of UHS is likely to be a regulatory and operational requirement for storage sites. It will ensure that infrastructure (e.g., wells and pipelines) and reservoir performance are within specifications, confirm containment and help manage adverse rock reactions and leakage, which could result in contamination and loss of the recovered hydrogen. Stored hydrogen can be monitored using atmospheric techniques, monitoring wells or geophysical methods. Monitoring wells are widely used in industry and are the most prospective means of confirming stored hydrogen and reservoir-cap rock conditions (pressure, temperature and chemistry). The number of wells will depend on a range of factors including, site conditions, desired resolution and implementation budget. Published studies emphasise the need for case-by-case evaluation using research tailored to a region or reservoir’s particular characteristics. Despite the large datasets available for many depleted oil and gas reservoirsin Taranaki, additional information may be required to support UHS operationalisation by reducing uncertainties. These investigations may include characterisation of 3D geological models, reservoir-cap rock properties, chemical reactions, microbiological activity, reservoir engineering performance and UHS monitoring requirements.