Cement-Rock Bond Integrity under Injection-Production Cycling in Underground Hydrogen Storage (UHS)

The shift from fossil fuels to renewable energy is essential for mitigating climate change and meeting the Paris Agreement goals. However, the inherent intermittency of solar and wind energy necessitates robust, large-scale storage solutions to balance supply and demand. Converting excess renewable energy into hydrogen for UHS in geological formations offers a high-capacity and long-duration storage option. Despite its potential, the introduction of hydrogen into the geological system presents uncertainties. One of these concerns is the long-term wellbore integrity, particularly at wellbore interfaces, which act as the primary barrier to fluid containment and leakage prevention.

This study investigates how hydrogen exposure and cyclic injection–production affect rock-cement bond integrity under simulated subsurface conditions. Experiments were performed on rock-cement composites prepared with Class G cement and two reservoir sandstone analogues representative of North Sea formations, with the rock-cement interface perpendicular to the plug cross section. Corsehill sandstone, clay rich ~50 mD, and Bentheimer sandstone, quartz rich ~600 mD. A subset of samples was exposed to hydrogen at 70 °C and 18 MPa for 50 days. To represent operational cyclicity relevant to UHS, samples were tested under triaxial loading and subjected to 5 and 20 pore pressure cycles. Nitrogen-exposed and unexposed sister plugs were used as controls to isolate hydrogen-specific effects. A comprehensive suite of measurements, including flow testing, fluid sampling, and acoustic velocity monitoring, was used to quantify interfacial degradation and assess the effects of cycling rate and cycle number. Following the experiments, several samples were analysed using micro-CT to characterise fracture patterns in the rock-cement interfacial transition zone.

Results show that permeability decreases during pore pressure cycling, with the largest reduction occurring during the first cycle, 30% for cement-Corsehill and 12% for cement-Bentheimer samples. Slower cycling rate result in a greater reduction in permeability over successive cycles. At higher cycle numbers, permeability increases at later stages, consistent with enhanced dissolution, as supported by the ICP results. In several samples, fractures observed within the cement and propagated parallel to the cement-rock interface, indicating that cement adjacent to the interface is the mechanically weakest zone, while the interface itself remains intact. These responses also controlled by the reservoir rock properties, and the higher permeability samples show stronger bond integrity.

These findings demonstrate that operational cycling conditions and site-specific geological properties are key controls on wellbore integrity, with direct implications for the safe and long-term deployment of UHS.