Capillary trapping in cyclic injection and withdrawal for geological hydrogen storage

Geological hydrogen storage in deep saline aquifers is a promising approach to supporting large-scale deployment of renewable energy. However, hydrogen recovery is strongly influenced by rock heterogeneity andcyclic injection–withdrawal behaviour, which remain poorly constrained. This study investigates the effect of rock type on two-phase flow characteristics and residual hydrogen trapping through miscible tracer tests and unsteady-state cyclic core flooding experiments. Four sandstones with different degrees of heterogeneity were examined under reservoir-representative high-pressure and high-temperature conditions using hydrogen and brine. Tracer tests ranked the samples from most homogeneous to least homogeneous, after which cyclic flow tests were performed. Initial gas saturations after the first injection cycle ranged from 0.54 to 0.71, with higher values in more homogeneous rocks. In contrast, heterogeneous rocks exhibited more pronounced viscous fingering, reduced displacement efficiency, and increased residual trapping. Residual trapping also depended on heterogeneity: in homogeneous samples, values stabilized after the first cycle (0.22–0.27), whereas in heterogeneous samples, residual trapping increased over successive cycles by up to 0.04 saturation units. The results highlight the advantage of homogeneous formations for hydrogen storage, offering higher storage capacity, reduced hysteresis, and more stable cyclic behaviour. In such reservoirs, extensive cyclic flow tests may be unnecessary, while in heterogeneous formations, increased residual trapping over multiple cycles must be accounted for in reservoir modelling. These findings provide key inputs for improving large-scale simulations and economic feasibility assessments of hydrogen storage projects.