Impact of hydrogen solubility on depleted gas field's caprock: an application for underground hydrogen storage

Abstract

Depleted gas fields are considered a low-risk location for underground hydrogen storage purposes to balance seasonal fluctuations in hydrogen supply and demand. The objective of this study was to identify any significant risk of hydrogen leakages stored in depleted gas fields. The capability of the storage area in terms of sealing efficiency varies with parameters such as rate of diffusion, solubility, thickness and capillary threshold pressure of the caprock. The most common caprock are shales, which contain organic material. The solubility of hydrogen into organic material could change the petrophysical properties of the rock, such as porosity and permeability. Any changes in these petrophysical characteristics can reduce the capillary threshold pressure thus reducing the caprock efficiency for the safe storage of hydrogen. There is about 20% of the remaining gas volume in the depleted gas field, which helps to prevent brine from entering the production streamlines and maintain reservoir pressure. The characteristic data of hydrogen at different high pressures and temperatures have been evaluated and imported into the simple finite element model using the Python programming language. Most of the parameters that influence reducing the strength of the caprock are identified. Crucial parameters are the rate of diffusion, the solubility of hydrogen in kerogen, geomechanical deformation, threshold capillary pressure, long period of injection and withdrawing of hydrogen. The model shows that the native gas production with hydrogen is low due to significant density variation and mobility ratio between methane and hydrogen. Finally, a wide range of parameters and reservoir conditions has been considered for minimising the potential risks of possible leakages.