A Study of Underground Hydrogen Storage in 2D Box Models with Properties Comparable to Typical Depleted Gas Fields in the Netherlands

Abstract A 2D numerical modeling study of underground hydrogen storage (UHS) in depleted gas fields, with properties from Dutch fields, is presented. The study quantifies the impact of reservoir properties (dispersivity, heterogeneity, permeability and dip), well configuration, and choice of hydrogen or methane as a cushion gas on the reservoir performance of a UHS. Reservoir performance is defined as production purity and recoverability of hydrogen. The results will support the selection of depleted gas fields for UHS in the Netherlands and for techno-economic analyses on UHS. A 2D numerical box model is initially filled with methane at an abandonment pressure of 60 bar. Cushion gas is injected at one end of the model, until a minimum cycle pressure (120 bar) has been reached. Then during a series of hydrogen injection and production cycles the reservoir pressure is varied between minimum (120 bar) and maximum (220 bar) values. Sensitivities described above are run for various cycle lengths (month, year) and for a total length of up to 40 years. The purity of production and recovery of stored hydrogen is quantified, and the hydrogen-methane interface movement is analyzed. The model results support the argument for choosing hydrogen as cushion gas, by showing an initially much better production purity thereby lowering the costs of the separation of co-produced methane from the hydrogen. It is observed that when choosing methane as the cushion gas, dispersion moves the methane cushion gas into the well where it is produced back and replaced in the reservoir by hydrogen over the first 10's of cycles, thereby offsetting the advantage in a lower initial hydrogen investment. After these cycles methane production is reduced to the point where the purity of the produced hydrogen is the same for both choices of cushion gas. Based on the model results, criteria for selecting a suitable UHS candidate field are derived. One of the main criteria is the absence of a layered permeability contrast. A sequence with a random lognormal distribution of permeability as well as a sequence of alternating low and high permeabilities with a factor 10-100 contrast, caused immediate breakthrough of the already present natural gas (methane) even for the hydrogen cushion gas case. UHS reservoir performance is also reduced for scenarios with longer timescales (longer cycle lengths of a quarter to half a year) and higher gravity numbers (high permeabilities), because gravity segregation causes the methane to underrun the hydrogen and reach the well. A partial completion at the top of the reservoir does not improve performance because of coning of the methane into the well, while a structure with a dipping top and bottom seal does prevent the methane from reaching the well. This could therefore be a criterion for field selection. Further study is proposed into strategies that might improve UHS performance (e.g. lowering the abandonment pressure) and into the impact of more detailed realistic scenarios of injection and production cycles (e.g. more fluctuating and intermittent storage demand and more reservoir property scenarios which are representative of Dutch gas fields (e.g. lower permeability ‘waste zone’ Ten Boer layer at the top of the Bunther).