Implications of Reservoir Anisotropy and Interlayer Heterogeneity for Hydrogen Recovery in Underground Storage

Underground hydrogen storage (UHS) in depleted reservoirs presents a promising solution for managing seasonal variations in renewable energy during the global energy transition. However, the impact of reservoir heterogeneity, particularly permeability anisotropy and interlayer characteristics, on hydrogen recovery efficiency remains insufficiently understood. To bridge this knowledge gap and improve storage site selection accuracy, we developed a systematic box model to evaluate the effects of reservoir heterogeneity and validated our findings using New Zealand's Ahuroa gas storage field.

Our investigation revealed that permeability anisotropy affects hydrogen recovery efficiency, with variations depending on well patterns. For well patterns with vertical wells only, both lateral (k_x/k_y) and horizontal-to-vertical (k_h/k_v) permeability anisotropy enhanced hydrogen recovery efficiency. For combined vertical and horizontal well patterns, the effect varied by anisotropy type. Lateral (k_x/k_y) anisotropy enhanced efficiency when horizontal wells aligned with the maximum permeability direction. In contrast, when horizontal wells aligned with the minimum permeability direction, k_h/k_v anisotropy exhibited an optimal ratio, beyond which efficiency began to decline. Analysis of interlayer effects revealed that reducing permeability from 1 mD to 10^-3 mD led to an enhancement in hydrogen recovery efficiency, increasing from 61% to 75%. Additionally, our investigation demonstrated that the presence of interlayer pinch-outs and discontinuities along vertical hydrogen migration pathways reduced hydrogen recovery efficiency. A realistic geological model corroborated the box model findings: hydrogen recovery efficiency improved from 66.6% to 77.9% as the k_x/k_y ratio increased from 1 to 10, and from 66.6% to 76.2% when the k_h/k_v ratio increased similarly. Furthermore, inaccurate estimation of interlayer permeability could result in an 11.3% deviation in hydrogen recovery predictions.

These results underscore the importance of accurately characterizing reservoir heterogeneity, including permeability anisotropy and interlayer properties, to ensure reliable hydrogen recovery predictions and improve site selection for UHS.