Experimental investigation of caprock sealing capacity for underground hydrogen storage

Hydrogen is a key energy carrier in the global transition to low-carbon systems, requiring scalable and secure storage solutions. While underground hydrogen storage (UHS) in salt caverns is proven, its cost and limited geographic availability have led to growing interest in depleted oil and gas reservoirs. A critical factor in evaluating these reservoirs is the sealing capacity of the overlying caprock. This study presents a novel experimental protocol for assessing caprock integrity under UHS conditions, using a custom-designed core-flooding apparatus integrated with a micro-capillary flow meter. This setup enables high-resolution measurements of ultra-low permeabilities (as low as 10 nano-Darcy), flow rates (down to 10 nano-liters/hour), threshold pressure, and breakthrough pressure. Benchmark tests with nitrogen and methane were followed by hydrogen experiments across caprocks with a wide range of permeability and porosity. The results demonstrate clear trends between caprock properties and sealing performance, providing a quantitative framework for evaluating UHS site suitability. Hydrogen showed slightly lower threshold and breakthrough pressures compared to other gases, reinforcing the need for accurate site-specific caprock evaluation. The proposed method offers a robust approach for characterizing candidate storage sites in depleted reservoirs. • A novel core-flooding setup enables measurements down to 10 nano-Darcy and nL/hr. • Threshold and breakthrough pressures rise with higher gas-brine interfacial tension. • Caprock permeability and porosity inversely correlate with sealing capacity. • Hydrogen shows lower threshold and breakthrough pressures than nitrogen and methane. • Method supports screening of depleted reservoirs for hydrogen storage feasibility.