Storage capacity comparison of hydrogen and carbon dioxide in heterogeneous aquifers

Underground gas storage is widely recognized as a crucial strategy in pursuing low-carbon and even zero-carbon emissions. This paper is motivated to investigate the injection process of underground gas storage. CO2 and H2 are prominent representative stored gases, also chosen as studied ones for their significant differences in physical properties, especially density. Simulations are conducted on models with different heterogeneities. Sensitive cases are designed with a vertical/horizontal well and under different injection strategies, to study the storage capacity of underground aquifers and the transportation process of injected gases during the injection process. Results show that the variations of injection mass and injection time of horizontal well systems have almost the same trends and values as those of the vertical well system for both H2 and CO2. As the injection rate increases, the effective storage efficiency (net injection mass friction) of H2 in the vertical well systems decreases while that in the horizontal increases, with relatively little difference for CO2. For dissolution trapping in a vertical well system, evaluation parameters (average concentration, dissolution mass, and fraction) increase as rising heterogeneity enhances the fingering gas-water contact but is relatively constant as the injection rate varies. Nevertheless, in horizontal well systems, they are relatively constant for CO2 but decrease for H2 with increasing injection rate, as less injection time limits the vertical gravity-induced migration, but are relatively constant with heterogeneity, as increased heterogeneity weakens the gravity-induced migration contact while improving the horizontal fingering contact. Besides, in these cases, the post-injection has a limited effect on the injected mass and overall storage efficiency and is not suggested due to its low time efficiency.