Reservoir Simulation-Informed Cost-Benefit Analysis for Structural Assessment: A Case Study of Hydrogen Geostorage in the Ahuroa Gas Field, Aotearoa New Zealand

Abstract As hydrogen production increases around the world, subsurface porous media such as depleted gas reservoirs are being considered for buffer storage of large volumes of renewable gas. In Aotearoa New Zealand, Ahuroa gas storage reservoir in the Taranaki region is a possible site to be converted into underground hydrogen storage (UHS). However, from a structural perspective, this site is incompletely characterized, with differing model interpretations arising because only 2D seismic data and well intercepts are available. In this study, based on the exiting data, two geological models (curved fault model and tear fault model) were built. The same hydrogen storage scenario was evaluated on both models to study the impact of geometric uncertainty on UHS performance. To model hydrogen migration scenarios in the reservoir-fault system, we use a workflow that integrates Leapfrog Energy for geological modeling and PFLOTRAN-OGS for reservoir simulation. Our model describes cyclic injection and production of hydrogen/methane gas mixtures from the water-wet reservoirs, quantifying the operational performance of the site with respect to the undetermined fault geometry and transmissibility. Simulation results indicate that in the tear fault model less cushion gas is required but more hydrogen was ultimately unrecoverable from a distant region beyond a spill point. Economically, the cost of additional cushion gas trades off against the cost of hydrogen loss, both of which are on the order of several tens of millions of dollars. Our findings highlight the value of robust geological modelling and seek to build an economic case for further data acquisition to reduce uncertainty.