A Pore-Scale Study on Cyclic Flows in Porous Underground Hydrogen Storage

A solution for energy storage is needed to balance supply and demand in future energy markets with an increased share of fluctuating renewable wind and solar energy. A key enabler is large-scale, seasonal energy storage, where porous underground formations are suitable storage solutions. An experimental pore-scale study was performed with the objective to investigate hydrogen flow, gas saturations, residual trapping and contact angles in porous media. Hydrogen injections were performed on a water saturated two- dimensional micromodel with pore network based on a natural sandstone relevant for underground hydrogen storage in an aquifer. The effect of capillary number on primary drainage and cyclic hydrogen injection was studied with a pore pressure of 40 bar and room temperature. An image segmentation algorithm was developed to extract gas sat- uration from microscope images of the pore space. A sensitivity analysis showed that the image segmentation algorithm had on average a relative uncertainty of 12%, pre- dominantly caused by water accumulations and uneven light source distribution. During primary drainage the hydrogen saturation increased for increasing capillary numbers. During imbibition the hydrogen saturation decreased until it stabilised at the residual gas saturation, where hydrogen was disconnected from the main flow, immobile and unable to recover. Static contact angles for primary drainage and imbibition varied between 19 to 71°. Local gas displacement through the field of view was observed during drainage and imbibition, resulting in fluctuating hydrogen saturation despite an expected increase for drainage and decrease for imbibition. Experiments showed that gas recovery should increase for higher capillary number, but a lower recovery was observed for the highest capillary number caused by local displacement through the field of view. Cyclic injections were performed at different capillary numbers and the development in hydrogen saturation was studied. Local gas displacements in and out of the field of view affected the end-point hydrogen saturation for each cycle. The end-point hydrogen satura- tion showed hysteresis where both gas development and end-points varied between cycles and repeated injections with equal capillary numbers. The gas saturation dependency on capillary number was weaker when gas was injected over several cycles. The relationship between increased gas saturation with increased capillary number was weaker for cyclic injections compared with primary drainage. Hydrogen loss during cyclic injection was studied by identifying the overlapping hydrogen distribution during drainage and imbibition. Trapped gas relative to initial gas satura- tion after drainage decreased with increasing capillary number. The results indicated increasing trapped gas with increasing cycles, but no clear trend was observed.

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